Test method and device for total protein assay

Abstract: A test device and method of determining the presence and concentration of the total pro­ tein content, including albumin and Bence Jones proteins, in a test sample. The test device in­ cludes a test pad comprising a new and improved carrier matrix incorporating an indicator reagent composition capable of interacting with proteins to produce a visually or instrumentally detectable and/or measurable response. The new and improved carrier matrix of the test pad comprises a fibrous, bibulous substrate, such as filter paper, homogeneously impregnated with a polymerized urethane-based compound. The new and improved carrier matrix provides improved color resolution and increased sensitivity to proteins in dry phase test strip assays, thereby affording a more accurate and trustworthy protein assay of liquid test samples, such as urine. Furthermore, by utilizing a carrier matrix comprising a fibrous, bibulous substrate homogeneously impreg­ nated with a polymerized urethane-based compound, a dry phase assay for low molecular weight pro­ teins, such as Bence Jones proteins, can be per­ formed on a liquid test sample.

Description: FIELD OF THE INVENTION The present invention relates to a test device and a method of assaying a test sample for the presence and concentration of proteins. More particularly, the present invention relates to a new and improved method and device for assaying a liquid, such as urine, for proteins by utilizing a test device having a test pad comprising an indicator dye (referred to hereafter as indicator reagent composition) incorporated into a new and improved carrier matrix. The new and improved carrier matrix comprises a fibrous, bibulous substrate, such as filter paper, homogeneously impregnated with a polymerized urethane-based compound, such that a detectable and/or measurable response occurs upon contact of the indicator reagent composition incorporated into the carrier matrix with a protein-containing test sample. The new carrier matrix of the present invention provides improved color resolution and increased protein sensitivity in order to more accurately detect and/or measure, either visually or by instrument, the total protein content of a liquid test sample. In addition, the present invention relates to using a carrier matrix, comprising a fibrous, bibulous substrate impregnated with a polymerized urethane-based compound, and further incorporating an indicator reagent composition, as the test pad of a test device in a method to determine the presence and/or concentration of proteins, like albumin and Bence Jones proteins, in a test sample by a dry phase, test strip assay procedure. BACKGROUND OF THE INVENTION AND PRIOR ART Albumin is the most abundant plasma protein, generally constituting slightly over one-half of the total protein in mammalian plasma. In the human body, albumin has the important role of regulating the water balance between blood and tissues, and of functioning as a transport molecule for various compounds, such as bilirubin, fatty acids, cortisol, thyroxine and drugs such as sulfonamides and barbiturates, that are only slightly soluble in water. An albumin deficiency can restrict the transport of slightly water soluble materials throughout the body and a deficiency is signaled in an individual by an abnormal accumulation of serous fluid, or edema. Therefore, it is clinically important to determine whether an individual has a deficiency of serum albumin. Likewise, it is clinically important to determine if an individual is excreting an excess amount of protein. A normal functioning kidney forms urine in an essentially two step process. Blood flows through the glomerulus, or glomerular region of the kidney. The capillary walls of the glomerulus are highly permeable to water and low molecular weight components of the blood plasma. Albumin and other high molecular weight proteins cannot pass through these capillary walls and are essentially filtered out of the urine so that the protein is available for use by the body. The liquid containing the low molecular weight components passes into the tubules, or tubular region, of the kidney where reabsorption of some urine components, such as low molecular weight proteins; secretion of other urine components; and concentration of the urine occurs. As a result, through the combined processes of the glomerulus and tubules, the concentration of proteins in urine should be minimal to non-existent. Therefore, abnormally high amounts of albumin and/or low-molecular weight proteins in urine must be detected and related to a physiological dysfunction. The relatively high concentration of albumin in the urine of an individual usually is indicative of a diseased condition. For example, the average normal concentration of protein in urine varies from about 2 mg/dL to about 8 mg/dL, with approximately one-third of the total urinary protein being serum albumin. However, in a majority of diseased states, urinary protein levels increase appreciably, such that albumin accounts for from about 60 percent to about 90 percent of the excreted protein. The presence of an abnormal increased amount of protein in the urine, known as proteinuria, is one of the most significant indicators of renal disease, and may be indicative of various other non-renal related diseases. Therefore, in order to determine if an individual has an albumin deficiency and/or to determine if an individual excretes an excess amount of protein, and in order to monitor the course of medical treatment to determine the effectiveness of the treatment, simple, accurate and inexpensive protein detection assays have been developed. Furthermore, of the several different assay methods developed for the detection and/or measurement of protein in urine and serum, the methods based on dye binding techniques have proven especially useful because dye binding methods are readily automated and provide reproducible and accurate results. In general, dye binding techniques utilize pH indicator dyes that are capable of interacting with a protein, such as albumin, and that are capable of changing color upon interaction with a protein absent any change in pH. When a pH indicator dye interacts with, or binds to, a protein, the apparent pK a (acid dissociation constant) of the indicator dye is altered and the dye undergoes a color transition, producing the so-called "protein-error" phenomenon. In methods utilizing the dye binding technique, an appropriate buffer maintains the pH indicator dye at a constant pH to prevent a color transition of the pH indicator dye due to a substantial shift in pH. Due to the "protein-error" phenomena, upon interaction with the protein, the pH indicator dye undergoes a color transition that is identical to the color change arising because of a change in the pH. Examples of pH indicator dyes used in the dry phase assay of proteins that are capable of interacting with or binding to proteins and exhibiting "protein-error" color transitions include tetrabromophenol blue and tetrachlorophenol-3,4,5,6-tetrabromosulfophthalein. Although pH indicator dyes have been used extensively in protein assays, several problems and disadvantages still exist in protein assay methods utilizing indicator dyes. For example, methods based upon pH indicator dyes either cannot detect or cannot quantitatively differentiate between protein concentrations below approximately 15 mg/dL. In addition, although several simple semiquantitative tests and several complex quantitative tests are available for the determination of the total protein content in a test sample, the majority of these assay methods, with the notable exception of the simple colorimetric reagent test strip, require the precipitation of protein to make quantitative protein determinations. The colorimetric reagent test strip utilizes the previously discussed ability of proteins to interact with certain acid-base indicators and to alter the color of the indicator without any change in the pH. For example, when the indicator tetrabromophenol blue is buffered to maintain a constant pH of approximately 3, the indicator imparts a yellow color to solutions that do not contain protein. However, for solutions containing protein, the presence of protein causes the buffered dye to impart either a green color or a blue color to solution, depending upon the concentration of protein in the solution. Some colorimetric test strips used in protein assays have a single test area consisting of a small square pad of a carrier matrix impregnated with a buffered pH indicator dye, such as tetrabromophenol blue. Other colorimetric test strips are multideterminant reagent strips that include one test area for protein assay as described above, and further include several additional test areas on the same strip to permit the simultaneous assay of other urinary constituents. For both types of colorimetric test strips, the assay for protein in urine is performed simply by dipping the colorimetric test strip into a well mixed, uncentrifuged urine sample, then comparing the resulting color of the test area of the test strip to a standardized color chart provided on the colorimetric test strip bottle. For test strips utilizing tetrabromophenol blue, buffered at pH 3, as the indicator dye, semiquantitative assays for protein can be performed and are reported as negative, trace, or one "plus" to four "plus". A negative reading, or yellow color, indicates that the urine contains no protein, as demonstrated by the lack of a color transition of the indicator dye. A trace reading may indicate from about 5 to about 20 mg/dL of protein in the urine. The one "plus" to four "plus" readings, signified by color transitions of green through increasingly dark shades of blue, are approximately equivalent to urine protein concentrations of 30, 100, 300, and over 2000 mg/dL, respectively, and serve as reliable indicators of increasingly severe proteinuria. In accordance with the above-described method, an individual can readily determine, visually, that the protein content of a urine sample is in the range of 0 mg/dL to about 30 mg/dL. However, the color differentiation afforded by the presently available commercial test strips is insufficient to allow an accurate determination of protein content in urine between 0 mg/dL and about 15 mg/dL. The inability to detect and differentiate between low protein concentrations is important clinically because a healthy person usually has a urine protein level in the range of about 10 mg/dL to about 20 mg/dL. Therefore, it could be clinically important to know more precisely the urine protein content of an individual, rather than merely estimating the protein content at some value less than about 30 mg/dL. Of course, the protein content of a urine sample can be determined more precisely by semiquantitative protein precipitation techniques or by quantitative 24 hour protein precipitation techniques. However, these tests are time consuming and relatively expensive. Furthermore, the precipitation tests must be run in a laboratory by trained personnel, and therefore are unavailable for the patient to perform at home to quickly determine urine protein content and to monitor the success or failure of a particular medical treatment. Therefore, it would be extremely advantageous to have a simple, accurate and trustworthy method of assaying urine for protein content that allows visual differentiation of protein levels in the ranges of 0 mg/dL to about 10 mg/dL, about 10 mg/dL to about 20 mg/dL, and about 20 mg/dL to about 30 mg/dL, and upwards to between about 100 mg/dL to about 300 mg/dL. By providing such an accurate method of determining urine protein concentration in an easy to use form, such as a dip-and-read test strip, the urine assay can be performed by laboratory personnel to afford immediate test results, such that a diagnosis can be made without having to wait up to one day for assay results and medical treatment can be commenced immediately. In addition, the test strip method can be performed by the patient at home to more precisely monitor low levels of protein in urine and/or the success of the medical treatment the patient is undergoing. As will be described more fully hereinafter, the method of the present invention allows the fast, accurate and trustworthy protein assay of urine by utilizing a test strip that includes a test pad comprising a new and improved carrier matrix incorporating an indicator reagent composition. The carrier matrix comprises a fibrous, bibulous substrate, such as filter paper, homogeneously impregnated with a polymerized urethane-based compound. The new and improved carrier matrix of the present invention enhances the visual color resolution, and therefore the sensitivity, of the assay, thereby allowing urine protein concentrations to be accurately determined at levels of approximately 30 mg/dL or less. In addition, the method of the present invention can be used to determine the presence and/or concentration of low molecular weight proteins, such as Bence Jones proteins, in a test sample. All prior art assay techniques for low molecular weight proteins involve immunoelectrophoresis methods or heat test methods that are time consuming, relatively expensive and are not amenable for use by the patient at home to detect low molecular weight proteins in urine. Bence Jones proteins belong to a class of urinary proteins having a low molecular weight of approximately 20,000 and that are small enough to pass through the glomerular filters of the kidney. However, the Bence Jones proteins usually are reabsorbed in the tubular section of the kidney. Therefore, the concentration of Bence Jones proteins is negligible in the urine of a healthy person. As a result, a significant amount of Bence Jones proteins in urine generally is clinically significant. Overall, the detection and measurement of the concentration of low molecular weight proteins in urine is important because certain diseases are characterized by the excretion of specific low molecular weight proteins (globulins) rather than by diffuse proteinuria characterized by elevated albumin levels. For example, the Bence Jones proteins represent a portion of the high molecular weight plasma myeloma globulin, and therefore are found in increased amounts in the urine of more than one-half of the patients suffering from multiple myeloma or leukemia. Bence Jones proteinuria also is found in the urine of many patients suffering from macroglobulinemia and primary systemic amyloidosis. In addition, an increased excretion of a specific globulin that is similar to Bence Jones proteins occurs in Franklin's disease; and patients with renal tubular disorders, such as the Fanconi syndrome, show a substantial increase in the quantity of globulins excreted in the urine. Accordingly, investigators have searched for a simple assay for low molecular weight proteins because the dye-binding method used in presently available commercial test strips is insensitive to low molecular weight proteins, like Bence Jones proteins. Surprisingly and unexpectedly, the method of the present invention provides a technique to detect and measure the concentration of low molecular weight proteins, like Bence Jones proteins, by incorporating an indicator reagent composition into a new and improved carrier matrix comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound. The Bence Jones proteins differ from all other urinary proteins in that they coagulate upon heating to temperatures between about 45° C and about 60° C, and then redissolve on further heating to the boiling point of test sample. This peculiar characteristic of Bence Jones proteins has been the basis of all qualitative and semiquantitative determinations for Bence Jones proteins. The dye binding technique used in commercially available test strips has proved insensitive to Bence Jones proteins because the much greater relative concentration of higher molecular weight proteins, such as albumin, in the urine of a healthy individual effectively interferes with and masks the presence of Bence Jones proteins. Furthermore, it is inconvenient and costly to separate the albumin from Bence Jones proteins, thereby negating the utility of separating the albumin from the Bence Jones proteins before using a dry phase test strip. As a result, dry phase test strips are presently unavailable to test for the presence and concentration of Bence Jones proteins in urine. However, incorporating an indicator reagent composition into the improved carrier matrix of the present invention provides an improved assay test pad for the detection and measurement of the total protein content, including Bence Jones protein, of liquid test samples, such as urine. The carrier matrix of the present invention allows improved color resolution of the color transition resulting upon contact of the protein-containing test sample with the indicator reagent composition, therefore improving assay sensitivity and allowing the detection and measurement of protein content in liquids to levels as low as 10 mg/dL. Proteinuria resulting either from abnormally high albumin levels or the presence of low-molecular weight proteins depends upon the precise nature of the clinical and pathological disorder and upon the severity of the specific disease. Proteinuria can be intermittent or continuous, with transient, intermittent proteinuria usually being caused by physiologic or functional conditions rather than by renal disorders. Therefore, accurate and thorough assays of urine and other test samples for protein must be available for both laboratory and home use. The assays must permit the detection and measurement of the proteins of interest, either albumin and/or Bence Jones proteins, such that a correct diagnosis can be made and correct medical treatment implemented, monitored and maintained. In addition, it would be advantageous if the protein assay method, both for high molecular weight proteins, like albumin, and low molecular weight proteins, like Bence Jones proteins, could be utilized in a dip-and-read format for the easy and economical, qualitative and/or semiquantitative determination of protein in urine or other test samples. Furthermore, any method of assaying for protein in urine or other test samples must yield accurate, trustworthy and reproducible results by utilizing a method that provides a detectable and measurable color transition as a result of an interaction between the indicator reagent composition and the protein, and not as a result of a competing chemical or physical interaction, such as a pH change or preferential interaction with a test sample component other than protein. Moreover, it would be advantageous if the protein assay method is suitable for use in dry reagent strips for the rapid, economical and accurate determination of protein in urine and other test samples. Additionally, the method and test pad, comprising the carrier matrix and the indicator reagent composition, utilized in the assay for protein and the indicator reagent composition should not adversely affect or interfere with the other test reagent pads that are present on multiple test pad strips. Prior to the present invention, no known method of assaying urine or other test samples for proteins utilized a test device including a test pad comprising a carrier matrix comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound that provides improved color resolution and increased assay sensitivity compared to test pads absent the homogeneous impregnation of the polymerized urethane-based compound, such that accurate and trustworthy protein assays can be made for protein concentrations of about 30 mg/dL and below. In addition, although a dry phase chemistry test strip utilizing a dye, such as tetrabromophenol blue or tetrachlorophenol-3,4,5,6-tetrabromosulfonephthalein, has been used extensively for several years, no dry phase test strip has utilized a test pad comprising a carrier matrix comprising a fibrous, bibulous substrate, such as filter paper, homogeneously impregnated with a polymerized urethane-based compound to improve visual color resolution, and therefore increase assay sensitivity, at lower protein concentration levels. Furthermore, until the method of the present invention, dry phase test strip procedures were available principally to test for total protein concentration, i.e., for albumin, only down to levels as low as 30 mg/dL. However, surprisingly and unexpectedly, the method of the present invention permits the dry phase test strip assay of urine and other test samples for albumin and for low molecular weight proteins, such as Bence Jones proteins, down to levels as low as 10 mg/dL. The prior art contains numerous references on the wet phase and the dry phase chemistry utilized in the pH indicator dye method of assaying urine for proteins. For example, Keston U.S. Patent No. 3,485,587 discloses the basic dye binding technique used to assay for proteins at a constant pH. Keston teaches utilizing a single indicator dye, maintained at a constant pH slightly below the pK a (acid dissociation constant) of the dye and impregnated into a dry test paper, like filter paper, to determine the presence and/or concentration of albumin by monitoring the color transition of the dye. Free, et al., in U.S. Patent No. 3,095,277, also discloses a method of detecting the albumin content of liquid test samples by incorporating a suitable indicator composition into a bibulous carrier, like untreated filter paper. Similarly, Atkinson et al. in U.S. Patent No. 3,438,737 discloses a test device comprising a test composition impregnated into an untreated bibulous matrix, such as filter paper, wood strips, synthetic plastic fibrous materials, non-woven fabrics and woven fabrics for detecting protein in fluids. The earlier filed EP 0 361 244 describes a test device for assaying proteins which contains a dual reagent indicator reagent system. In contrast to the prior art, and in contrast to the presently available commercial test strips, the method of the present invention provides increased sensitivity in the detection and measurement of proteins in urine. Surprisingly and unexpectedly, by utilizing a carrier matrix of the present invention, comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound in a dry phase reagent test strip, protein levels of about 30 mg/dL and below can be determined accurately. In addition, the method of the present invention also allows the simple and essentially immediate detection and measurement of Bence Jones proteins. Hence, in accordance with the method of the present invention, new and unexpected results are achieved in the dry phase reagent strip assay of urine and other test samples for proteins, including low molecular weight proteins, by utilizing a test pad, comprising a carrier matrix comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound, incorporating an indicator reagent composition. SUMMARY OF THE INVENTION In brief, the present invention is directed to a new and improved test device, method of manufacturing the test device, and method of determining the presence and/or concentration of proteins in a test sample. The device includes an improved carrier matrix that is capable of incorporating an indicator reagent composition that interacts with a protein in a test sample to produce a detectable response. The improved carrier matrix of the test device comprises a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound. For home use, the reagent composition produces a visually detectable response. For laboratory use, the reagent composition produces a response that is detectable visually or by instrument. The new and improved carrier matrix of the device of the present invention comprises such fibrous, bibulous materials as filter paper that are homogeneously impregnated with a polymerized urethane-based compound. An indicator reagent composition then is homogeneously incorporated into the carrier matrix after homogeneous impregnation of the fibrous, bibulous substrate with a polymerized urethane-based compound. The carrier matrix then holds the indicator reagent composition homogeneously throughout the carrier matrix while maintaining carrier matrix penetrability by the test sample and by the test sample component to be assayed. More particularly, the present invention is directed to a method of assaying urine or other test samples for proteins by utilizing a test device including a test pad comprising a new and improved carrier matrix for the indicator reagent composition. It has been demonstrated that incorporating an indicator reagent composition into a carrier matrix comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound affords improved color resolution and increased sensitivity at low protein concentration ranges. In accordance with an important feature of the present invention, the qualitative and/or semiquantitative determination of protein levels between 0 mg/dL and about 2000 mg/dL, and especially between 0 mg/dL and about 30 mg/dL, in urine and other test samples is accomplished. By utilizing the carrier matrix of the present invention in clinical test methods, the qualitative and/or semi-quantitative concentration of proteins, such as albumin, in urine or other test samples can be more accurately determined because the improved color resolution afforded by the fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound increases the sensitivity of the dry phase assay method to low concentrations of protein. Furthermore, surprisingly and unexpectedly, the new and improved carrier matrix of the test device allows the detection and measurement of low molecular weight proteins, such as Bence Jones proteins, in urine and other test samples. Therefore, it is an object of the present invention to provide a new and improved test device and method for determining the relative concentration of proteins in a liquid. Another object of the present invention is to provide a simple, trustworthy, accurate and reproducible method of assaying urine or other test samples for proteins. Another object of the present invention is to provide a new and improved protein interactive test device for interaction with protein in a test fluid to produce a visible change, such as a change in color, of the test device, indicative of the protein concentration in the test fluid. Another object of the present invention to provide a method of assaying urine or other liquid test samples for albumin or low molecular weight proteins, such as Bence Jones proteins. Another object of the present invention is to provide a method of assaying urine or other liquid test samples that provides improved visual color resolution and increased sensitivity to low protein concentrations. Yet another object of the present invention is to provide a method of assaying urine or other liquid test samples that is sensitive to protein concentrations as low as about 10 mg/dL and that semiquantitatively discriminates between protein levels of from 0 mg/dL to about 2000 mg/dL, and especially from 0 mg/dL to about 30 mg/dL. Another object of the present invention is to provide a method of assaying urine or other test liquids that utilizes a test device including a test pad comprising a carrier matrix comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound. Another object of the present invention is to provide a method of assaying urine or other test liquids by utilizing an indicator reagent composition that, when incorporated into a carrier matrix comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound, can interact with proteins and undergo a detectable and measurable color transition to establish the presence and to measure the concentration of low levels of protein in the test sample. Another object of the present invention is to provide a test device including a test pad comprising an indicator reagent composition incorporated into a new and improved carrier matrix including a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound that can interact with proteins and undergo a visually and/or instrumentally differentiable color transition to allow the semiquantitative determination of the concentration of protein in the urine or other liquid samples at levels between 0 mg/dL and about 2000 mg/dL, and especially between 0 mg/dL and about 30 mg/dL. Another object of the present invention is to provide a method of assaying urine or other test samples for the presence and concentration of low molecular weight proteins. Still another object of the present invention is to provide a method of assaying a liquid sample for low molecular weight proteins, including Bence Jones proteins, by utilizing a test device including a test pad comprising a carrier matrix, comprising a fibrous, bibulous matrix impregnated with a polymerized urethane-based compound, incorporating an indicator reagent composition. Another object of the present invention is to provide a method of assaying for Bence Jones proteins by incorporating an indicator reagent composition into a dry phase detection device comprising a carrier matrix of sufficient sensitivity to allow the detection and measurement of low molecular weight proteins, such as Bence Jones proteins. Another object of the present invention is to provide a method of manufacturing a detection device for proteins comprising a test pad including a carrier matrix comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound. Another object of the present invention is to provide a new and improved test device and a method of manufacturing the test device including a test pad comprising a carrier matrix having incorporated therein after or during manufacture thereof, an indicator reagent composition capable of interacting with a protein in a test sample, wherein the carrier matrix comprises a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound. Another object of the present invention is to provide a new and improved test device and method of manufacturing the test device for sensing the presence of proteins in a liquid, where the chemical compound is capable of permeating a carrier matrix comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound and is capable of reacting with an indicator reagent composition that is incorporated into the carrier matrix. A still further object of the present invention is to provide a new and improved dry phase test strip capable of incorporating an indicator reagent composition into a carrier matrix comprising a fibrous, bibulous matrix homogeneously impregnated with a polymerized urethane-based compound to achieve a test strip of new and unexpected precision in protein response. Another object of the present invention is to provide a new and improved test device for the quantitative analysis of proteins, including low molecular weight proteins. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and advantages and novel features of the present invention will become apparent from the following detailed description of the preferred embodiments of the invention illustrated in the accompanying figures illustrating the enhanced color resolution of the color transition in the reagent test strips and the increased sensitivity to proteins, permitting more accurate semiquantitative analyte determinations: FIG. 1 is a color space plot showing the assay of liquid samples containing 0, 10, 50 and 100 mg/dL of albumin respectively and 100 mg/dL of Bence Jones proteins using a dry phase test strip utilizing a carrier matrix of untreated filter paper incorporating the indicator dye tetrabromophenol blue (TBPB); FIG. 2 is a color space plot showing the assay of liquid samples containing 0, 10, 20, 30, 100 and 500 mg/dL of albumin respectively and 100 mg/dL of Bence Jones proteins using a dry phase test strip utilizing a carrier matrix of the present invention comprising a filter paper substrate homogeneously impregnated with a polymerized urethane-based compound, and incorporating the indicator dye tetrabromophenol blue (TBPB). DETAILED DESCRIPTION OF THE INVENTION In accordance with the method of the present invention, the qualitative and/or semi-quantitative assay for proteins, including albumin and/or low molecular weight proteins, in urine and other test samples is accomplished by utilizing a test device including a test pad comprising a carrier matrix incorporating an indicator reagent composition. The carrier matrix comprises a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound. By employing a test device that includes a test pad comprising the new and improved carrier matrix, visual color resolution is improved over assays employing a test device including untreated filter paper as the carrier matrix of the test pad. In addition, the sensitivity of the assay to low protein concentrations is increased. The improved color resolution and increased sensitivity to low protein levels afforded by the method of the present invention are especially useful in urine assays. Present-day commercial assays are incapable of differentiating between protein levels ranging from 0 mg/dL to about 30 mg/dL, and especially from 0 mg/dL to about 15 mg/dL. Differentiating between low protein concentration levels is clinically important in the art because a range of from about 10 mg/dL to about 20 mg/dL is used as the normal urine protein level for a healthy individual, therefore urine protein levels from 0 mg/dL to about 10 mg/dL may indicate a potential protein deficiency that can cause physiological imbalances and urine protein levels greater than about 20 mg/dL may indicate an excessive excretion of proteins that can signify a diseased state. It should be noted that in regard to urine protein concentrations in the relatively high range, such as from about 100 mg/dL to about 2000 mg/dL, the method of the present invention still affords improved color resolution and increased sensitivity to urine protein concentration, however such clinical benefits are less critical in this concentration range since such high protein levels are definitely indicative of an abnormal physiological state that must be investigated further. In further regard to urine assays, the presence of low levels of low molecular weight proteins, such as Bence Jones proteins, is indicative of specific diseased states, such as leukemia or multiple myeloma. Therefore, in accordance with another important feature of the device and method of the present invention, the improved color resolution afforded by the use of a carrier matrix comprising a fibrous, bibulous matrix homogeneously impregnated with a polymerized urethane-based compound, and incorporating an indicator reagent composition, and the resulting increased sensitivity to low levels of protein in urine, provides a technique to detect and measure the concentration of low molecular weight proteins present in urine. Therefore, as will be discussed more fully hereinafter in the detailed description of the invention, a method and device is available to test either for total urine protein content in urine, for the albumin content in urine, or for the low molecular weight protein content in urine. Furthermore, it will become apparent that in addition to assaying urine, the method and device of the present invention also can be used to determine the presence and semiquantitive concentration of albumin in blood plasma and serums; and more generally, the albumin content of many other albumin-containing fluids as well. In accordance with another important feature of the present invention, the method and device of the present invention is employed in dry phase, test strip assays to determine the presence and/or concentration of proteins in urine or other liquid test samples. Surprisingly and unexpectedly, it has been found that a test pad incorporating a suitable indicator reagent composition into a carrier matrix comprising a fibrous, bibulous substrate impregnated with a polymerized urethane-based compound has demonstrated improved color resolution and increased sensitivity to low protein concentrations when used in a dye-binding technique to determine the presence and/or concentration of proteins in a test sample. The dye-binding technique using an indicator reagent composition incorporated into the carrier matrix of the present invention, provides a more accurate, trustworthy and clinically significant semiquantitative assay for protein. Presently, commercially available dry phase, test strip assays utilize untreated bibulous substrates, such as filter paper, as the carrier matrix of a test device used to determine the presence and/or semiquantitative concentration of protein in a test sample. According to present day assay methods, the indicator reagent compositions used in assays for protein interact with proteins and undergo a color transition due to the protein-error phenomena when maintained at the proper, constant pH. The protein-error phenomena is fully described in Free et al U.S. Patent No. 3,095,277; Atkinson et al U.S. Patent No. 3,438,737; and Keston U.S. Patent No. 3,485,587, wherein the various dyes, the correct pH ranges, the buffers and the untreated carrier matrices, such as bibulous substrates, like filter paper, required to observe the protein-error phenomena are disclosed. The three above-identified patents basically describe the present day dry phase test strips employed to assay for total protein content in urine. These total protein test strips generally include an indicator reagent composition comprising an indicator dye that normally undergoes a color transition at a strongly acidic pH of 5 or below and a buffer to maintain the pH of the indicator dye slightly below the pH of color transition for the dye. A sufficient buffering of the indicator dye essentially assures that the dye changes color due to an interaction with protein rather than due to a pH change occurring upon contact with the test sample. The present day total protein test strips further include a carrier matrix, usually untreated filter paper, for incorporation of the indicator reagent composition. In accordance with an important feature of the present invention, it has been demonstrated that a new and improved carrier matrix, comprising a fibrous, bibulous substrate, such as filter paper, homogeneously impregnated with a polymerized urethane-based compound, provides a more accurate and trustworthy assay for total protein content in liquid samples. Furthermore, both suprisingly and unexpectedly, by incorporating an indicator reagent composition in a dry phase test strip comprising a carrier matrix comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound, the detection and measurement of low molecular weight proteins in a test sample is possible. Furthermore, a method of fast, accurate, reproducible and trustworthy assays, performable at home or in the laboratory to yield essentially immediate assay results for albumin or low molecular weight proteins, is achieved. In order to achieve the benefits afforded by the present invention, it is imperative that the carrier matrix of the present invention incorporates an indicator reagent composition including a suitable indicator dye. A suitable indicator dye is capable of interacting with proteins, and is capable of undergoing a sufficient color transition due to the protein-error phenomena upon interaction with a protein, to give a detectable and/or measurable response. However, in accordance with the present invention, it has been found that incorporating a suitable indicator reagent composition into a carrier matrix comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound surprisingly and unexpectedly improves the color resolution and differentiation, both visually and instrumentally, of the color transition occurring upon interaction of the indicator dye with proteins. Therefore, the sensitivity of the protein assay, especially at relatively low protein concentrations, is increased because of the carrier matrix of the present invention. The method of the present invention utilizes the "protein-error" phenomena previously discussed. However, the incorporation of a suitable indicator reagent composition into the carrier matrix of the present invention improves the color resolution and differentiation of the color transition occurring because of the dye-protein interaction. As previously described, when a pH indicator dye interacts with a protein, the apparent pKa of the dye is altered and a color transition occurs producing the so-called "protein-error" phenomenon. However, by employing the carrier matrix of the present invention, a more spectacular color development is achieved, therefore improving color resolution and differentiation upon interaction of the indicator dye with proteins, and accordingly increasing assay sensitivity. It has been demonstrated that the carrier matrix of the present invention provides a better blank assay (for a sample containing no protein) by shifting the color transition of a blank, control assay towards the yellow region of the color space. As a result, by providing a better blank test, the color resolution of an albumin-containing test sample is improved because a more spectacular color development is achieved in comparison to the albumin-free control assay. In general, any pH indicator dye can be utilized in the method of the present invention, provided that the dye is capable of interacting with proteins and undergoing a detectable and measurable color transition in response to the protein interaction. Such indicator dyes as described above are well-known and are utilized in indicator reagent compositions in methods to determine the presence and/or the semiquantitative concentration of protein in urine or other liquid test samples. In addition to the indicator dyes, it is known that the indicator reagent composition also may require a sufficient amount of a proper buffer, such that the indicator dye will not change color as a result of a pH shift, but will change color upon contact and interaction with proteins to accurately establish the presence and/or semiquantitative concentration of protein in the test sample. Further, it has been demonstrated that any of various known types of buffers can be used in the indicator reagent composition. In addition, it has been found that for optimum results, the pH of the indicator reagent composition generally should be maintained at a pH value only slightly below the pH range wherein the indicator dye of the indicator reagent composition undergoes a color transition. A method of determining a suitable buffered pH value for the particular indicator dye of the indicator reagent composition and of determining the particular buffer than can be used in the indicator reagent composition is found in Keston, U.S. Patent No. 3,485,587. Upon contact with the urine or other test sample, a color transition of the indicator reagent composition demonstrates the presence of protein. Furthermore, the intensity and degree of the color transition can be used to determine the semiquantitative concentration of protein in the test sample by comparing or correlating the color produced by the test sample to colors produced by solutions having a known concentration of protein. In accordance with an important feature of the present invention, it has been demonstrated that the intensity and degree of color transition of the indicator reagent composition is surprisingly and unexpectedly increased when the indicator reagent composition is incorporated into a carrier matrix comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound. The resulting color transition is sufficiently resolved and differentiated such that the amount of protein in the test sample can be measured and accurately determined without the use of color-measuring instruments, such as spectrophotometers or colorimeters. However, if desired, such color-measuring instruments can be used to measure the difference in color degree and intensity between the test sample and a solution of known albumin concentration. Accordingly, an assay for protein that utilizes a test pad comprising an indicator reagent composition incorporated into the new and improved carrier matrix of the present invention improves the accuracy and reliability of the assay and also increases physician confidence in the assay. Additionally, because of the number of urine assays for protein being performed at home by the untrained patient, as opposed to trained physicians or technicians in the laboratory, it is imperative to provide accurate and reliable semiquantitative assay methods for protein content in the urine. The dry phase, test strip assay for protein that utilizes a test pad comprising an indicator reagent composition incorporated into the new and improved carrier matrix of the present invention is performed in accordance with methods well known in the art. In general, the assay for protein is performed by contacting the urine or other test sample with an analyte detection device that includes a test pad comprising an indicator reagent composition incorporated into the carrier matrix of the present invention. The analyte detection device can be dipped into the test sample, or the test sample can be applied to the analyte detection device dropwise. The resulting change in color of the analyte detection device demonstrates the presence of protein; and, if so designed, the resulting color transition can be compared to a standardized color chart to provide a semiquantitative measurement of the concentration of protein in the urine or test sample. Typically, the prior art describes the analyte detection device as a test strip designed either as a single test pad test strip (to assay only for a single analyte) or as a multiple test pad test strip (to assay for several analytes simultaneously). For either type of test strip, the test strip includes a support strip, or handle, normally constructed from a hydrophobic plastic, and at least one test pad, comprising a bibulous or nonbibulous substrate, incorporating the indicator reagent composition. In general, the bibulous or nonbibulous substrate is an absorbent material that allows the test sample to move, in response to capillary forces, through the substrate to contact the indicator reagent composition and produce a detectable and measurable color transition. The test pads of the prior art could be any substance capable of incorporating the chemical reagents required to perform the assay of interest, as long as the substrate, either bibulous or nonbibulous in nature, is substantially inert with respect to the chemical reagents, and is porous and/or absorbent relative to the liquid test sample. However, in accordance with an important feature of the present invention, the test pad comprises a carrier matrix comprising a fibrous, bibulous substrate. The expression "fibrous, bibulous substrate" refers to bibulous materials that are insoluble in water and other physiological fluids and that maintain their structural integrity when exposed to water and other physiological fluids. Suitable fibrous, bibulous substrates include filter paper, cellulose, wood, woven fabrics, nonwoven fabrics and the like. Other suitable fibrous, bibulous substrates include cloth; hydrophilic natural polymeric materials, particularly cellulosic material, and especially fiber-containing papers such as filter paper or chromatographic paper; synthetic or modified naturally-occuring polymers, such as cellulose acetate, polyvinyl chloride, polyacrylamide, polyacrylates, crosslinked dextran, agarose, and other such crosslinked and noncrosslinked water-insoluble fibrous, hydrophilic polymers. Hydrophobic and non-absorptive substances are not suitable for use as the fibrous, bibulous substrate included in the carrier matrix of the present invention. The fibrous, bibulous substrate can be of different chemical compositions or a mixture of chemical compositions. The fibrous, bibulous substrate also can vary in regards to smoothness and roughness combined with hardness and softness. However, in every instance, the carrier matrix comprises a fibrous, bibulous, hydrophilic or absorptive material. The fibrous, bibulous substrate of the carrier matrix is most advantageously constructed from fibrous, bibulous filter paper. In contrast, the handle of the test strip usually is formed from hydrophobic, non-absorptive materials such as cellulose acetate, polyethylene, terephthalate, polycarbonate or polystyrene. To achieve the full advantage of the present invention, the indicator reagent composition is incorporated into a carrier matrix comprising filter paper as the fibrous, bibulous substrate. The filter paper substrate is homogeneously impregnated with a polymerized urethane-based compound to form the carrier matrix of the present invention before the indicator reagent composition is incorporated into the carrier matrix. The test pad comprising the carrier matrix of the present invention incorporating the indicator reagent composition then is utilized in a dry phase test strip for the assay of protein in a test sample. The method of the present invention affords an economical, accurate and reliable assay for the total concentration of protein in test samples that can be performed at home or in the laboratory. In addition, the method of the present invention allows detection, differentiation and measurement of low protein concentrations in the test sample therefore making the assay more useful clinically. In accordance with the preferred embodiment of the present invention, to perform a dry phase, test strip assay for protein, the carrier matrix first is prepared. A fibrous, bibulous substrate, such as filter paper, first is cut into sheets or strips. The filter paper strips or sheets then are impregnated with a composition containing a urethane compound. As will be described more fully hereinafter, the filter paper can be impregnated with a composition containing a polymerizable urethane compound or with a composition containing a polymerized urethane compound. If a polymerizable urethane compound is impregnated onto the filter paper, it is polymerized by a suitable method, then cured, to provide the new and improved carrier matrix of the present invention comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound. Alternatively, if the fibrous, bibulous substrate is impregnated with a composition including a polymerized urethane compound, the polymerization step can be omitted and after curing the new and improved carrier matrix of the present invention is provided. A suitable indicator reagent composition then is incorporated into the carrier matrix by immersing the carrier matrix into a solution of the indicator reagent composition, or by spraying or spreading the indicator reagent composition onto the carrier matrix to form the test pad of the test device. The solvent of the indicator reagent composition is removed by oven drying in an air oven maintained at about 50° C. for about 20 to 30 minutes. Alternatively, the indicator reagent composition can be included in the composition containing a urethane compound, and therefore incorporated into the fibrous, bibulous substrate simultaneously with the urethane compound to form a test pad of the test device. The test pad, comprising the carrier matrix incorporating the indicator reagent composition, then is cut to an appropriate size, such as a test pad having dimensions from about 0.25 cm by about 0.5 cm to about 0.5 cm by about 1.0 cm. The test pad comprising the carrier matrix incorporating the indicator reagent composition then is secured to an opaque or transparent hydrophobic plastic handle with double sided adhesive tape. The resulting test device, or dry phase test strip, then is dipped into a fresh, uncentrifuged urine sample for a sufficient time to saturate the test pad comprising a carrier matrix incorporating the indicator reagent composition with the sample. After waiting a predetermined time, such as from about 15 secs. to about 60 secs., the test strip is examined, either visually or by instrument, for a response. The color transition, if any, of the test pad comprising the carrier matrix incorporating the indicator reagent composition reveals the presence and/or concentration of protein in the urine sample. Analogous to the prior art, it is well within the experimental techniques of those skilled in the art of preparing test devices to determine the proper balance between size of carrier matrix, the strength of indicator reagent composition solution, the amount of test sample, and the method of introducing the test sample to the test strip, such as by pipetting rather than dipping, in order to design a semiquantitative assay for protein utilizing the method and composition the present invention. In many cases simple visual observation of the test strip provides the desired information. If more accurate information is required, a color chart bearing color spots corresponding to various known protein concentrations, can be prepared for the particular indicator reagent composition used in the test strip. The resulting color of the test strip after contact with the urine sample then can be compared with the color spots on the chart to determine the protein concentration of the test sample. If a still more accurate determination is required, a spectrophotometer or colorimeter can be used to more precisely determine the degree of color transition. In addition, the dry phase, reagent strip assay can be made semiquantitative by employing spectrophotometric or colorimetric techniques, as opposed to visual techniques, in order to more reliably and more accurately measure the degree of color transition, and therefore more accurately measure the concentration of protein in the test sample, especially at lower protein concentrations, such as below 30 mg/dL. As will be discussed more fully hereinafter in the detailed description of FIGS. 1 and 2, the ability to detect, differentiate between and measure low concentrations of proteins in a test sample by employing the carrier matrix of the present invention, surprisingly and unexpectedly provides a method of assaying for hard-to-detect low molecular weight proteins that may be present in the test sample. For example, the presence of low molecular weight Bence Jones proteins in urine is a diagnostic indication that the patient suffers from leukemia or multiple myeloma. However, according to present day methods, the detection of Bence Jones proteins in urine requires a heat and precipitation technique that is expensive and time-consuming. Accordingly, until the method of the present invention, no dry phase, test strip technique was available to accurately detect and measure the low concentrations of Bence Jones proteins usually found in urine. Therefore, in accordance with an important feature of the present invention, it has been demonstrated that by utilizing a dry phase test strip comprising a test pad incorporating a suitable indicator reagent composition into a carrier matrix of the present invention, that the presence and concentration of low levels of albumin or of Bence Jones proteins in a urine sample can be achieved. Surprisingly and unexpectedly, the carrier matrix of the present invention provides improved color resolution and differentiation, and hence improved assay sensitivity, compared to matrices used in the prior art. As previously discussed, a dry phase test strip used for the assay of proteins in test samples generally includes a test pad comprising a bibulous or nonbibulous substrate that is amenable to treatment and incorporation of a suitable indicator reagent composition; that permits the urine or other test sample to permeate the substrate rapidly enough to obtain protein assays relatively quickly; and that does not contaminate the urine or other test sample either by test sample extraction of components comprising the substrate or by appreciably altering the urine or test sample in a way to make the subsequent assays inconclusive, inaccurate or doubtful. Such bibulous and nonbibulous substrates, upon incorporation of a suitable indicator reagent composition, provide a test pad that allows for the detection and measurement of protein in liquid test samples. However, the prior art test pads, comprising untreated bibulous and nonbibulous substrates incorporating an indicator reagent composition did not allow the accurate protein determination of test samples containing from 0 mg/dL to about 30 mg/dL of protein. Surprisingly and unexpectedly, unlike the untreated bibulous and nonbibulous substrates of the prior art, the new and improved carrier matrix used in the method and test device of the present invention, wherein the carrier matrix comprises a fibrous bibulous substrate homogeneously impregnated with a polymerized urethane-based compound, allows the measurement and detection of low levels of albumin in a test sample, such as from 0 mg/dL to about 30 mg/dL. For the test strip of the present invention, designed to assay for the total protein content of a test sample, the carrier matrix can be any fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound that allows permeation by the test sample to saturate the assay area of the test strip that is incorporated with the indicator reagent composition. To achieve the full advantage of the present invention, in the assay for the total protein content of a test sample, the carrier matrix comprises a fibrous, bibulous substrate, including cellulosic materials, such as paper, and preferably filter paper. Filter paper possesses all of the qualities required of a bibulous matrix of the present invention, plus the advantages of abundant supply, favorable economics, and a variety of suitable grades for impregnation by the polymerized urethane-based compound. However, it has been found that to achieve the benefits and advantages of the present invention, the fibrous, bibulous substrate, such as filter paper and other cellulosic bibulous substrates, is homogeneously impregnated with a polymerized urethane-based compound. The untreated filter paper bibulous substrates and the related untreated bibulous substrates of the prior art possess sufficient porosity to allow proteins, such as albumin, to penetrate the bibulous substrate, and contact and interact with the incorporated indicator reagent composition to produce a color transition. By homogeneously impregnating a polymerized urethane-based compound onto a fibrous, bibulous substrate, a carrier matrix possessing sufficient porosity and unexpectedly improved color resolution and differentiation of the color transition results. Therefore, a protein assay of increased sensitivity is provided. In accordance with an important feature of the present invention, the carrier matrix can be prepared by various methods. For example, the fibrous, bibulous substrate can be homogeneously impregnated with a composition including polymerized urethane compound, that then is cured to provide a carrier matrix homogeneously impregnated with a polymerized urethane-based compound of the desired configuration, such as pore size distribution. Alternatively, the fibrous, bibulous substrate can be homogeneously impregnated with a composition including a polymerizable urethane compound. Then, after polymerization of the polymerizable urethane compound, a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane compound results. Similarly, this carrier matrix then is cured to provide a carrier matrix of the desired configuration, such as pore size distribution and pore shape. Consequently, by then incorporating a suitable indicator reagent composition into a carrier matrix of the present invention, a test device and method of detecting and/or differentiating between low levels of albumin, such as from 0 mg/dL to about 30 mg/dL, and between low levels of low molecular weight proteins in a test sample is provided. In accordance with another important feature of the present invention, it has been found that impregnating a polymerized urethane-based compound onto a fibrous, bibulous subtrate provides a carrier matrix of the present invention that demonstrates improved color resolution and improved color differentiation when employed in a dry phase, test strip assay for high molecular weight proteins, such as albumin, and for low molecular weight proteins, such as Bence Jones proteins. As will be demonstrated in the embodiments of the present invention described hereinafter, if the fibrous bibulous substrate is impregnated with a composition including a polymerizable urethane compound, the polymerizable urethane compound first is polymerized to yield a polymerized urethane compound. Then the polymerized urethane compound is cured either by a water bath, by a sonicator bath containing water or by heating the polymerized urethane compound to produce the polymerized urethane-based compound. If a polymerized urethane compound is impregnated onto a fibrous, bibulous substrate, the polymerization step is omitted, but the curing step is included in order to provide a carrier matrix having a polymerized urethane-based compound of the preferred configuration. Furthermore, in each case, the carrier matrix comprising a fibrous, bibulous matrix homogeneously impregnated with a polymerized urethane-based compound must be treated with a suitable indicator reagent composition before the carrier matrix can be used in a test device to detect proteins. It has been found that in order to provide the carrier matrix of the present invention, a polymerizable urethane compound or a polymerized urethane compound, such as a urethane prepolymer, first is impregnated onto the fibrous, bibulous substrate. Similarly, a mixture of a polymerizable urethane compound and a polymerized urethane compound can be impregnated onto the fibrous, bibulous substrate. In order to impregnate the fibrous, bibulous substrate, the polymerizable urethane compound or polymerized urethane compound first is dispersed or dissolved in a liquid vehicle. The liquid vehicle is removable from the dispersion or solution during curing of the urethane-containing composition. Removing the liquid vehicle during curing allows the urethane compound to dry and coagulate into the preferred pore size and pore shape on the fibrous, bibulous substrate. The resulting polymerized urethane-based compound, homogeneously impregnated onto the fibrous, bibulous substrate, therefore provides an improved carrier matrix for use in a dry phase test strip designed for the assay of proteins. The urethane compound dispersed or dissolved in the continuous liquid vehicle phase can be polymerizable or polymerized, and includes oligomers, prepolymers, incompletely cured polymers and mixtures thereof. In addition, depending upon the solubility and chemical properties of the indicator reagent composition, the urethane-containing composition can be mixed with the indicator reagent composition prior to curing, and the test pad then is formed by curing the urethane-containing composition. The test pad comprising the carrier matrix incorporating the indicator reagent composition is cut into strips, then into pads, and secured to a plastic handle. It has been found that the urethane-containing composition, including a polymerizable or polymerized urethane compound like an oligomer, prepolymer, incompletely cured polymer or mixtures thereof forms a cured film on the fibrous, bibulous substrate upon removal of the continuous liquid vehicle phase during the curing process. The urethane compound, after dissolving or dispersing in a continuous phase, such as by including an emulsifier, can be cured in any known manner. Further, the solution or dispersion of the urethane compound can include a suitable curing catalyst or can be heat cured provided that the solution or dispersion of the urethane compound is homogeneously impregnated onto the fibrous, bibulous substrate in the form of an incompletely cured solution or dispersion. Generally, the urethane compounds that are useful in accordance with the present invention are those that can be dissolved or dispersed in a liquid vehicle, such as an organic solvent, like dimethylformamide, and that can be cured to yield an essentially colorless film. In accordance with one embodiment of the present invention, the urethane compound is a polymerizable urethane prepolymer, and particularly a urethane prepolymer comprising essentially repeating urethane units wherein the prepolymer chain is terminated at each end with isocyanate functionalities. To achieve the full advantage of the present invention, it has been found that the polymerizable urethane compound, or a polymerized urethane compound, is preferentially neutral in electronic character. An example of a suitable commercial urethane prepolymer is DESMODERM® KBH GRANULATE, available commercially from BAYER AG. The expression "urethane prepolymer" is understood to describe an essentially linear polymer of repeating urethane units. The urethane prepolymer has at least two isocyanate functionalities per molecule, and the polyurethane prepolymer should have a weight average molecular weight (M w ) of at least 50,000. Urethane prepolymers with weight average molecular weights below 50,000, for example down to about 30,000, also are useful as long as the prepolymers are soluble or dispersible in a liquid vehicle and can be cured on the fibrous, bibulous substrate. The maximum M w is one wherein the urethane prepolymer can be solubilized or otherwise dispersed in a liquid vehicle or continuous phase, such as an organic solvent, like dimethylformamide. For the incompletely-cured dispersed urethane prepolymer, weight average molecular weights of up to about 500,000 are expected to be practical for the present invention. Upon curing, there is no upper limit to the molecular weight of formed polymerized urethane-based compound on the fibrous, bibulous substrate. It has been found that, to achieve the full advantage of the present invention, the M w for the polymerizable urethane prepolymer is within the M w range of about 70,000 to about 80,000. The polymerizable or polymerized urethane compound, such as a urethane prepolymer, useful in the method of the present invention can include other monomeric units that are incorporated into the polymerizable urethane compound by copolymerizing an isocyanate containing monomer, hydroxyl containing monomer and a suitable third monomeric unit into the urethane prepolymer. In addition, although the polymerizable or polymerized urethane compound useful in the method of the present invention is preferentially neutral in nature, anionic or cationic polymerizable urethane compounds also are envisioned as being useful in the method of the present invention. More particularly, a prepolymer found useful in the method of the present invention, DESMODERM® KBH, is a neutral thermoplastic granular polymerized urethane material, obtained by reacting 75 parts of a polyester of adipic acid, including 70 mol % ethylene glycol and 30 mol % 1,4-butanediol (M w = 2,000); 25 parts of a polyester of adipic acid and 1,4-butanediol (M w = 2,250); 25 parts 1,4-butanediol; and 85 parts diphenylmethanediisocyanate. In accordance with the present invention, the particular urethane compound utilized in the present invention, after mixing with the other components of the urethane-containing composition, is cured after impregnation onto the fibrous, bibulous substrate to produce a polymeric film that has a physical structure that is permeable both to high molecular weight and to low molecular weight proteins. Generally, the urethane compound is present in the urethane-containing composition in a range of from about 0.1% by weight to about 10% by weight, and preferably from about 1% by weight to about 5% by weight, based upon the total weight of the urethane-containing composition. As will be discussed more fully hereinafter, the carrier matrix of the present invention, comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound, affects the degree of color resolution and the sensitivity of the protein assay. Therefore, in accordance with the method of the present invention, analyte test devices including a carrier matrix of the present invention can be designed for improved color resolution and increased assay sensitivity. In addition to the urethane compound, the urethane-containing composition used to impregnate the fibrous, bibulous substrate to form the carrier matrix of the present invention may include surfactants to help solubilize or emulsify the urethane compound in the liquid vehicle or to improve the ability of the urethane-containing composition to impregnate the fibrous, bibulous substrate. The surfactants can be present from 0% by weight up to approximately 5% by weight, based on the total weight of the urethane-containing composition. Anionic surfactants found useful in the urethane-containing composition are not necessarily limited to a particular type, and include ammonium, alkylammonium, potassium and/or sodium dodecylbenzene sulfonate, alkyl sulfonates, silylalkyl sulfonates, alkyl sulfates, alkyl ether sulfates, dioctyl sulfosuccinate, alpha olefin sulfonates, and alkyl sarcosinates; or mixtures thereof. Similarly, nonionic surfactants, such as the nonoxynols or octoxynols, as are well known in the art, can be used in the urethane-containing composition. In addition, other surface active agents, such as silicon-containing materials, like a dimethylpolysiloxane fluid, may be incorporated into the urethane-containing composition in weight percentages of up to 2% based upon the total weight of the urethane-containing composition. These silicon-containing materials possess a low surface tension and therefore further assist in impregnating the fibrous, bibulous substrate and also act to alter the surface tension of the urethane-containing composition to provide a leveling affect to produce a smooth polymerized urethane-based film on the fibrous, bibulous substrate. As discussed previously, the urethane-containing composition includes a liquid vehicle, like an organic solvent, capable of solubilizing and/or dispersing the urethane compound and any surfactants or silicon-containing materials that may be present. The organic solvent should be relatively inert such that it will not react with the urethane compound, and the solvent should evaporate at relatively low temperatures to provide a dry carrier matrix after curing of the urethane-containing composition. It has been demonstrated that organic aprotic solvents, such as dimethylformamide, N-methyl pyrrolidone, and dimethyl sulfoxide, or mixtures thereof, provide the required solvency to dissolve and disperse the components of the urethane-containing composition, provide the required inertness to preclude reaction of the solvent with the urethane compound, and possess the required vapor pressure to yield a solvent-free polymerized urethane-based film impregnated onto the fibrous, bibulous substrate. Similarly, the lower alcohols, such as methyl alcohol, ethyl alcohol and isopropyl alcohol, have been found to have the necessary solvency, inertness and vapor pressure to yield a solvent-free polymerized urethane-based film. Such lower alcohols can be used alone, in combination or in combination with the aprotic solvents described above. The liquid vehicle, removed during curing, is included in the urethane-containing composition in an amount of at least 85%, and preferably is present in an amount of at least 90%, and up to about 99% by weight, based on the total weight of the urethane-containing composition. It also has been found that liquid vehicle selection determines the length of curing time needed to remove the liquid vehicle from the urethane-containing composition. For example, a 10 minute curing time is needed to sufficiently remove N-methylpyrrolidone; a 2 minute curing time is needed to sufficiently remove dimethylformamide; and only about 30 seconds is needed to sufficiently remove a 50:38 dimethylformamide:methyl alcohol (v/v) solvent mixture. Therefore, the choice of liquid vehicle can significantly reduce the time needed to complete the curing step. In accordance with one embodiment of the present invention, a urethane-containing composition was mixed according to the formulation presented in Example 1. As will be discussed more fully hereinafter, the urethane-containing composition of Example 1, then was used to impregnate a fibrous, bibulous substrate of filter paper. After curing, a suitable indicator reagent composition was incorporated into the resulting carrier matrix of the present invention to form a test pad used to test standard solutions for albumin or Bence Jones proteins by the normal dip-and-read test strip procedure. EXAMPLE 1 Urethane-Containing Composition DESMODERM® KBH (Neutral Urethane) 2.0% (by weight) Dimethylformamide 98.0% Total 100.0% In the manufacture of the urethane-containing composition of Example 1, the components were thoroughly mixed until the composition was homogeneous. Then a fibrous, bibulous substrate, such as WHATMAN® CCP500 filter paper, available from Whatman Ltd., Maidstone, Kent, U.K., was impregnated with the urethane-containing composition of Example 1 by dipping the filter paper into the composition of Example 1 for approximately 30 seconds. After impregnating the filter paper with the urethane-containing composition of Example 1, the urethane-containing composition was cured onto the fibrous, bibulous filter paper by placing the impregnated filter paper into a 42° C. water bath for about 2 minutes. After removing the impregnated filter paper from the water bath, the impregnated filter paper was dried in an 80° C. oven for about 20 min. to complete the curing process. The urethane-containing compositions of Examples 2 through 4 were similarly mixed, then impregnated into a filter paper substrate and cured in a water bath to form a carrier matrix of the present invention comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound. EXAMPLE 2 DESMODERM® KBH 2.0% (by weight) Dimethylformamide 60.0% Methyl Alcohol 38.0% TOTAL 100.0% EXAMPLE 3 DESMODERM® KBH 2.0% (by weight) Dimethylformamide 50.3% Ethyl Alcohol 47.7% TOTAL 100.0% EXAMPLE 4 DESMODERM® KBH 2.0% (by weight) Dimethylformamide 53.0% Isopropyl Alcohol 45.0% TOTAL 100.0% Although the above-described process is the preferred curing process, alternatively, the urethane-containing composition impregnated onto the fibrous, bibulous substrate can be cured either by placing the impregnated bibulous substrate into a sonicator bath containing water, then sonicating for about two minutes, followed by oven drying at 80° C. for about 20 minutes; or by eliminating the water curing step and curing the urethane-containing composition onto the fibrous, bibulous substrate by oven drying at 80° C. for about 20 minutes. It should be noted that the water curing step can be eliminated entirely. However, the performance of a test pad comprising a carrier matrix of the present invention that has been cured is superior to a test pad comprising a carrier matrix of the present invention that has not been cured. The superior results demonstrated by the cured carrier matrix may be attributed to more complete liquid vehicle removal during curing and to a more preferred pore shape and pore size distribution that occurs upon curing. After curing the urethane-containing composition onto the fibrous, bibulous substrate to form a polymerized urethane-based compound, the resulting carrier matrix of the present invention is incorporated with an indicator reagent composition to form a test pad. Alternatively, if the reagents comprising the indicator reagent composition are soluble in the organic solvent used in the manufacture of the urethane-containing composition, like dimethylformamide, and if the reagents comprising the indicator reagent composition are insoluble in water, the reagents can be incorporated into the urethane-containing composition and impregnated onto the fibrous, bibulous substrate with the urethane-containing composition prior to curing. To show the new and unexpected results arising from using a test pad comprising the carrier matrix of the present invention incorporating a suitable indicator reagent composition in a test device to detect and measure the amount of protein in a test sample, color space plots were made for total protein assays and for Bence Jones protein assays obtained from dry phase test strips including a test pad comprising an indicator reagent composition incorporated into an untreated filter paper bibulous matrix and from dry phase test strips including a test pad comprising an indicator reagent composition incorporated into a carrier matrix of the present invention, comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound. FIGS. 1 and 2 are color space plots obtained from contacting four (FIG. 1) or five (FIG. 2) standardized albumin solutions and from contacting a standardized solution of Bence Jones proteins with various dry phase test strips including a test pad comprising an indicator reagent composition impregnated into either a carrier matrix comprising untreated filter paper (FIG. 1) or a carrier matrix of the present invention, comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound (FIG. 2). For example, FIG. 1 is the color space plot resulting from contacting a dry phase test strip including a test pad comprising the single indicator tetrabromophenol blue (TBPB), buffered with a citrate buffer, incorporated into an untreated filter paper carrier matrix with standardized solutions containing no albumin (0), 10 mg/dL albumin (10), 50 mg/dL albumin (50), 100 mg/dL albumin (100) and 100 mg/dL Bence Jones proteins (BJ). FIG. 2 is a color space plot for a dry phase test strip including a test pad comprising the identical indicator reagent composition incorporated into a carrier matrix of the present invention including WHATMAN® CCP500 filter paper homogeneously impregnated with the urethane-containing composition of Example 1. The impregnated filter paper was subjected to a water curing process. The dry phase test strips, including a test pad comprising an indicator reagent composition impregnated onto carrier matrices of the present invention, contacted standardized albumin-containing solutions including no albumin (0), 10 mg/dL albumin (10), 20 mg/dL albumin (20), 30 mg/dL albumin (30), 100 mg/dL albumin (100), 500 mg/dL albumin (500) and 100 mg/dL Bence Jones proteins (BJ). As illustrated in FIGS. 1 and 2, a color space plot includes three axes, the L*, A* and B* axes. The values of L* plotted on the vertical axis are a measure of the intensity of color, whereby a large L* value denotes a light color and L*=0 denotes a completely black color. The horizontal A* axis is a measure of the color transition from green to red, whereby the more positive the A* value, the more red the color, and analogously, the more negative the A* value, the more green the color. Similarly, the third axis, B*, is a measure of the color transition from blue to yellow, whereby the greater the value of B*, the more yellow the color, and analogously the smaller the value of B*, the more blue the color. The color space difference ( E) is calculated from the following equation: E = (L₁* - L₂*)² + (A₁* - A₂*)² + (B₁* - B₂*)² Eq. 1 wherein: L₁*, A₁*, and B₁* are the color space values determined for a first standardized protein solution; L₂*, A₂* and B₂* are the color space values determined for a second standardized protein solution having a different protein concentration from the first standardized protein solution; and E is the color space difference between the color space plots of the first and second standardized protein solutions. The color space difference ( E) is the straight line distance between two points in a three-dimensional color space plot. Theoretically, a color space difference of 1 is the smallest color difference the human eye can distinguish. However, because of the inherent differences between the visual capabilities of individuals, a color space difference ( E) of about 5 is required in order to practically and confidently distinguish between colors. The L*, A* and B* values plotted on the color space plots of FIGS. 1 and 2 are calculated from the percent reflectance measurements taken at sixteen different wavelengths evenly spaced between 400 nm (nanometers) and 700 nm using standard equations well-known in the art. In general, the percent reflectance at each of the sixteen different wavelengths is multiplied by the intensity of the light at that wavelength. These values then are multiplied by standard weighing functions for the colors red, green and blue, and finally added together. These calculations yield three tristimulus values X, Y and Z, and L*, A* and B* are calculated from the X, Y and Z tristimulus values using the following equations: L* = 116 x [(Y/Yo) 1/3 - 16)] (Eq. 2) A* = 500 x [(X/Xo) 1/3 - (Y/Yo) 1/3 ] (Eq. 3) B* = 200 x [(Y/Yo) 1/3 - (Z/Zo) 1/3 ] (Eq. 4) wherein: Xo, Yo and Zo are the tristimulus values for perfect white (i.e. reflectance = 100% at all wavelengths), and X, Y and Z are the tristimulus values calculated as described above from the sixteen wavelengths between 400 nm and 700 nm. From the color space plots of FIGS. 1 and 2, the color space differences ( E) were calculated, and summarized in TABLE I. In interpreting TABLE I, the term, E(Alb 10-0) is the color space difference between protein assays for protein solutions containing 10 mg/dL of albumin and 0 mg/dL of albumin. Similarly, the term E(Alb50-0) is the color space difference between protein assays for protein solutions containing 50 mg/dL of protein and 0 mg/dL of protein. The terms E (Alb100-0), E (Alb 500-0) and E (BJ100-0) are analogously defined. As illustrated in the color space plot of FIG. 1 and in TABLE I, protein assays were conducted on standardized solutions including albumin and Bence Jones proteins using a dry phase test strip including a test pad having the indicator, tetrabromophenol blue, incorporated into an untreated filter paper matrix. From FIG. 1 and TABLE I, it was found that the color space difference between a solution containing 10 mg/dL of albumin and a solution containing no albumin is 4.8. Because the human eye normally can differentiate only between colors having a color space difference of approximately 5, this assay would be inconclusive as to whether the sample contained any albumin because the color differentiation between the test strip contacting the 0 mg/dL albumin solution and the test strip contacting the 10 mg/dL test strip could not be accurately determined. At best, the assayer could estimate that the sample contained from 0 mg/dL albumin to about 10 mg/dL albumin. Similarly, FIG. 1 and TABLE I demonstrate that an assayer could not determine the concentration of Bence Jones proteins in a test sample containing from 0 mg/dL of Bence Jones proteins to about 100 mg/dL of Bence Jones proteins because the color space difference between a sample containing 0 mg/dL of Bence Jones proteins and a sample containing 100 mg/dL of Bence Jones protein provided by an analyte device having an indicator reagent composition impregnated into an untreated filter paper matrix is only 4.4, or a color space difference that is barely detectable by a normal human eye. TABLE I and FIG. 1 further show that the human eye can detect color differences resulting from the presence of 50 mg/dL and 100 mg/dL of albumin because the color space differences are 19.2 and 25.5, respectively. However, surprisingly and unexpectedly, by homogeneously impregnating a fibrous, bibulous filter paper substrate with a polymerized urethane-based compound, the carrier matrix of the present invention improves the color resolution and differentiation of the color transition to permit an assayer to visually differentiate between samples containing 0 mg/dL of albumin and 10 mg/dL albumin. From FIG. 2 and TABLE I, a color space difference ( E) between a solution containing 10 mg/dL of albumin and a solution containing no albumin is 6.6 when using a test device including a test pad incorporating an indicator reagent composition into a carrier matrix of the present invention. Such a color space difference is sufficient to be discernible by the human eye, and shows a substantial improvement (37%) over the color space difference of 4.8 afforded by the untreated filter paper matrix of FIG. 1. Similarly, an assayer can visually detect Bence Jones proteins in a test sample because the color space difference between a 100 mg/dL solution of Bence Jones proteins and a 0 mg/dL solution of Bence Jones proteins is 13.0, compared to a color space difference of 4.4 when an indicator reagent composition is incorporated into untreated filter paper, or a 195% improvement. Such a color difference is more than sufficient to allow color differentiation by the human eye. Similarly, TABLE I and FIG. 2 shows enhanced color differentiation for the 20 mg/dL, 30 mg/dL, 100 mg/dL and 500 mg/dL albumin solutions compared to the solution containing no albumin. Overall, FIGS. 1 and 2 and TABLE I show that an indicator reagent composition impregnated into a carrier matrix comprising a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound improves color resolution and assay sensitivity in the assay for the total protein content of a liquid test sample, especially at low protein levels of less than 30 mg/dL. The method and device of the present invention allow visual differentiation of color transitions resulting from contact of the indicator reagent-containing carrier matrix with a test sample containing protein at levels of between 0 mg/dL and 10 mg/dL, thereby providing more accurate and trustworthy assays. The present invention further provides a method to quickly and accurately test for Bence Jones proteins and other low molecular weight proteins in a test sample by providing a carrier matrix that sufficiently improves assay sensitivity to allow detection and measurement of low concentrations of low molecular weight proteins. It has been demonstrated that color space differences are improved by using the carrier matrix of the present invention. In addition, employing the carrier matrix of the present invention dramatically increased protein assay sensitivity to low molecular weight proteins therefore providing a simple, dry phase test strip procedure to assay for low molecular weight proteins. As demonstrated in FIGS. 1 and 2 and in TABLE I, assaying a solution containing 100 mg/dL of Bence Jones proteins with a test device having an indicator reagent composition incorporated into an untreated filter paper matrix gave an imperceptible color difference of 4.4 compared to assaying a solution containing no Bence Jones proteins. However, color resolution and assay sensitivity is improved by incorporating the same indicator reagent composition into a carrier matrix of the present invention such that the color difference is a readily perceptible 13.0. Similar results are demonstrated for liquid test samples containing low albumin levels of 10 mg/dL, wherein the color space difference increased from 4.8 to 6.6 for test devices using the carrier matrix of the present invention compared to test devices using untreated filter paper matrices. Therefore, in accordance with an important feature of the present invention, more accurate and reliable assays for total protein content, or for low molecular weight protein content, in urine and other liquid test samples can be performed by utilizing the carrier matrix of the present invention in a dry phase test strip assay for proteins. The carrier matrix of the present invention improves the color resolution of the assay and therefore improves assay sensitivity, especially at low albumin levels of approximately 30 mg/dL and below. Furthermore, by performing the assay with a dry phase test strip that includes the carrier matrix of the present invention, a new and unexpectedly accurate method of determining the presence and/or concentration of low molecular weight proteins, like Bence Jones proteins, in the test sample is provided.

Claims: An analyte detection device to determine the presence or concentration of protein in a liquid test sample comprising a carrier matrix incorporating therein an indicator dye, wherein the carrier matrix is permeable to the liquid test sample and comprises a fibrous, bibulous substrate homogeneously impregnated with a polymerized urethane-based compound, and wherein the indicator dye is capable of interacting with a protein to produce a detectable and measureable color change in the carrier matrix. The analyte detection device of claim 1 wherein the fibrous, bibulous substrate is selected from the group consisting of filter paper, wood, chromatographic paper, cloth, cellulose acetate, polyvinyl chloride, polyacrylamide, polyacrylates, crosslinked dextran, and agarose; or mixtures thereof. The analyte detection device of any of claims 1 and 2 wherein the polymerized urethane-based compound is homogenously impregnated onto the fibrous, bibulous substrate from a composition including from about 0.1 % to about 10% by weight of the polymerized urethane-based compound and from about 85% to about 99% by weight of a siutable liquid carrier. The analyte device of any of claims 1 to 3 wherein the liquid carrier is selected from the group consisting of dimethylforamide, N-methyl pyrrolidone, dimethyl sulfoxide, methyl alcohol, ethyl alcohol and isopropyl alcohol; or mixtures thereof. The analyte detection device of any of claim 1 to 4 wherein the liquid test sample comprises a biological fluid. The analyte detection device of any of claims 1 to 5 to determine the presence and/or concentration of albumin in a biological fluid. A method of manufacturing a test pad for determining the presence or concentration of a protein in a test fluid comprising: contacting a fibrous, bibulous substrate with a urethane containing composition comprising a urethane compound dispersed in a suitable carrier liquid in an amount sufficient to retention of a portion of the urethane compound in the substrate; drying the fibrous, bibulous substrate to remove the carrier liquid and to form a carrier matrix; incorporating an indicator dye capable of interacting with a protein to produce a detectable and measurable color change, into the carrier matrix to form a test pad; and drying the test pad. A method of determining the presence or concentration of protein in a test fluid comprising: (a) contacting the test fluid with an analyte detection device comprising a test pad including a carrier matrix and an indicator dye, wherein the indicator dye is capable of exhibiting a detectable response upon interaction with a protein, and wherein the carrier matrix comprises a fibrous, bibulous substrate impregnated with a polymerized urethane compound; and (b) examining the analyte detection device for a response to the protein content of the test fluid.

Inventors: CORYN TIMOTHY M, LAU ARTHUR L Y, RITUCCI CARRIE A, CORYN, TIMOTHY M., LAU, ARTHUR L. Y., RITUCCI, CARRIE A.
Applicants: MILES INC, MILES INC.
Application year: 1989
Main classification: G01N33
IPC codes: G01N33