Abstract:
A chromatographic lateral-flow assay system for rapid, high sensitivity method of detecting low levels of ligands in body fluids, with few false positives and few false negatives. The lateral-flow assay may have a membrane strip in ribbon form, which increases detection on the order of 2 to 10 fold over the conventional chromatographic specific binding assay techniques by placing a dried or lyophilized conjugate in colloidal spheres opposite side of the lateral flow membrane strip. A chromatographic specific binding assay strip device, comprising: a laminate strip having a first side and an opposite second side; a conjugate pad or membrane disposed on said first side of said laminate; a sample receiving pad or membrane strip and reservoir pad or membrane disposed on said second side of said laminate; and a detection pad or membrane strip disposed between the sample pad or membrane and the reservoir pad or membrane on said second side of said laminate. The assay system comprises a housing device, such as a test tube or cassettes to facilitate the mixing of a sample solution with the dried or lyophilized conjugate, and kits.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to an assay system and apparatus involving specific binding of analytes and/or ligands, and specifically relates to chromatographic flow binding assays with a colored conjugate, and a novel lateral-flow-platform that includes a dried or lyophilized colloidal sphere conjugate, such as gold, to the back of the lateral flow test. 
     BACKGROUND OF THE INVENTION 
     It is increasingly desirable to provide a rapid high sensitivity system to detect low levels of ligands in body fluids, plant extracts, environmental samples, tissue samples and enrichment broths. Ideally, such systems should have a minimal number of procedural steps and yield reliable results, even when used by untrained persons. A ligand may be a specific sequence of amino acids or molecule found on proteins such as an antibody, protein receptors, bacterial/microbial peptides, hormone, or drug that binds to a receptor. A receptor is any of various specific protein molecules in surface membranes of cells and organelles to which complementary molecules, such as hormones, neurotransmitters, antigens, or antibodies, may become bound. The ligand may also be a chemical intermediate or reactant with an analyte. Analytes are substances that bind to a ligand. Ligands or analytes may also be peptides, drugs, carbohydrates, haptens, chemicals, chemical reaction with an intermediate compound, and the like. 
     To a significant extent, many known tests presently available for detecting ligands are either time consuming, labor intensive, or in need of instrumental assistance to read results. Most known tests also lack an acceptable degree of sensitivity or specificity. This is unfortunate since rapid testing is important for diagnosis and treatment of various physiological conditions; detection of certain strains of micro-organisms; determination of the most appropriate antibiotic treatment of a patient; detection of drug analytes in individuals; detection of cancer cells in a patient&#39;s bio-fluid; detection of antibody to a microbial agent; detection of a disease-state protein; and the like. 
     Although known types of ligand-receptor assays have been used to detect the presence of various substances, such as ligands, there is a need in the art to provide a rapid, high sensitivity assay requiring a minimum degree of skill from a user. Rapid test assay devices for field use, such as in a home or doctor&#39;s office are known in the art for detecting proteins, peptides, drugs, carbohydrates, haptens, chemicals, chemical reaction with intermediate compounds, and the like. Such devices are referred to as one-step lateral flow or one-step immuno-chromatographic assays. These types of assay devices require a minimal number of steps and can be performed by an untrained person. Examples of these test devices abound, and the individual components of a typical one-step lateral flow test are described below: 
     A first type of test device involves a test strip of rectangular or square dimensions made of a vinyl, polypropylene, or other pliable or non-pliable plastic laminate to serve as a backing to hold in place other test components that are on an adhesive bond on the backing. A ligand, protein or analyte binding membrane having discreet zones of immuno-reactive proteins or substances immobilized or attached, are usually in linear impregnation or spotted through a dropper onto the membrane. The membrane may be composed of nylon, nitrocellulose, mixed cellulose esters, polysulfones, and the like. If the assay is used to detect an animal antibody, the immuno-reactive protein coated on the membrane may be a ligand to which antibody contained in a positive sample reacts. These immuno-reactive proteins used to coat the membrane are typically native or recombinant proteins derived from Human Immunodeficiency Virus (HIV), human T-cell lymphotropic viruses (HTLV),  Mycobacterium tuberculosis  (TB), and the like. In a second construct, the membrane may also be impregnated with anti-antibody to capture total class and subclass immunoglobulins. In this case, the specific antibody in the sample may react with native or recombinant ligand proteins, such as HIV, HTLV, or TB, coated onto the surface of the conjugate particles. A third construct may have native or recombinant protein antigens coated on both conjugate and membrane surfaces that react with separate binding domains on the antibody. If the membrane is detecting antigen as the analyte, the surface may be impregnated with antibody or ligand reactive with the antigen. Constructs one, two and three are known as “sandwich” type rapid assays, since the analyte being detected is captured (sandwiched in between) by both the immuno-conjugate and the membrane surface. A fourth method is a competitive inhibition assay, wherein sample analyte at detectable levels either saturates the anti-analyte conjugate or saturates the immobilized anti-analyte capture line on the membrane resulting in no visible line formation. A negative reaction in this case results in a visible line formation, due to the binding of the conjugate with the test line. 
     A second group of test devices involve a fibrous membrane, such as, for example, glass, polyester, cotton, or spun polyethylene, in contact with a membrane containing ligand and bound to densely colored particles such as latex, gold, silver, selenium, carbon, and the like. The bound ligand is complementary to the assay being constructed and reacts with the analyte being detected. The coated colored particles are often described as an immuno-conjugate. Sufficient molecules of ligand are coated onto the surface of the colored particles so that when a positive reaction does occur the discreet, striped, or spotted zones on the membrane surface are visible to the naked eye. A sample negative for the ligand being detected may leave a white zone in a sandwich type immuno-assay. If the assay is a competitive inhibition type, a negative sample yields a visible line or spot. The colored particles may be dried down onto the fibrous pad or membrane and placed at the dorsal end (at the opposite end of the absorbent pad or membrane) of the membrane. Release agents may be contained in the dried down colored particles to facilitate re-hydration of the particles, allowing them to react with the analyte being detected. 
     A third group of test devices include a fibrous sample receiving pad or membrane, such as glass, polyester, cotton, or spun polyethylene, that is partially in contact with the immuno-conjugate and serves as a reservoir for absorbing and releasing sample. The sample may contain chemicals to facilitate reactive qualities of the assay. The sample may be any biological fluid (bio-fluid) such as tissue extracts, blood, serum, plasma, tears, perspiration, urine, or saliva. The sample may also be derived from an environmental extract, plant extract, or microbial enrichment broth. When a sample or diluted sample is applied to the sample receiving pad or membrane, the movement of liquid is chromatographic and unidirectional towards the absorbent pad or membrane. During migration, the sample re-hydrates the colored particles and reacts with ligand bound to the particles. 
     Other test devices are known. For example, one has a bulbous absorbent pad or membrane, which serves as a reservoir for absorbing all liquid components that pass through the membrane. The absorbent material is typically cotton or paper. Another has a zone applied to the membrane containing a control line indicating sample was applied, or that the assay&#39;s immuno-conjugate is functional. The control can react with the sample or conjugate. Still another has buffered diluent often used to dilute and condition the sample being detected, allowing for exotic chemistries to occur, thus improving assay performance. The diluent is composed of salt solutions, detergents, and the like and may be applied from a dropper tip, vials, or is contained in a sample vial. And finally another has a plastic or cardboard housing to contain, provide support, and allow ease of use for the whole strip. 
     It is important to note that in the known prior art the conjugate pad or membrane is in the same plane as the membrane, and overlaps, or is in constant contact with the membrane. Further, the conjugate is in dry form disposed or deposited in the flow path upstream of the test site. This gives rise to a very specific flow path wherein the sample re-hydrates the dried conjugate and travels through the absorbent pad or membrane. The flow path is known as an absorbent flow path since the liquid sample is absorbed by the sample receiving pad or membrane, and next wets the conjugate pad or membrane. Typically, the amount of sample required to wet the conjugate pad or membrane is about 10 to 20 microliters (μl). The sample and conjugate mixture subsequently travels through the conjugate pad or membrane, then contacts the membrane and flows onto the test (flow) membrane. 
     In use, in most lateral flow assays using antigen-antibody reactions, the immuno conjugate is composed of synthetic conjugates having latex microparticles, enzymatic, fluorescent, or visually observable metal sol tags. In all these assays, there is a receptor, for example an antibody, which is specific for the selected ligand or antigen, a means for detecting the presence, and often a quantity of the ligand-receptor reaction product. Examples of such qualitative assays include blood typing and most types of urinalysis. For these semi-quantitative type positive/negative tests, visually observable indicia such, as the presence of agglutination or a color change are preferred. 
     Positive/negative assays (or semi-quantitative) must still be very sensitive because of the often small concentrations of the ligands of interest in the test fluid. False positives may be troublesome, particularly with agglutination and other rapid detection methods such as dipstick and color change tests. Because of these problems, sandwich and competitive inhibition assays and other detection methods that use metal sols or other types of colored particles have been developed in an attempt to increase sensitivity of detection. In addition, the use of direct labels attached to one of the specific binding ligands produces instant analytical results without the need to add further reagents. Nevertheless, these techniques have not solved all problems related to sensitivity, neither have they been reliable in obtaining results with a minimal number of method steps encountered in these rapid detection methods. 
     Various other techniques have been devised for labeling one member of a specific binding pair so that the binding reaction may be indirectly observed. The results of specific binding reactions are generally not directly observable. Useful labels include radiolabels, chromophores, fluorophores, (the presence of which may be detected by means of radiation detectors), spectrophotometers, the naked eye, and the like. Where members of a specific binding pair are tagged with an enzyme label, their presence may be detected by the enzymatic activation of a reaction system including a signal generating a substrate/cofactor group wherein a compound, such as a dye, is activated to produce a detectable signal. 
     Other specific binding assay devices are known in the art having vertically arranged elements including the following: a porous capture material impregnated at a reaction site with a member of a specific binding pair such as an antibody or an antigen; a removable prefilter disposed above the capture material; and, a blotter disposed below the capture material. A sample liquid such as blood, serum or other bio-fluid is added to the device wherein the prefilter removes particulates and other impurities from the sample. These impurities would otherwise be trapped on top of the specific binding capture material. Analyte substances within the sample are trapped by means of specific binding reactions with their specific binding partners on the capture material. Non-analyte components of the sample solution pass through the capture material and are absorbed by the blotter. Wash steps may be carried out to remove non-analyte components from the capture material and additional reagents such as enzyme substrates, cofactors and dye precursors may be added to the capture material to indicate the presence or absence of analyte at the reaction site. The prefilter should then be removed so that the presence or absence of analyte at the reaction site may be visually determined. Unfortunately, while these devices are somewhat useful, they suffer from limitations in capture efficiency and sensitivity because most of the analyte in the sample  10  material flows around, rather than through, the reaction site on the capture material. 
     Several one-step lateral flow immunoassay devices, having a strip capable of transporting a developing liquid by capillary action having a first zone for receiving a sample, a second zone impregnated with a first reagent capable of being transported by the developing liquid, and a third zone impregnated with a second reagent, are known in the art (See generally, U.S. Pat. Nos. 4,366,241; 4,094,647; 4,235,601; 4,361,537; and 6,841,159). U.S. Pat. Nos. 6,352,862; 5,622,871; and 5,120,643 also relate to one-step lateral flow tests wherein the conjugate is in dry form along the flow path. 
     A major disadvantage of all of the known prior art is that the sample rehydrates the dried conjugate along the flow path. Thus, the majority of sample applied is not involved in the immunological reaction. For example, between 10 μl to 20 μl of sample is all that is required to fully rehydrate a 5 by 5 mm gold pad or membrane with the conjugate pad or membrane composed of polyester, glass, nylon, cellulose or other fibers. Since the conjugate pad or membrane is in the device flow path and in contact with the membrane, a minimal remaining sample is involved in the immunological reaction. The remaining 50 μl to 150 μl of sample acts as a liquid front by pushing the reacted analyte conjugate complexes through the flow path. Thus, a severe limitation of the one-step lateral flow assay is the limited amount of sample actually involved in the reaction with the conjugate. None of the known prior art teach or suggest locating the conjugate pad or membrane on the reverse side of the strip, and not in contact with either the sample receiving pad or the membrane. By locating the conjugate pad or membrane on the reverse side of the membrane, the entire sample first rehydrates the conjugate resulting in a uniform dispersion of conjugate through the entire applied sample. Further, in a one-step lateral flow assay, as a sample mixes with a dry conjugate, incomplete mixing often yields conjugate particles with vastly different numbers of captured analyte molecules. This alone may be a source of decreased sensitivity since the maximum number of analyte molecules are not captured on the surface of the conjugate particles. 
     Other designs are known in the art also include two-step assays. These often require the conjugate pad or membrane to be physically and spacially removed from the test strip. For example, there are assays known in the art where the conjugate is contained in book form, or a “male to female” molded apparatus. In the book form, the test strip is located on one panel of the book while the conjugate is located on the other panel. The test requires the user to close both halves for the test to begin. With the “male to female” apparatus, the sample is applied to the male portion of the apparatus, and the two halves are closed to initiate the reaction. Another format has the conjugate dried and located on a separate sampling stick. The sampling stick is mixed with a sample and is then fixed in the cassette sample well. For example, some two step assay platforms are known in the art, such as indicated in U.S. Pat. No. 6,824,997 and U.S. Pat. No. 5,418,171. These tests have a significant drawback though. The end user must perform an additional step to physically bring the membrane and conjugate into contact after the sample has been adequately mixed. 
     Thus, there is a desire and need in the art for a high sensitivity assay system and apparatus to rapidly detect low levels of ligands in a small sample size of fluid. Such new tests should involve a minimal number of procedural steps while at the same time, yielding reliable results, even when used by untrained persons. Such a system and apparatus would be useful in areas of infectious disease, pregnancy, microbial detection, ovulation, cancer marker identification, autoimmunity, cardiac markers, biowarfare agents, allergy, drugs of abuse and environmental monitoring, and the like. 
     SUMMARY OF THE INVENTION 
     The present invention generally relates to a rapid high sensitivity system apparatus and method for detecting low levels of ligands in body bio-fluids, environmental and tissue culture extracts, with high sensitivity and specificity. 
     The invention includes a system and method for a semi-qualitative (e.g., positive/negative) indicator, such as the presence of agglutination or a color change, as well as quantitative determination. Accordingly, it is an important aspect of one embodiment of the present invention to provide a system and method of analyte detection having a membrane strip that increases detection on the order of 2 to 10 fold over the conventional chromatographic specific binding assay techniques by placing a dried or lyophilized colloidal sphere conjugate to the back of a lateral flow test. Preferably gold colloidal spheres are used, but other metal sols and latex microparticles may be used as well. 
     Another important aspect of the invention is the inclusion of a novel conjugate pad or membrane solution dried onto the conjugate pad or membrane. This solution is composed of negatively charged detergents that are coated onto the conjugate pad or membrane. When sample contacts a conjugate pad or membrane, the labeled colored particles are rapidly expressed into the sample where the test ligand has an increased opportunity to react with said conjugate particles. 
     Other important aspects of the present invention may include several cassettes for use with the test strip. The cassettes are designed to allow for sample loading either on the reverse of the cassette, the cassette side, or on the front of the cassette. When loading the sample on the front or side of the cassette, the cassette contains ridges or channels designed both to transport the sample directly to the conjugate pad or membrane and to pre-mix the sample and conjugate. 
     Additional aspects and advantages of the invention will become apparent from the following detailed description, the drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The foregoing features, as well as other features, will become apparent with reference to the description and figures below, in which like numerals represent like elements, and in which: 
         FIG. 1  is an expanded side view of chromatographic elements assembled into a test device in accordance with the objects&#39; present invention; 
         FIG. 2  is an expanded side view of prior art chromatographic elements assembled into a test device; 
         FIGS. 3   a - 3   c  are side views of chromatographic elements assembled into a test device with alternative layouts of the conjugate pad or membrane in accordance with the objects&#39; present invention; 
         FIG. 4  is a perspective view of a chromatographic assay device enclosed in a chamber in accordance with the objects&#39; present invention; and 
         FIG. 5  is an expanded side view of a cassette holder in accordance with the objects&#39; present invention. 
         FIG. 6  is an example of a cassette configuration of the present invention adding detail showing homogeneous mixing. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention generally relates to a rapid, high sensitivity chromatographic assay for detecting low levels of ligands in bio-fluids, environment, plant and tissue culture extracts, using a minimal number of procedural steps even when used by untrained persons. The present invention encompasses diagnostic kits that may contain a chromatographic specific binding assay system, and preferably an immunochromatographic specific binding assay system. Furthermore, the system and apparatus, because of its accuracy and simple method steps, make it appropriate for field use such as a home, clinic, point of care setting, or doctor&#39;s office. Test results may be visually read or read by an instrument known in the art and readily available to give either a semi-qualitative (e.g., a positive/negative result) or a quantitative result. In use, the present invention is simple to use and requires a minimum degree of user skill and involvement. 
     To assist in understanding the present invention,  FIG. 2  illustrates a side view of an expanded prior art assay platform device having chromatographic elements and is generally indicated at  42 . The main components of device  42  are a sample receiving pad or membrane  13  composed of a fibrous membrane such as cotton, glass fibers, polyster, nitrocellulose or nylon (where the sample fluid  10  is initially applied) and a cassette  16  shown in expandable halves and a laminate  11 . A liquid sample  10  is applied through holes (cassette  16  openings)  16   b  onto a sample receiving pad or membrane  13 . The sample fluid  10  flows out of the sample receiving pad or membrane  13  toward an adjacent dried or lyophilized mobile conjugate pad  15  and is along a flow path  20 , composed of a fibrous membrane, where the sample  10  and mobile conjugate  22  has an opportunity to mix and flow through capillary migration into an elongated lateral flow analyte detection fibrous membrane strip  12  toward the direction of reservoir pad or membrane  14 , located at the opposite end of the sample receiving pad or membrane  13  conjugate. Sample  10  and conjugate mixture  22  then continue to migrate to an immobilized capture reagent membrane at region  12   a  of detection membrane strip  12 , where the sample/conjugate mixture can bind or stick to the immobilized capture reagent, causing a color reaction, indicating the presence of a specific ligand or analyte. The presence and/or amount of analyte in the sample  10  may be determined by the visibility of a line formed by the capture reagent  12   a , specific for the analyte-label reagent conjugate being tested. Detection may be observed through a viewing window area  16   a  of cassette  16  having a transparent cover. There is also a control reagent region  12   b , which is used to verify that the reagents are reacting as they should. The membrane strips or pads are attached to laminate  11  (semi-rigid) to maintain its elongated structure. A cassette case  16  is also used to maintain rigidity and prevent contamination of fluids that may alter the capillary flow of the foreign fluids toward the reservoir pad or membrane  14 . Membrane  14  is typically composed of cotton, paper or glass fiber. Membrane  12  regions may be composed of nitrocellulose, mixed cellulose ester, nylon and the like. A major shortcoming of all of the known prior art is that sample  10  rehydrates the dried conjugate  22  along the flow path  20  at the conjugate pad or membrane  15 , starting from the sample application area  16   b , flowing in the direction of the reservoir absorbent pad as shown at  20  in  FIG. 2 . Thus, the majority of sample  10  applied is not involved in the immunological reaction. In fact, most of the sample  10  acts as a liquid front, pushing the reacted analyte conjugate complexes through flow path  20  and is actually not involved in the reaction with the conjugate  22 . 
       FIG. 1  illustrates a possible embodiment of the present invention assay platform device and is generally indicated at  34 . Device  34  is shown in expanded side view to illustrate the individual component configuration of the chromatographic elements. Noted differences over the prior art are the change in location of cassette opening (sample well)  16   b  to the underside of cassette  16  and its immediate contact with conjugate pad or membrane  15 . Device  34  may be configured to perform at least four types of assays including: secondary antibody sandwich for measurement of sample antibody; antibody-antigen-antibody sandwich for measurement of antigen in either competitive (inhibition) or non-competitive mode; antigen-antibody-antigen sandwich for measurement of antibody; and competitive inhibition assay involving antigen bound to the conjugate microparticles, anti-antigen on the membrane, with detectable analyte inhibiting the anti-antigen reaction. 
     In the proposed immunoassay device of the present invention, the dried or lyophilized conjugate pad or membrane  15  may contain metal sols, enzymatic, fluorescent, latex microparticles, and the like, preferable colloidal gold particles. Conjugate pad or membrane  15  is attached to the side of laminate or semi-rigid material  11 , opposite sample receiving pad or membrane  13 . The entire elongated assay strip and components are enclosed in a plastic cassette  16 . Liquid sample  10  is applied to holes or openings  16   b  directly unto the conjugate pad or membrane  15 , allowing a complete mixing of the conjugate particles  22  with sample fluid  10 . The completely mixed sample  10  and conjugate  22  next flows by gravity and/or capillary action around laminate  11  (as shown in  FIG. 3 ) onto sample receiving pad or membrane  13  along flow path  30 . This allows sample fluid  10  to rehydrate conjugate particles  22 , creating a more complete mixture of sample  10  and conjugate  22  compared to the application methods in the prior art. Sample  10  and mobile conjugate mixture next flows through capillary migration into the elongated lateral flow analyte detection fibrous membrane strip  12  along flow path  30  (toward the reservoir pad or membrane  14 , located at the opposite end of the sample receiving pad or membrane  13 ). Sample  10  and conjugate  22  mixture then continues to migrate to an immobilized capture reagent membrane region  12   a , where the sample/conjugate mixture can bind or stick to the capture reagent, causing a color reaction, indicating the presence of a specific ligand or analyte. The presence and/or amount of analyte in the sample  10  may be determined by the visibility of a line formed by the capture reagent  12   a , specific for the analyte-label reagent conjugate, which is detected in the viewing window area  16   a  which may be a transparent cover over an opening in the cassette cover  16  and known in the art. There is also a control reagent feature  12   b , which is used to verify that the reagents are reacting as they should. The major advantage of the present invention over the prior art is the flow path  30  may now be one in which the liquid phase containing sample  10  and conjugate  22  is first pre-mixed prior to contacting sample receiving pad or membrane  13 . The dried or lyophilized conjugate  22  in the conjugate membrane  15  may consist of latex microparticles, enzymatic, fluorescent, or visually observable tags such as silver, selenium, carbon, other metal sol tags, preferable colloidal gold spheres. The detection membrane strip  12 , sample receiving pad or membrane  13 , conjugate pad or membrane  15 , or reservoir pad or membrane  14  may be composed of a series of porous material pieces such as, paper, cotton, polyester, glass, nylon, mixed cellulose esters, spun polyethylene, polysulfones, and the like. Preferably, nitrocellulose, nylon or mixed cellulose esters are used for the analyte detection membrane strip  12 , while paper, cotton, polyester, glass fiber or polyethylene are preferred for the conjugate pad  15 , sample receiving pad or membrane  13  and reservoir absorbent  14 . 
     In a test tube  40  version of the assay illustrated at  FIG. 4  conjugate pad or membrane  15  and sample  10  are mixed prior to contact with the sample receiving pad or membrane  13  and flows along a path  46 . Thus, in both the test tube  40  version and cassette  16  version the flow path is one in which sample  10  and conjugate  22  go through a liquid, non-absorbent phase prior to being absorbed onto a sample receiving pad or membrane  13 . 
     In one embodiment of the present invention the detection membrane  12  and sample receiving pad or membrane  13  and conjugate pad or membrane  15  may optionally contain blocked bovine serum albumin (BSA) and detergent to prevent loss of human antibody, by non-specific attachment. 
     A reservoir pad or membrane  14  at the distal end of the device receives and absorbs liquid sample  10  and conjugate  22  material to facilitate capillary migration through the detection membrane  12  along the pre-configured flow path. A buffered diluent may be used to dilute and condition sample  10  being detected, allowing for binding and/or chemical reactions to occur to improve assay performance. The diluent may be composed of salt solutions, detergents, and the like and may be applied from a dropper tip vials or contained in a sample vial. A capture ligand  12   a  is attached to the detection membrane  12 , and a second region  12   b  performs a built in control feature. 
     The whole strip should be contained in a plastic or cardboard housing (cassette  16 ) to provide support and for ease of use. The cassette  16  may contain ridges designed to both mix the sample  10  and conjugate  22  and to facilitate eventual contact with the sample receiving pad or membrane  13 . The cassette may have ridges or channels to facilitate (channeling) of sample  10  to contact the conjugate pad or membrane  15 . 
       FIGS. 3(   a - c ) illustrate alternative embodiments of the present invention wherein the chromatographic elements assembled into an assay platform device. The location of the conjugate pad or membrane  15  is varied as is the shape of laminate  11  to illustrate different placement locations of the dried or lyophilized mobile conjugate pad or membrane  15 . These variations illustrate a key technical advantage of the present invention over the prior art in that it is not dependant upon the exact placement of the dried or lyophilized mobile conjugate pad or membrane  15 . Rather the concept of the application of a liquid sample  10  directly to the dried or lyophilized mobile conjugate pad or membrane  15 , located on the opposite side and upstream of the sample receiving pad or membrane  13 , enabling a more complete mixture with the conjugate  22  and sample fluid  10 . 
     Applying the present invention provides membrane strips and improved methods for the practice of chromatographic specific binding assay techniques. Detection membrane strip  12  is selected to have a sufficient pore size such that the conjugate  22  reagent may be comprised of latex, gold, silver, selenium, carbon, but are not limited to these elements. Sample  10  migrates by capillary action. Sample  10  size is determined by the capacity of the wick pad  14  and may be as much as 500 μl compared to 10 μl to 20 μl of effective sample  10  as constrained by one-step lateral flow assays in the prior art. Test samples  10  interact with the conjugate  22  to produce a near complete homogeneous mixture of conjugate  22  and sample  10 . Thus each conjugate particle  22  contains a uniform number of captured analyte molecules, and further each conjugate particle  22  contains about the same number of captured analyte molecules as shown in  FIG. 1 . 
     By placing the conjugate pad or membrane  15  containing ligand bound to densely colored particles such as latex, gold, silver, selenium, carbon, and the like on a reverse or opposite side of the sample receiving membrane  13 , but not in contact with the sample receiving pad or membrane  13 ; the analyte detection improves on the order of 2 to 10 fold over the conventional chromatographic specific binding assay techniques. In simplest practice as illustrated in  FIG. 4  the test strip  44  is placed in test tube  40  containing approximately 110.mu.l to 500.mu.l of sample  10 . When the sample  10  solution contacts the conjugate pad or membrane  15 , particle conjugate  22  is rapidly expressed into the sample  10  where the test ligand has opportunity to react with said conjugate particles  22 . As sample  10  migrates into the flow membrane, the ligand-particle conjugate is carried along flow path  46  where it is given opportunity to react with the capture ligand attached to the detection membrane  12 . To better understand the Figs., it is noted that cassette  16  of  FIGS. 1 ,  2 ,  5 A and  5 B can be the functional equivalent of test tube  40  of  FIG. 4 . In all embodiments cassette  16  and test tube  40  allow conjugate membrane  15  and receiving membrane  13  to be in liquid communication despite being on opposite sides of laminate  11 . 
     The detection membrane strip  12  of this invention is characterized by a high capture efficiency, which is particularly advantageous in the use of the invention for diagnosis of pediatric and geriatric patients where the volume of sample  10  fluids obtainable from such patients may be limited. 
     This lateral flow assay platform is also suitable for analysis of samples  10  with heavy loads of particulate matter without the necessity of a prefilter. Particulate matter does not interfere with analyte determination at the location of the reaction site  12   a , but instead accumulates at the interface of the sample application means and the chromatographic material. Nevertheless, prefilters, may be used and fitted into sample application means for samples having especially heavy loads of particulate matter, for example, whole blood. 
       FIG. 5  shows a diagram of an assembled strip holder (cassette  16 ).  FIG. 5   a  shows the viewing side of cassette  16  and corresponding laminate  11  configuration.  FIG. 5   b  shows a view of the cassette  16  having a sample receiving opening  16   b  and the corresponding view of laminate  11 . This apparatus performs two functions, first the sample  10  and conjugate  22  mix to homogeneity, and second, the mixture flows through the detection membrane  12 , for detection at the viewing window  16   a . Cassette  16  or test tube  40  are critical to proper mixing of the conjugate and sample  10 . This also is distinguishinable over the one-step lateral flow assay in that cassette  16  or test tube  40  is required for the entire sample  10  to mix with the dried conjugate at conjugate pad or membrane  15 . 
     Test Tube  40  Version of the Invention 
     One embodiment of the present invention allows the chromatographic test assay to be performed in a test tube  40  as shown in  FIG. 4 . In this case, sample  10  is added to tube  40  and detection strip  44  is inserted. The properties of the conjugate  22  are such that it immediately goes into solution upon contact with sample  10 . Conjugate  22  and sample  10  come to homogeneity rapidly, and the mixture then flows to the sample receiving pad or membrane  13  and analysis of the detection window  16   a  is performed. 
     Cassette  16  Version of the Invention 
     In another embodiment a sample  10  is applied on the reverse side of detection strip  12  via an opening  16   b  in the strip cassette  16 . As sample  10  mixes the conjugate  22  in the conjugate pad or membrane  15 , it quickly collects in the bottom of cassette  16 . Further, cassette  16  may contain plastic rods, chambers, or channels which further allow for a homogeneous mixing of the conjugate  22  and sample  10  as shown in  FIG. 6 . The conjugate-sample mixture may be moved to the detection membrane strip  12  via channels or ridges  48  located in the cassette as shown in  FIG. 6 . 
     In one embodiment of the present invention, the assay strip  34  has detecting antibodies. The immuno-reactive protein may be a ligand to which antibodies contained in a positive sample  10  reacts with such as HIV, HTLV, TB, and the like. Detection membrane strip  12  may be impregnated with anti-antibody to capture total, class and subclass immunoglobulins. In this case, the specific antibody in sample  10  reacts with ligand peptides (HIV, HTLV, TB, and the like) coated onto the surface of the particles  22 . 
     In another embodiment of this invention, device  34  may have antigen on both the conjugate pad or membrane  15  and detection membrane strip  12  surfaces to which specific antibody may then be able to react with separate binding domains on the antibody. If detection membrane strip  12  is detecting antigen as the analyte, the membrane  12  surface may be impregnated with antibody or ligand reactive with the antigen. Typical antigen capture examples are specific antigen peptides, HIV p24 antigen, Hepatitis B surface antigen, cardiac markers, bacterial cells and the like and are known in the art. A colored line formed in this area may indicate detectable levels of analyte. A control zone, which indicates to the user whether or not the test was successfully run may also be included and illustrated in figures. In this example, the control in region  12   b  reacts with the conjugate. Often for human samples the control line is an antibody fragment, such as Fragment antigen binding (FAB)′2 goat anti human IgG (H&amp;L) from Jackson Immunoresearch or KPL. (FAB)′2 has two antigen-binding arms that remain linked. Using FAB material prevents the binding of the protein A coated gold particles  22 , and indicates the addition of human sample  10  (containing antibody). If buffer alone is added to the test strip, no control line develops resulting in an invalid assay. A colored conjugate to which purified proteins, haptens, or antibody to human immunoglobulins is chemically attached. The dried or lyophilized conjugate  22  on the conjugate pad or conjugate membrane  15  is mixed with special solubilizing and releasing compounds in a buffer; facilitating any interaction with the sample  10  bio-fluid. 
     While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the present invention attempts to embrace all such alternatives, modifications and variations that fall within the spirit and scope of the appended claims.