Patent Publication Number: US-2004053423-A1

Title: Immunochromatographic assay method and apparatus

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] This invention relates generally to immunoassays, and more specifically to an improved chromatographic assay, often referred to as a lateral flow assay, having a test strip employing susperparamagnetic particles as the labels for the analytes to be detected, where, as an additional feature, the analytical strip is removable for reading the quantity of analytes captured therein and for archival purposes.  
       [0003] 2. Discussion of Related Art  
       [0004] Various chromatographic immunoassay techniques have been available for many years. One common aspect of known devices, particularly in the lateral flow technology is that the assay is read visually, that is, by means of one or more optically readable lines on a test strip, typically held in a carrier, which may have various configurations. One end of the test strip is exposed to the sample, normally a body fluid of some type, being tested for the particular target analytes of interest. It is known that particular analytes are indicative of particular biological, environmental, and biohazard conditions, among others. For example, urine may be tested for pregnancy or ovulation and if the target analytes are present, the test is positive. Body fluids may be tested for the presence of other analytes indicative of biological conditions or they may be indicative of the presence of substances, such as drugs. Another example would be for testing water for contaminates. Examples of lateral flow assay methods and apparatuses, where the reading is normally conducted optically, are shown in U.S. Pat. Nos. 5,591,645; 5,798,273; 5,622,871; 5,602,040; 5,714,389; 5,879,951; 4,632,901; and 5,958,790.  
       [0005] A different technology is employed in other types of biological technologies employing magnetic particles or micobeads, sometimes more specifically termed superparamagnetic iron oxide impregnated polymer beads. These beads are employed to bind with the target analytes in the sample being tested and are then typically isolated or separated out magnetically. Once isolation has occurred, other testing may be conducted, including observing particular images, whether directly optically or by means of a camera. Examples of these technologies are disclosed in U.S. Pat. Nos. 3,981,776; 5,395,498; 5,476,796; 5,817,526; and 5,922,284. Another apparatus for detecting target molecules in a liquid phase is shown in U.S. Pat. No. 5,981,297 where magnetizable particles are employed and the output of magnetic field sensors indicates the presence and concentration of target molecules in the sample being tested. Other examples to sense magnetically using physical forces are disclosed in U.S. Pat. Nos. 5,445,970; 5,981,297 and 5,925,573.  
       [0006] There are several limitations or disadvantages to the known optically detected assays. Because they are optical, only surface changes (coloration, typically) can be detected. The target analytes may be in the sample solution but of such a low concentration that only a relatively few are captured in the capture zone in the porous membrane of the assay. This provides a faint or even non-optically detectable line, and a resultant false negative reading. Quantitative assessments are really only an estimation based on color intensity of the detection line. Because the prior art assays are optically read, they are subject to contamination by exposure, and light-caused degradation. Optical assays have a limited archival shelf life.  
       [0007] None of the known prior art employs magnetic particles in conjunction with lateral flow assay technology.  
       SUMMARY OF THE INVENTION  
       [0008] Broadly speaking, the invention relates to lateral flow immunoassay technology employing superparamagnetic particles as the labels for the analytes to be detected. The bound complexes of labeled particles and analytes are captured in predetermined areas or regions on the test strip and the presence and quantity of labeled analytes are then readable by magnetic means. An advantageous additional feature of the invention is that the test strip can be removable from the support member for archival purposes or for reading by an appropriate magnetic sensing device, or both.  
       [0009] A relatively standard lateral flow assay structure is employed but the invention greatly improves the sensitivity of the device over known lateral flow techniques. It provides a very rapid (a few seconds) measurement of the analytical region in the test strip. There are many advantages of using magnetic particles over known colored particles or other optical indicators in the prior art. These include linearity because magnetic detection is linear with respect to the amount of magnetic material present over a wide range, through at least four orders of magnitude. Time stability is also significant because magnetic particles are stable. The developed assay is available to be archived and retested as necessary. Further, magnetic particles are generally inert to biological systems and the environment so they not only remain stable, they are environmentally and biologically safe. Further, magnetic particles are already in widespread use throughout the diagnostics industry with other technologies so they are readily available. Other benefits of magnetic detection are that since the particles are superparamagnetic, they are magnetic only when in a magnetic field. This allows them to be freely manipulated in solution without aggregating.  
       [0010] Another significant advantage over the prior art optical lateral flow devices is that with this invention the total amount of analytes in the capture region of the test strip is measured as a single mass in one volumetric measurement by magnetic means. The permeability of magnetic fields is such that any analyte contained within the active region of the detector will be measured. This contrasts with optical sensing techniques in which only reporter-analyte interactions on or very near the surface are detectable. In this invention the strength of the magnetic signal increases directly with the mass of iron involved. This inherent linearity of magnetic detection contributes to sensitivity, accuracy and dynamic range. Finally, superparamagnetic particles are physically similar to collodial gold with regard to size, and may be easily adapted to a wide range of lateral flow assays. It is noted that collodial gold, as well as fluorescent latex particles, are typically employed in the prior art optically sensed immunological assay techniques.  
       [0011] In lateral flow technology, at one end of the porous membrane (the active part of the test strip) is the sample introduction area conventionally comprising a sample pad and a conjugate pad. In the prior art, the conjugate pad was the source of freely moveable colored particles, typically gold sols from collodial gold, or fluorescent latex particles. In the present invention, the moveable particles are the superparamagnetic particles which label the target analytes from the sample being introduced through the sample pad. The sample, together with the bound magnetic particle labels and target analytes, move with capillary action along the porous membrane and are captured in a predefined location called a capture region or capture zone. There may be more than one capture zone to enable multiplexing, that is, testing for more than one type of analyte at the same time in the same test strip. Excess analytes and the carrying liquid continue to move on through the capture zone to the other end of the porous membrane, sometimes forming a control line or zone separate from the capture zone. An added feature is that typically a wicking pad is mounted on the far end of the porous membrane to enhance the capillary action which drives the flow from the introduction at one end of the porous membrane through the entire length of the membrane.  
       [0012] The porous membrane typically is mounted on a relatively rigid support or base member, but in an advantageous embodiment a separation sheet, or adhesive layer, exists between the base member and the porous membrane. This enables very easy removal of the test strip, which normally would include the separation sheet, so that the test strip is a very thin element which may be magnetically sensed in an appropriate device. Since optical means are not employed and color at the capture zone has no meaning, the top of the porous member is preferably covered by another protective sheet or membrane which is not transparent. It may be completely opaque. This top sheet may also include pre-printed standards, which are employed for calibrating purposes so that the magnetic detector can be calibrated for each test to ensure complete accuracy. The protective sheet may not be a separate element in some cases, but may only be the upper surface of the membrane properly treated to function as a protective sheet or surface. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0013] The objects, advantages and features of this invention will be more clearly perceived from the following detailed description, when read in conjunction with the accompanying drawing, is which:  
     [0014]FIG. 1 is a schematic perspective, partially phantom, view of an embodiment of the invention;  
     [0015]FIG. 2 is a schematic side view of the FIG. 1 embodiment;  
     [0016]FIG. 3 is a schematic side view of a preferred embodiment of the invention;  
     [0017]FIG. 4 is a top view of the FIG. 3 embodiment;  
     [0018]FIG. 5 shows how the test strip of FIG. 3 is removed;  
     [0019]FIG. 6 is a perspective of a reader device used with the assay apparatus of FIG. 1 or  3 ; and  
     [0020]FIG. 7 schematically shows how the assay strip is read by a magnetic reader device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0021] With reference now to the drawing, and more particularly to FIGS. 1 and 2, there is shown an embodiment of the present invention, a lateral flow test strip for an immunochromatographic assay. A relatively rigid assay support membrane  11  serves as a base and provides support for the test strip, which in this embodiment is removable from the support. On top of the support membrane is backing member  12  on which is mounted porous membrane  13 . The backing member is removably adhered to the support, or it may itself be an adhesive layer. Within the porous membrane is capture zone  14 . The capature zone is formed by striping with antigens or antibodies, for example, as is well known in the art. On top of the porous membrane is non-light transmissive cover or surface  15 . The cover or surface may be considered to be optically opaque, at least to the extent that capture zone indicia would not be visible through cover  15 . The protective membrane may be made of plastic, glass or paper, for example, or any practical combination thereof. A printed standard or calibration line  16  is situated on top of the cover and provides information utilized by the assay reader after the test has been accomplished. This is contemplated to be a magnetic stripe with the information the reader needs.  
     [0022] At the left end, as shown in FIGS. 1 and 2, is sample pad  17 , through which an analyte-containing sample solution  20  is administered to the porous membrane, with analytes  20 A shown in the sample pad. The sample pad may also include conjugate pad  19  which is in communication with the porous membrane. Within the conjugate pad are superparamagnetic beads or particles which are coupled with antibodies, the combination of a bead and an antibody being referred to as a conjugate, a plurality of them being labeled with reference numeral  20 B. These conjugates are configured to combine with target analytes in the sample solution in a known manner to create a sandwich assay, well known in the art, where the beads provide labels for the target analytes. Competitive assay techniques, also well known in the art, could also be employed.  
     [0023] The porous membrane will, as a feature of some embodiments, have a wicking pad  18  at the opposite, or right, end of the test strip. This is a conventional element.  
     [0024] The operation of the lateral flow assay is well known. The labeled analytes (in a sandwich assay, for example) move by capillary action from the left to the right as seen in FIG. 1. Labeled analytes are captured in the capture zone where reading of the assay results is accomplished. Wicking pad  18  enhances capillary flow in the porous membrane by “pulling,” or “driving” the fluid through the porous membrane. A typical material from which the porous membrane is made is nitrocellulose.  
     [0025] While the capillary action and the existence of a capture zone are well known and conventional, the manner in which the described embodiments of the invention detect the presence and the quantity of the target analytes differs greatly from the prior art. The known lateral flow assays depend upon color or fluorescence to provide a visual or optical indication of the presence of target analytes in the capture zone, but the ability of optical techniques to detect the presence of the target analytes is limited. A relatively low concentration of target analytes in the sample can result in so few captured analytes as to be optically undetectable on the surface of the porous membrane at the capture zone. Further, the optical intensity of the capture zone with the captured analytes is only a rough function of the quantity of target analytes captured. However, there is no way to accurately measure the total quantity of captured analytes within the capture zone because only the surface is optically readable.  
     [0026] The present embodiment provides greatly enhanced sensitivity and quantitative accuracy because the magnetic labeled analytes in the capture zone are detectable by a suitable magnetic detector to the extent of the target analytes within the entire volume of the capture zone.  
     [0027] Additional features may be added, including additional capture zones  14  (two are shown in FIGS. 3 and 5) and additional calibration lines  16 . In FIGS.  3 - 5  there are four calibration lines, one before and one after each capture zone. There could be several capture zones and equivalent calibration lines.  
     [0028] A significant aspect of an embodiment of the invention is the means and manner of magnetically reading the assay. A magnetic reader  21  of the type contemplated is shown in FIG. 6. This employs the technology disclosed in PCT publication WO 99/27369, to determine the presence of target analytes and their quantity. The reader of the present invention is contemplated to be portable, that is, approximately pocket size, so that it is easily employed in the field. It will provide accurate assay readings even under stressful conditions and in poor light. The apparatus of FIG. 6 has a narrow slit opening  22  into which an assay strip can be inserted for reading by electromagnetic means. The analyte quantity may be shown in window  23 , which could be an LED or an LCD screen, for example. The manner in which the capture zone with analytes bonded therein is read is shown schematically in FIG. 7. Lower electromagnetic head  31  has a top surface  32  on which is mounted detection coil  34  and across which strip  13 / 15  is positioned with capture zone  14  centered over the detection coil. The strip is shown in this example with surface or cover  15  of the strip actually touching the surface of the detection coil. Cover  15  thus protects the delicate structure of the porous membrane during the reading process, as well as long term if the strip is archived. Top magnet  33  is closely adjacent to the opposite side of the strip, having stripping layer  12  thereon. The structure of the electromagnetic head shown in FIG. 7 is generally equivalent to that in reader  21 .  
     [0029] To enable the reader of FIG. 6 to be employed to read the analytes in the capture zone, the test strip is made readily removable from support membrane  11 , and from sample pad  17  and wicking pad  18 . As seen in FIGS.  3 - 5 , pull tab  24  is secured to cover  15 , which is, in turn, secured to porous membrane  13 . The porous membrane is removably secured to the support by means of a removable backing  12  membrane and secured to the bottom of the porous membrane. It will be noted that the assay is essentially inverted as shown here. The sample could be applied to sample pad  17  with the strip turned over from the position of FIGS. 3 and 5. The conjugate pad  19  and the wicking pad  18  are positioned between support membrane  11  and the peelable strip  12 ,  13 ,  15 . Element  11 A may be a non-active polymer fill membrane. Sheet or adhesive  12  is readily removable from the surfaces of elements  18 ,  19  and  11 A.  
     [0030] It is contemplated that the test strip, primarily consisting of the porous membrane and protective surfaces thereof, may be made sufficiently rigid to not need a support membrane. Such a configuration would not need to be stripped from anything. The reader apparatus might have to be modified somewhat to handle the different configuration of the test strip, but the principle of magnetic reading shown in FIG. 7 still applies.  
     [0031]FIG. 5 shows how the test strip, comprised of the cover, porous membrane and removable backing, is removed from the support membrane. In actuality, this removed test strip is typically about 3-12 mm wide, and only about 150-500μ thick. This strip is easily fed into reader  21  for a digital readout, which may be shown on the screen or printed on paper in any desired form by reader  21 . The exposed test strip is stable and can be archived either before or after being read. Since the superparamagnetic beads are magnetized only during the reading process, the exposed test strip is not subject to degradation. The analytes contained in the capture zone remain there, labeled with the conjugate combination.  
     [0032] Alternative features of the embodiment discussed above contemplate the test strip being slid off the support membrane or peeled off, either manner of removable being physically possible.  
     [0033] The opaque surface or cover  15  has several positive functions. Contrary to prior art optical lateral flow assays, where very faint lines can easily be misinterpreted in the field, especially in stressful situations or low light conditions, there is no possibility of misinterpretation of test results with this invention. Optically read assays, especially those visually read, are also subject to operator bias. In the present invention the reader reads the total number of labeled analytes in the capture zone without inherent sources of error as mentioned above. Further, the thickness of the opaque cover precisely positions the porous membrane and thus, the capture zone, with respect to the magnet head and detection coil  34 . Since the test strip actually touches the detection coil, without the protective surface the porous nitrocellulose membrane would be damaged by rubbing across detection coil  34 , thereby possibly producing incorrect or unreliable readings, or both. Although being very thin, in the range of 30-50μ the cover protects against physical damage and environmental contamination as well as providing precise positioning for accurate electromagnetic readings.  
     [0034] While the present invention has been illustrated and described by means of a specific embodiment, it is to be understood that numerous changes and modifications can be made therein without departing from the scope of the claims and equivalents thereto.