Method and apparatus for assaying whole blood

Abstract: A method of assaying a sample of whole blood for one member of a specific binding pair, the method involving treating the sample with salt to alter the red blood cells in the sample to decrease their ability to pass through filter media, exposing the resulting sample to a filter which retains the red blood cells but allows the passage therethrough of filtrate of the sample containing the member of the specific binding pair, and mea suring the member of the filtrate. A kit includes a hypertonic solution in one container, a second container for mixing the solution with a sample, a filter positioned with respect to the second container to permit the mixture to pass from the second container through the filter; and a means for collecting the sample. The filter is capable of retaining red blood cells in the mixture.

Description: Background of the Invention This invention relates to the performance of immunoassays on whole blood. Immunoassays are carried out to detect a particular antigen, hapten, or antibody in blood. Since red blood cells interfere with such assays, they are normally removed from the blood prior to performing the assay, e.g., by centrifugation or filtration. The resulting plasma solution can then be used in any standard immunoassay, cf. e.g. US - A - 4 136 161 (GERBER et al.), which describes a colorimetric method of detection of bindable substances such as antigens, using chromogenic agents such as tetramethyl benzidine. The state of the art for assays of whole blood involves the following disadvantages : a) tolerating the presence of red blood cells, thus limiting the flexibility of assaying and detection techniques; b) more complex separations, using chemical binding reagents or mechanical techniques such as centrifugation; c) attempts to filter, with attendant problems of clogging or slowness or undesired passage of red cells. WO - A - 79 / 01120 (GOVERNMENT ... UNITED STATES ...) discloses a laminar flow device for separating red blood cells from plasma, i.e. a flow regime having two forces : a force perpendicular to the membrane to force the plasma through; and a force tangential to the membrane to prevent blood cell/membrane clogging. FR - A - 2 344 842 (ORTHO DIAGNOSTICS INC.) discloses a method for stabilizing erythrocytes. Incidentally, the treatment of blood with isotonic and hypertonic salt solutions is known, FR - A - 2 344 842 (ORTHO DIAGNOSTICS INC.), and EP - A - 0 190 488 (BECTON DICKINSON & CO.). Summary of the Invention In general, the invention features a method of assaying a sample of whole blood for one member of a specific binding pair, involving treating the sample with a salt to alter the red blood cells in the sample to decrease their ability to pass through filter media, exposing the resultant sample to a filter which retains the red blood cells but allows the passage therethrough of filtrate containing the member of the specific binding pair being assayed, and measuring the member of the filtrate. As used herein, the term "specific binding pair" means any pair of molecules which bind to each other but do not bind to a significant degree to other molecules; examples of such pairs are antigens and antibodies specific therefore. It is believed that the salt causes the red blood cells in the sample to exhibit a change in physical properties, e.g., deformability, so that they are unable to pass through filter media which would have permitted their passage prior to treatment with the salt. A filter having an average pore size small enough to retain untreated red blood cells would become clogged and prevent passage of liquid as well. According to the invention, a filter can be used which has an average pore size (nominal size 0.3-0.6 µm (microns)) large enough to prevent clogging, but which still retains the treated red blood cells. In preferred embodiments of the method, the step of treating the sample with a salt involves mixing the sample with a hypertonic solution to form a dilute solution, and following the exposing of the sample to the filter, the filtrate is deposited on a solid support on which the measuring of the member of the specific binding pair is carried out; preferably, the solid support and the filter are in contact with one another when the sample is exposed to the filter. Alternatively, the filtrate is removed from the filtration device and placed in contact with a measuring device comprising a separate solid support on which measuring is carried out. Preferably, the hypertonic solution contains a salt of ionic strength of at least 0.5 M. The method of the invention is preferably carried out using a kit composed of a first container holding a hypertonic solution, a second container for holding the hypertonic solution and mixing it with the sample, a filter positioned with respect to the second container so as to permit the mixture of the sample and the hypertonic solution to pass from the container through the filter, such filter being capable of retaining the treated red blood cells in the sample, and means e.g., a solid support, for collecting the sample after it has passed through the filter. In preferred embodiments of the kit, the filter is flexible, and the kit comprises means to hold the filter in intimate contact with the solid support. The solid support comprises a surface bound to one of the components of the specific binding pair, e.g., an antigen, a virus, or an antibody. The filter is positioned within a holder, which may comprise a fluid chamber having sloping sides to improve flow consistency. The filter is sealed to the holder. The kit includes a filter holder having an annular member comprising an annular exterior wall and an annular interior wall, attached by a connecting member; the exterior wall and the interior wall are spaced apart a distance selected to form a cavity for receiving an co-operatively shaped seating member connected to the measuring device. Either the interior wall or the said exterior wall comprises a surface irregularity along the cavity to seat the measuring device, and the surface irregularity comprises means for venting air as said filter device is seated on said measuring device. In another aspect, the invention features the above-described filter device. The filter device preferably includes means to impart tension to the filter, e.g., an annular screen positioned to contact the filter. The invention allows the rapid and simple assay of whole blood samples for any immunologically detectable compound, without prior removal of red blood cells. Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims. Description of the Preferred Embodiment Fig. 1 is a diagrammatic representation of an assay filter assembly of the invention. Fig. 2 is a section of an alternative filter assembly. Structure Referring to the Fig. 1, cylindrical membrane assembly 10 includes filter holder 12 fitted with filter 14 to form fluid chamber 11, and an assay membrane holder 16 fitted with assay membrane 18. Membrane holder 16 and filter holder 12 may be constructed of any suitable nonporous material, for example, PVC. Filter holder 12 has an annular groove which mates with annular ridge 22 on assay membrane holder 16 so that filter 14 is positioned in intimate and homogeneous contact with assay membrane 18, which is supported by porous support disc 19, which in turn overlies absorbent bed 21. Assay membrane 18 bears immunologically reactive material 24 which consists, for example, of latex microspheres 1 micron in diameter coated with a suitable immunologically reactive compound, for example, an antibody, or of a bioreactive zone prepared by binding the immunologically reactive compound directly to a solid support consisting of either an activated surface or an inert surface capable of irreversibly binding the immunologically reactive compound. Assembly 10 is of a diameter suitable for holding membrane 18, for example, 1 cm. Membrane 18 and filter 14 are attached to their respective holders by glue, or other means which prevents leakage between the holder and membrane. Filter 14 is capable of retaining red blood cells in a mixture of whole blood and salt while allowing passage therethrough of red blood cell-free liquid. Filter 14 can e.g., be a filter having fibers lying on top of one another with a thickness of 1/4 to 1/2 mm and a nominal pore size rating of 0.4 microns, for example, a glass AE filter (Gelman). Assay membrane 18 can be any standard assay membrane, e.g., a glass AE filter. A second filter embodiment is shown in Fig. 2. The features of filter membrane assembly 10′ are generally the same as those shown in Fig. 1, and corresponding primed members are used to designate those features in Fig. 2. Filter membrane assembly 10′ also includes a protrusion 30′ on the inside of the outer wall 32′ of filter holder 12′. The depth of outer wall 32′ and support wall 34′ are carefully selected so that filter 14′ is in intimate contact with porous support disc to support capillary action. In Fig. 2, filter 14′ is a glass fibre filter disc sealed to assembly 10′ by snap ring 36′ which has an annular protrusion 42′ that fits within an indentation in the inner wall 38′ of holder 12′. A polypropylene screen 39′ (150 mm) which includes a 1/4″ central opening, is molded into filter 14′. The screen assures that the glass fiber filter disc retains the desired convex shape. The convex form of the screen transfers a kinetic advantage to the weaker glass fiber disc. As the center point of the filter contacts the assay membrane, positive tension is generated as the rest of the glass fiber filter is seated against the assay membrane. This tension ensures an intimate contact between both surfaces. As the required tension is slight, material has been removed from the center of the screen to prevent stressing of the glass fiber filter. The dimensions of the filter holder have been carefully determined to mate with the solid support. Specifically, support wall 34′ and filter support 12′ form a close fit. There is a risk that an increase in air pressure between the glass fiber filter and assay membrane during seating could distort the shape of the filter. Additionally, any unexpelled air could remain trapped between the surfaces resulting in non-intimate contact. To avoid these problems, venting is accomplished as the filter and support are fitted together, by the placement of three small protrusions (one of which is shown as 30′) around the circumference of the interior of the outside wall 32′ of the filter holder 12′. These protrusions raise the outer wall 32′ away from the exterior of the solid support (see arrow A) until contact is made by the respective annular groove and ridge. Air is thus allowed to exit the space between the filter holder and solid support until contact is made. The salt (not shown) is provided in any suitable container, e.g. a cylindrical PVC container, preferably as a buffer solution. The buffer is hypertonic, preferably with an ionic strength greater than 0.5 M. In order to reduce the background level of reactivity of the assay membrane with blood solutions, the buffer also contains proteins unreactive with the assay membrane, for example, bovine serum albumin and non-fat dry milk. Preferably, the buffer also contains nonionic detergents, e.g., Tween® 80 or Triton® X-100 which increase the sensitivity of the assay. Assay In the first step of the assay, filter 14 is wetted with buffer. Whole blood from the animal or human patient to be tested (approximately 3 drops (0.1 ml)) is gently mixed with 0.5 ml of buffer and the resulting treated whole blood solution is poured into the assembled membrane filter assembly having filter 14 in contact with assay membrane 18. The red blood cells in the blood are retained by the filter, and the red cell-free liquid containing the component to be detected passes through the filter to the assay membrane. After the filtration process, which takes approximately 1-5 minutes, is complete the filter assembly 10 is removed from the assay membrane and the assay membrane is processed using standard technique to detect the blood component being assayed. The following example is illustrative. Example: Detection of Feline Leukemia Antigen Antibodies to feline leukemia antigen (commercially available) are bound to latex microspheres by standard techniques and then spotted onto an assay membrane (Gelman AE filter) fixed to a membrane holder as in the Figure. A Gelman AE filter is mounted above and in contact with membrane 18, as shown in the Figure. Blood is drawn from a cat by a standard procedure and 3 drops (approximately 0.1 ml) mixed with 0.5 ml of a buffer containing 0.25 molar sodium citrate, one molar KCl, 5% bovine serum albumin, 2.5% non-fat dry milk, and 0.1% Tween® 80. The resulting dilute whole blood solution is then poured into the assembly and given 3 minutes to pass through the filter onto the assay membrane. The filter assembly is then removed and the assay membrane treated to determine the presence or absence of feline leukemia antigen using conventional techniques. Generally, after the leukem ia antigen has bound to anti-leukemia antibody on the membrane, enzyme-labeled anti-leukemia antibody is added to form a "sandwich", and color is developed using an enzyme substrate and colormetric indicator. Other embodiments are within the following claims. For example, although it is preferred that the red cell-free liquid be collected on a solid support as described above, the liquid can also be collected in a container and assayed using, e.g., a homogeneous assay method. Any hypertonic solution can be used which causes the required change in shape and deformability of the red cells in the blood, e.g., other salts, and sugar solutions. Any member of a specific binding pair can be measured, for example the assay can be used to screen whole blood samples for HTLV-3 virus or antibodies to this virus to test for Acquired Immune Deficiency Syndrome (AIDS).

Claims: Method of separating red blood cells from a whole blood sample to yield a sample fraction comprising plasma in preparation for assaying the resulting sample fraction for one member of a specific binding pair , characterized by treating said sample with a salt so as to render said sample hypertonic and thereby to alter the red blood cells in said sample to decrease their ability to pass through filter media, exposing the resultant sample to a filter (14; 14') which retains said red blood cells but allows the passage therethrough of filtrate of said sample containing said member of said specific binding pair for measuring said member in the filtrate (Figs. 1;2). Method of claim 1 , characterized in that said treating said sample with said salt comprises mixing said sample with a hypertonic solution having an ionic strength of at least 0.5 M . Method of claim 1 , characterized in that a non-ionic detergent is added to said sample prior to filtering . Method of assaying a filtered blood sample fraction comprising plasma, yielded by ay preceding claim, for one member of a specific binding pair , characterized by following exposing of said sample to filter (14; 14'), depositing said filtrate on a solid support (24, 18, 19, 21) on which measuring of said member of said specific binding pair is carried out (Figs. 1; 2). Method of claim 4 , characterized in that said solid support (24, 18, 19, 21) and said filter (14; 14') are in contact with one another when said sample is exposed to said filte r (14; 14'). Method of any preceding claim , characterized in that one member of said specific binding pair is a virus or an antibody against a virus . Method of claim 6, characterized in that said virus is HTLV-3 . Method of assaying a sample of whole blood for feline leukemia antigen , characterized by treating said sample with a salt so as to render said sample hypertonic and thereby to alter the red blood cells in said sample to decrease their ability to pass through filter media, exposing the resultant sample to a filter (14; 14') which retains said red blood cells but allows the passage therethrough of filtrate of said sample containing said antigen , and measuring said antigen in the filtrate (Figs. 1; 2) . Method of claim 8 , characterized in that said treating said sample with said salt comprises mixing said sample with a hypertonic solution having an ionic strength of at least 0.5 M , so as to render said sample hypertonic. Method of claim 9 , characterized in that said hypertonic solution comprises KCl and sodium citrate . Method of claim 10 , characterized in that said hypertonic solution comprises nonionic detergent . Method of claim 11 , characterized in that said hypertonic solution comprises bovine serum albumin and non-fat dry milk . Method of assaying a filtered blood sample fraction comprising plasma , yielded by any of claims 1 - 3 , for one member of a specific binding pair , characterized in that filtration is accomplished in a filtration device comprising a filter (14; 14'), and said filtrate is removed from said filtration device and placed in contact with a measuring device comprising a solid support (24, 18, 19, 21) on which said measuring of said member of said specific binding pair is carried out, said filter (14; 14') and said support being separated from each other (Figs. 1; 2). Kit for assaying a sample of whole blood for one member of a specific binding pair , characterized by a first container holding a hypertonic solution , a second container for holding said hypertonic solution and mixing said hypertonic solution with said sample , a filter (14; 14') positioned with respect to said second container so as to permit said mixture of said sample and said hypertonic solution to pass from said second container through said filter (14; 14'), said filter (14; 14') being capable of retaining the treated red blood cells in said sample, and means for collecting said sample after it has passed through said filter (14; 14') (Figs. 1; 2). Kit of claim 14 , characterized in that said collecting means comprises a solid support (24, 18, 19, 21). Kit of claim 15, characterized in that said filter (14; 14') is flexible , and said kit comprises means to hold said filter (14; 14') in intimate contact with said solid support (24, 18, 19, 21). Kit of claim 15 , characterized in that said solid support (24, 18, 19, 21) comprises a surface bound to one of the components of said specific binding pair . Kit of claim 17 , characterized in that said one member of said specific binding pair is an antigen , a virus , or an antibody . Kit of claim 14 , characterized in that said filter (14; 14') is positioned within a holder (12; 12'). Kit of claim 19 , characterized in that said holder (12; 12') comprises a fluid chamber (11) having sloping sides to improve flow consistency. Kit of claim 19 , characterized in that said filter (14; 14') is sealed to said holder (12; 12'). Kit of claim 19 , characterized by a measuring device , said filter holder (12; 12') comprising an annular member configured to seat over said measuring device. Kit of claim 22 , characterized in that said annular member comprises an annular exterior wall (32') and an annular interior wall (38') , attached by a connecting member , said exterior wall (32') and said interior wall (38') being spaced apart a distance selected to form a cavity for receiving a co-operatively shaped seating member connected to said measuring device , to insure intimate contact between said filter (14; 14') and said solid support (24, 18, 19, 21) (Fig. 2) . Kit of claim 23 , characterized in that at least one of said interior wall (38') and said exterior wall (32') comprises a surface irregularity (30') along said cavity to seat said measuring device (Fig. 2) . Kit of claim 24 , characterized in that said surface irregularity comprises means (30') for venting air as said filter device (10; 10') is seated on said measuring device (Fig. 2) . Kit of claim 14 , characterized by a means for reducing non-specific binding . Filter device (10; 10') characterized by a filter (14; 14') and a filter holder (12; 12'), said filter holder (12; 12') comprising an annular member configured to seat over a solid-phase assay member (24, 18, 19, 21), and annular means comprising an annular exterior wall (32') and an annular interior wall (38'), attached by a connecting member , said exterior wall (32') and said interior wall (38') being spaced apart a distance selected to form a cavity sized and shaped for receiving a co-operatively shaped measuring device (Figs. 1; 2) . Filter device of claim 27 , characterized in that at least one of said interior wall (38') and said exterior wall (32') comprises a surface irregularity (30') along said cavity to seat said measuring device (24, 18, 19, 21) (Fig. 2) . Filter device of claim 28 , characterized in that said surface irregularity comprises means (30') for venting air as said filter device (10; 10') is seated on said measuring device (24, 18, 19, 21) (Fig. 2) . Filter device of claim 27 , characterized in that said filter holder (12; 12') comprises means (39') to impart tension to said filter (14; 14') to insure intimate contact between said filter (14; 14') and a measuring device (24, 18, 19, 21) (Fig. 2) . Filter device of claim 30 , characterized in that said means to impart tension comprises a screen (39') positioned to contact the filter (14; 14') (Fig. 2) . Filter device of claim 31 , characterized in that said screen (39') is annular (Fig. 2) .

Inventors: TONELLI QUENTIN J, TONELLI, QUENTIN J.
Applicants: IDEXX LAB INC, IDEXX LABORATORIES, INC.
Application year: 1987
Main classification: G01N33
IPC codes: G01N33