The gel test was developed in Switzerland in the late 1980s as a way to standardize the method of obtaining agglutination and to provide a simple and reliable way to read it. (Lapierre Y, Rigal D, Adam J, et al. The gel test: a new way to detect red cell antigen-antibody reactions. Transfusion 1990;30:109-13 ) Besides these benefits, the test also provided a way to do antiglobulin tests without having to wash them. Also, the test uses small volumes of serum and cells and is capable of using automated reading.
Unlike in tube tests, in the gel method agglutination does not take place in a liquid phase but rather in a gel contained in a special microtube. Briefly, the method is this: patient plasma and a 1% suspension of red cells (e.g., screen cells) in a low ionic medium (LIM) are dispensed into the microtube and incubated at 37∞C for 15 mins; the card containing the microtubes is then centrifuged at a controlled speed for 10 minutes. At the start of centrifugation the cells are separated from the serum; then they meet the antiglobulin serum contained in the microtube; finally the cells are trapped by the gel (if agglutinated) or pellet to the bottom of the tube.
Review pictures of a technologist performing the steps used in the gel method.
A commercial site for the gel test marketed as the "ID-Micro Typing System" can be found here.
Today gel technology is but one method of a group of methods known collectively as column agglutination technology (CAT). Another is red cell affinity column technology.
Whereas gel agglutination is based on size exclusion of agglutinated red cells in an inert matrix, red cell affinity column technology (ReACT) is based on affinity adherence of red cells in an immunologically active matrix.
In ReACT, antibody-sensitized red cells bind to ligands attached to an agarose matrix. The main ligand is Protein G (prepared from Group C or G Streptococcus or by recombinant technology), which has high affinity for all four IgG subclasses. Another ReACT ligand is Protein A (from Group A Staphlococcus), which binds to IgG 1, 2, and 4.
Visit this commercial site for an overview of ReACT.
Solid phase adherence assays (SPAA) for the detecting and identifying unexpected antibodies are based on research done in the early 1980s (Plapp FV, Sinor LT, Rachel JM, et al. A solid phase antibody screen. Am J Clin Pathol 1984;82:719). Like other methods in this category, SPAA is rapid, sensitive and specific, uses small volumes of sera and reagents, and can incorporate automated reading.
SPAA for antibody identification uses red cell membranes that have been bound to the surfaces of polystyrene microtitration strip wells, and which capture IgG antibodies (if present) in patient sera. The method in brief is as follows: patient serum is added to wells coated with screen cells; tests are incubated at 37o C for 15 mins; unbound IgG is washed away (as in standard antiglobulin tests); anti-IgG-coated indicator red cells are added; tests are centrifuged and read.
If patient IgG antibodies have attached to the cell membranes, the anti-IgG-coated indicator red cells form anti-IgG-IgG complexes. As a result, the indicator cells adhere to the wells as a second immobilized cell layer and form a dispersed conflux ("lawn") of cells, constituting a positive test. Conversely, in negative tests, the indicator cells pellet to the bottom of the wells forming a distinct, smaller cell button.
This commercial site markets the SPAA product "Capture-R Ready Screen TM" , as well the "ABS 2000," which uses SPAA for antibody screening.
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