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Immunologic tests generally detect antibodies to pathogens. Handling varies, but if testing is to be delayed, the specimen should typically be refrigerated or frozen to prevent overgrowth of bacterial contaminants.
Agglutination
tests:
In agglutination tests (eg, latex agglutination, coaggregation), a particle (latex bead or bacterium) is coupled to a reagent antigen or antibody. The resulting particle complex is mixed with the specimen (eg, CSF, serum); if the target antibody or antigen is present in the specimen, it cross-links the particles, producing measurable agglutination.
If results are positive, the body fluid is serially diluted and tested. Agglutination with more dilute solutions indicates higher concentrations of the target antigen or antibody. The titer is correctly reported as the reciprocal of the most dilute solution yielding agglutination; eg, 32 indicates that agglutination occurred in a solution diluted to 1/32 the starting concentration.
Agglutination tests generally are rapid but less sensitive than many other methods. They can also determine serotypes of some bacteria.
Complement
fixation:
The complement fixation test measures complement-consuming (complement-fixing) antibody in serum or CSF. The test is used for diagnosis of some viral and fungal infections, particularly coccidioidomycosis. The specimen is incubated with known quantities of complement and the antigen that is the target of the antibody being measured. The degree of complement fixation indicates the quantity of this antibody in the specimen. The test can measure IgM and IgG antibody titers or can be modified to detect certain antigens. It is accurate but has limited applications, is labor intensive, and requires numerous controls.
Enzyme
immunoassays:
Enzyme immunoassays are tests that use antibodies linked to enzymes. The tests include the enzyme immunoassay (EIA) and enzyme-linked immunosorbent assay (ELISA). These tests detect antigens and detect and quantify antibodies. Because sensitivities of most enzyme immunoassays are high, they are usually used for screening. Titers can be determined by serially diluting the specimen as with agglutination tests.
Test sensitivities, although generally high, can vary, sometimes according to patient age, microbial serotype, or stage of clinical disease.
Precipitation
tests:
Precipitation tests are tests that measure an antigen or antibody in body fluids by the degree of visible precipitation of antigen-antibody complexes within a gel (agarose) or in solution. There are many types of precipitation tests (eg, Ouchterlony double diffusion, counter immunoelectrophoresis), but their applications are limited. Usually, a blood specimen is mixed with test antigen to detect patient antibodies, most often in suspected fungal infection or pyogenic meningitis. Because a positive result requires a large amount of antibody or antigen, sensitivity is low.
Western
blot test:
The Western blot detects viral or other antibodies in the patient's sample (eg, serum or other body fluid) by their reaction with target antigens (eg, viral components) that have been immobilized onto a membrane by blotting.
The Western blot typically has good sensitivity, although often less than screening tests such as ELISA, but generally is highly specific. Thus, it is usually used to confirm a positive result obtained with a screening test.
Technical modifications of the Western blot are the line immunoassay (LIA) and the recombinant immunoblot assay (RIBA), which use synthetic or recombinant-produced antigens.
Last full review/revision November 2005
Content last modified November 2005
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