Basic tests for antibody detection measure the antibody produced by a host to determinants on the surface of a bacterial agent in response to infection. Specific antibodies bind to surface antigens of the bacteria in a thick suspension and cause the bacteria to clump in visible aggregates. Such antibodies are called agglutinins, and the test is referred to as bacterial agglutination. Electro static and additional chemical interactions influence the formation of aggregates in solutions. Because most bacterial surfaces have a negative charge, they tend to repel each other. Performance of agglutination tests in sterile physiologic saline (0.9% sodium chloride in distilled water), which contains free positive ions, enhances the ability of antibody to cause aggregation of bacteria. Although bacterial agglutination tests can be performed on the surface of both glass slides and in test tubes, tube agglutination tests are often more sensitive, because a longer incubation period can be used, allowing more antigen and antibody to interact. The small volume of liquid used for slide tests requires a rather rapid reading of the result, before the liquid evaporates, causing erroneous results.
Examples of bacterial agglutination tests include assays for antibodies to Francisella tularensis and Brucella spp., which are part of a panel referred to as febrile agglutinin tests. Bacterial agglutination tests are often used to diagnose diseases in which the bacterial agent is difficult to cultivate in vitro. Diseases diagnosed by this technique include tetanus, yersiniosis, leptospirosis, brucellosis, and tularemia. The reagents necessary to perform many of these tests are commercially available, singly or as complete systems. Because most laboratories are able to culture and identify the causative agent, agglutination tests for certain diseases, such as typhoid fever, are seldom used today. Furthermore, the typhoid febrile agglutinin test (called the Widal test) is often positive in patients with infections caused by other bacteria because of cross-reacting antibodies or a previous immunization against typhoid. Appropriate specimens from patients suspected of having typhoid fever should be cultured for the presence of salmonellae.
Whole cells of parasites, including Plasmodium and Leishmania spp., or Toxoplasma gondii, have also been used for direct detection of antibody by agglutination. In addition to using the actual infecting bacteria or parasites as the agglutinating particles, certain bacteria may be agglutinated by antibodies produced against another infectious agent. Many patients infected with one of the rickettsiae produce antibodies capable of nonspecifically agglutinating bacteria of the genus Proteus, specifically Proteus vulgaris. The Weil-Felix test detects these cross-reacting antibodies. Because newer, more specific serologic methods of diagnosing rickettsial disease have become more widely available, the use of the Proteus agglutinating test is no longer offered in many laboratories.
