These methods are described in more detail in Chapter 64. There are essentially two basic approaches for how serologic reactions aid the microbiologic diagnosis.: (1) using known antibody to identify the microorganism, and (2) using known antigens to detect antibodies in the patient’s serum.
Identification of an Organism with Known Antiserum Slide Agglutination Test—Antisera can be used to identify Salmonella and Shigella by causing agglutination (clumping) of the unknown organism. Antisera directed against the cell wall O antigens of Salmonella and Shigella are commonly used in hospital laboratories. Antisera against the flagellar H antigens and the capsular Vi antigen of Salmonella are used in public health laboratories for epidemiologic purposes.
Latex Agglutination Test—Latex beads coated with specific antibody are agglutinated in the presence of the homologous bacteria or antigen. This test is used to determine the presence of the capsular antigen of the yeast Cryptococcus neoformans.
Enzyme-Linked Immunosorbent Assay—In this test, a specific antibody that is linked to an easily assayed enzyme is used to detect the presence of the homologous antigen. This test is useful in detecting a wide variety of bacterial, viral, and fungal infections and is discussed in greater detail in Chapter 64.
Fluorescent Antibody Tests—A variety of bacteria can be identified by exposure to known antibody labeled with fluorescent dye, which is detected visually in the ultraviolet microscope. Various methods can be used, such as the direct and indirect techniques (see Chapter 64).
Identification of Serum Antibodies with Known Antigens
Slide or Tube Agglutination Test—In this test, serial two fold dilutions of a sample of the patient’s serum are mixed with standard bacterial antigen suspensions. The highest dilution of serum capable of agglutination is the titer of the antibody. As with most tests of a patient’s antibody, at least a fourfold rise in titer between the early and late samples must be demonstrated for a diagnosis to be made. This test is used primarily to aid in the diagnosis of typhoid fever, brucellosis, tularemia, plague, leptospirosis, and rickettsial diseases.
Serologic Tests for Syphilis—The detection of antibody in the patient’s serum is frequently used to diagnose syphilis because Treponema pallidum does not grow on laboratory media. There are two kinds of tests.
(1) The VDRL and RPR tests are nontreponemal tests that use a cardiolipin–lecithin–cholesterol mixture extracted from normal beef heart as the antigen (not a Treponemal-specific antigen). Clumping of the cardiolipin occurs in the presence of antibody induced during T. pallidum infection. While these tests are not specific for syphilis, they are inexpensive, easy to perform, and are thus used as screening tests.
(2) Treponemal-specific tests use T. pallidum as the anti gen. The two most widely used tests are the FTA-ABS and the T. pallidum particle agglutination (TPPA) tests. In the FTA-ABS test, the patient’s serum is first absorbed with treponemes other than T. pallidum to remove nonspecific antibodies. Then, it is reacted with nonviable T. pallidum on a slide. Fluorescein-labeled antibody against human IgG is used to determine whether IgG antibody against T. pallidum is bound to the organism. In the TPPA test, the patient’s serum sample is mixed with gelatin particles that have been sensitized with T. pallidum whole-cell anti gens. Patient serum that contains antibodies to T. pallidum will react with the gel particle and appear as a uniformly distributed smooth mat of particles in the microtiter plate. A negative result appears as a compact button at the bottom of the microtiter plate.
Cold Agglutinin Test—Patients with Mycoplasma pneumoniae infections develop autoimmune antibodies that agglutinate human red blood cells in the cold (4°C) but not at 37°C. These antibodies occur in certain diseases other than Mycoplasma infections; thus, false-positive results can occur. Nucleic acid amplification tests (NAATs) are preferred for the diagnosis of pneumonia caused by M. pneumoniae.
