Serodiagnosis of bacterial diseases is of value only in specific circumstances. IgG antibody is long-lived, and its presence, although indicative of previous infection or immunization, gives little or no information on current bacterial infection. IgM antibody is usually produced within days to a few weeks after exposure to antigen, and thus its presence suggests recent exposure in most cases. As with viral diseases, serial determinations of antibody levels with rising titers are of greater diagnostic value, but because of the time intervals required, they are usually of little clinical value.
In general, culture of specific pathogens is required to confirm the diagnosis guidelines of a bacterial disease. Serologic tests may aid in diagnosis when diseases are caused by bacteria that are difficult to grow. Brucella is one such species. These organisms are difficult to culture from patients’ specimens, and no useful delayed hypersensitivity skin test is available.
A serum agglutination test for antibodies using B abortus antigen is often used to help diagnose brucellosis. Most mycobacteria are also difficult to grow, but antibody titers are not helpful in diagnosis. Thus, in contrast to viral and fungal pathogens, the serologic tests in bacterial infections remain primarily a tool for epidemiologic studies rather than for clinical diagnosis. Nevertheless, most bacteria induce specific antibody reaction sponses, which, in most cases, can be easily measured in serum. As discussed, these antibody responses are often critical in determining the host response to an infecting agent. This is why serodiagnosis of bacterial diseases are playing important role.
