Serology

Serology has been used historically to confirm infections with bacteria, fungi, and
viruses that are difficult to detect by other methods. The difficulty with serology is that
some immunocompromised patients will not mount an adequate antibody response to
infection, a significant increase in antibody titer may not be detected until weeks or
months after the initial presentation, persistence of antibodies may make it difficult to
differentiate between a recent and a past infection, and cross-reactions may
compromise the specificity of the antibody response. In general, serology should be
used to confirm other diagnostic tests whenever possible.

Serology has been the method most commonly used for diagnosis of C.
pneumoniae infections. The need for paired samples to show seroconversion or titer
rise considerably diminishes the value of serology in acute situations.
Microimmunofluorescence (MIF) testing has been the most appropriate test for
the serologic diagnosis of an acute C. pneumoniae infection.26 In primary infections,
the diagnosis can be obtained from the first sample that contains IgM
antibodies specific for C. pneumoniae. The possibility of a false-positive reaction due
to IgM rheumatoid factor should always be kept in mind. In patients undergoing reinfections, the potentially rapid serologic IgG response can be missed if the first serum
sample is not collected early enough after the onset of disease. In MIF testing, strains
of C. pneumoniae react much more uniformly than do those of C. trachomatis. EIA

tests for chlamydial species-specific serology are currently replacing MIF due to their
easy laboratory performance. They seem to be more sensitive than MIF, especially in
acute infections of children. In C. pneumoniae infections, the CF test shows sensitivity
only in primary infections of young adults.