To circumvent the shortcomings of cell culture, antigen-I detection methods are commercially available.
Direct fluorescent antibody (DFA) staining methods use fluorescein-isothiocyanate conjugated mono* clonal antibodies to either outer membrane proteins tjj lipopolysaccharides of C. trachomatis to detect elementary bodies in smears of clinical material (Figure 46-3). The MicroTrak DFA (Syva Co., San Jose, Calif.) is one suds commercial system.
Chlamydial antigen can also be detected by enzyme-linked immunosorbent assays (ELISA). Nuro¿|! ous U.S. Food and Drug Administration (FDA)-approv^
kits are commercially available. These assays use poly-el raal or monoclonal antibodies that detect chlamydial rjps. These tests are not species-specific for C. trachomatis and may cross-react with LPS of other bacterial species present in the vagina or urinary tract and thereby produce a false-positive result. Also available are nucleic arid hybridization tests that use a chemiluminescent tyre afDNAprobe (PACE, Gen-Probe, San Diego, Calif.) that is complementary to a sequence of ribosomal RNA (rRNA) in the chlamydial genome. Once formed, the DNA-rRNA complex is absorbed onto a magnetic bead and detected by a luminometer. This assay is species-specific for C. trachomatis.
Based on numerous studies, these nonculture tests are more reliable in patients who are symptomatic and shedding large numbers of organisms than in those who are asymptomatic and most likely shedding fewer organisms. For the most part, these assays have sensitivities of greater than 70% and specificities of 97% to 99% in populations with prevalences of C. trachomatis infection of 5% or more.3 In a low-prevalence population, that is, less than 5%, a significant proportion of positive tests will be falsely positive. Therefore, a positive result in a low-prevalence population should be handled with care, and a positive result should be verified.3-4 Positive results can be validated by the following methods:
Amplification Assays. At the time of this writing, five commercial assays using nucleic acid amplification are FDA-approved for the laboratory diagnosis of C. trachomatis infection. These assays use different formats: polymerase chain reaction (PCR), ligase chain reaction, standard displacement amplification, and transcription-mediated amplification. The first three assay formats amplify or target DNA sequences present in the cryptic plasmid that is present in 7 to 10 copies in the chlamydial EB, whereas the last format amplifies ribosomal RNA sequences. Studies clearly indicate that these tests are more sensitive than culture and other nonnucleic acid amplification assays.515,21 Because of the increased sensitivity of detection, first-voided urine specimens from symptomatic and asymptomatic men and women are acceptable specimens to detect C. trachomatis, thereby affording a noninvasive means of chlamydia testing.
Table 46-3 summarizes the various uses of the different C trachomatis laboratory diagnostic tests. Unfortunately, there is currently no straightforward approach to the laboratory diagnosis of chlamydial infections. Each laboratory lest format has its own advantages and limitations with regard not only to sensitivity, specificity, and cost, but also to the type of patient population tested (i.e., low- or high-prevalence populations and symptomatic vs. asymptomatic individuals).21 Moreover, wide variations in sensitivities with these nonculture tests are reported in the literature. The Centers for Disease Control and Prevention developed new guidelines for C. trachomatis laboratory tests, recommending additional testing after a positive C. trachomatis screening test by nonnucleic acid amplification tests.4 At the time of this writing, however, confirming a positive C. trachomatis result obtained by nucleic acid amplification has only received limited evaluation.
Serodlagnosis. Serologic testing has limited value for diagnosis of urogenital infections in adults. Most adults with chlamydial infection have had a previous exposure to C. trachomatis and are therefore seropositive. Serology can be used to diagnose LGV. Antibodies to a genus-specific antigen can be detected by complement fixation, and a single-point titer greater than 1:64 is indicative of LGV. This test is not useful in diagnosing trachoma, inclusion conjunctivitis, or neonatal infections. The microimmunofluorescence assay (micro-IF), a tedious and difficult test, is used for type-specific antibodies of C. trachomatis and can also be used to diagnose LGV. A high titer of IgM (1:32) suggests a recent infection; however, not all patients produce IgM. In contrast to CF, micro-IF may be used to diagnose trachoma and inclusion conjunctivitis using acute and convalescent phase sera. Detection of C. trachomatisspedfic IgM is useful in the diagnosis of neonatal infections. Negative serology can reliably exclude chlamydial infection.
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