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Introduction: Laboratory diagnosis of chlamydial infections

Chlamydial infections are frequently asymptomatic. Confirmation of chlamydial infection usually depends on taking an appropriate clinical sample from the patient [see: sample collection] followed by the direct detection of the organism using a suitable laboratory-based diagnostic test. The demonstration of chlamydial antibodies in an individual is rarely diagnostic [see: serology]. For many years, the optimum method of confirming the presence of chlamydial infection was the growth of the infecting organism in cell culture and the demonstration of characteristic chlamydial inclusions. However, this method necessitated the availability of good transport and cold-storage facilities in order to maintain the viability of the organism prior to inoculation. Moreover growth and isolation of the organisms in cell culture was relatively tedious and difficult to quality control. Chlamydial cell culture facilities were available to relatively few clinical centres, other than those where there happened to be groups interested in chlamydial research, most notably in Seattle, San Francisco and London. It is now widely recognized that cell culture techniques were, at best, only 60 to 80% sensitive. Nevertheless in rare medico-legal cases, for example concerning sexual abuse in children, the unarguable confirmation of infection provided by a carefully processed positive chlamydial isolate is a distinct advantage. Cell culture can also be useful for the diagnosis of chlamydial infections at non genital sites, for which commercial tests are often not licensed [Stary 2000].

A key advance in the laboratory diagnosis of chlamydial infections has been the development of non-viability dependent tests which place less demands on specimen transport. The first of these tests were the chlamydial antigen detection tests, which relied either on the direct detection of chlamydial elementary bodies in clinical material using fluorochrome-labelled, chlamydial specific, monoclonal antibodies [see: direct immunofluorescence] or the capture and detection of chlamydial antigen in an extract of clinical material using enzyme immunoassay based procedures [see: enzyme immunoassay]. These methods are still appropriate in some settings and they remain in widespread use. However they are gradually being superseded by newer, methods based on the detection of chlamydial nucleic acid, either by direct hybridization or by  nucleic acid amplification.  The latter use a variety of amplification reactions, including the polymerase chain reaction (PCR), ligase chain reaction (LCR), strand displacement amplification or transcription mediated amplification. Nucleic acid-based methods generally offer superior sensitivity and specificity to the antigen detection tests, but at greater cost and a greater requirement for trained staff. However, depending on the prevalence of infection in the test population, costs may be reduced by combining different specimens [see: specimen pooling]. 

Generally speaking, in populations where the prevalence of chlamydial infection is low, say <5%, a test of high specificity and thus high positive predictive ability is required [see: prevalence, specificity and predictive value]. Fortunately a number of commercial tests are now available, based on chlamydial DNA or RNA, which offer excellent specificity and sensitivity, although there has been some debate as to how these tests should be assessed [see: discrepant analysis].

A second major advance has been the recognition that cervical or urethral specimens requiring invasive genital sampling are by no means essential for the laboratory diagnosis of chlamydial infection. Non invasive and or self collected samples which may be adequate include male and female urine, various vulval-introital samples or vaginal fluid expressed from a tampon [see: non-invasive sampling]. 

[MEW]

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Diagnostics index

Reference

Stary, A. (2000). Diagnosis of genital Chlamydia trachomatis infections. In: Proceedings of the Fourth Meeting of the European Society for Chlamydia Research (Saikku, P. ed.), pages 94-97, published Esculapio, Bologna.