- Author: Hugh Ringland Taylor, AC, MD, MBBS, BMedSc(Melb), DO(Melb), FRANZCO, FRACS, FAAO, FACS, FAICD; Chief Editor: Hampton Roy, Sr, MD more...
In endemic areas, the diagnosis is almost always based on the clinical appearance.
Laboratory assays are principally used in research.
To confirm that the clinical diagnosis of active trachoma is the result of ocular C trachomatis infection, the best laboratory techniques are the nucleic acid amplification tests (NAATs), of which the polymerase chain reaction (PCR) is one example.
The NAATs have high sensitivity and specificity but are too expensive and too technically complex to be widely available in most settings where trachoma is endemic.
On any assay, apparent false-positive and false-negative results (compared with the clinical signs of active trachoma) may be related to the natural history of infection and disease. Individuals become infected several weeks before specific clinical signs appear, and evidence of conjunctival inflammation persists for weeks to months after the infection resolves.
Other useful techniques are direct fluorescein-labeled monoclonal antibody (direct fluorescent antibody [DFA]) assay and enzyme immunoassay (EIA) of conjunctival smears. These tests are less sensitive than the NAATs. However, to determine whether antibiotics are needed by a community, the prevalence of infection may be an important parameter, so a test with high specificity may be useful even if it has lower sensitivity.
Newer diagnostic methods have superseded cell culture, which was the criterion standard for laboratory diagnosis. Cell culture requires a highly specialized laboratory and is expensive and technically demanding. Cell culture has virtually 100% specificity but only moderate sensitivity.
Giemsa cytology is microscopic examination of stained conjunctival scrapings for intracytoplasmic inclusions. Giemsa cytology is technically demanding. This test has high specificity but low sensitivity.
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