Microbiological diagnosis of gonorrhoea.

Gonorrhoea is caused by Neisseria gonorrhoeae. This is a delicate and fastidious organism which dies rapidly if exposed to desiccating or oxidising conditions and requires a moist carbon dioxide enriched atmosphere and a nutrient medium if it is to be cultured successfully. For these reasons diagnosis by culture was regarded as difficult and uncertain, but many decades ago the problems were overcome, and culture became the method of first choice for diagnosis, and remains the "gold standard" against which others are measured.' However, culture is by no means the only method available for diagnosis of gonorrhoea, and others offer alternative advantages such as speed, robustness, or technical simplicity, and no single method is appropriate in all situations. In the typical UK microbiology laboratory specimens to be examined for gonococci will usually have been taken from patients in whom there is a significant likelihood of infection, and, on the results obtained, therapy is likely to be administered. Prevalences will vary, but are likely to be highest in patients attending genitourinary medicine clinics and somewhat lower in other patient groups. However, specimens may also be received as part of continuous or intermittent monitoring of particular population groups such as antenatal clinic patients. Here the aim, while including the treatment of any infected patients identified, is primarily to establish background knowledge of the prevalence of infection in that community. Tests employed in these differing situations will need to meet different performance criteria. Where a clinician is seeking to identify infection in an individual patient the sensitivity of the test (that is, the likelihood of a genuine infection being detected by the test) is of paramount importance, whereas the risk of encountering a false positive result-which relates to the specificity of the test, will be of lesser importance.2 However, in a situation where infection rates are low the problems of encountering false positive results will become more significant, so that the specificity of the test system employed will take on increasing importance and some sensitivity may have to be sacrificed. Moreover, in any situation the likelihood of any test result, either positive or negative, being accurate (the so called positive and negative predictive values) will depend on the number of genuine cases present in the population tested, as well as on the sensitivity and specificity of the test.2 Thus, when numbers are low specificity becomes increasingly important, whereas in a population with a high prevalence of infection, optimal sensitivity should be sought. Elsewhere in the world the availability of laboratory facilities, and the technical expertise available in these, can be very different from those normally found in the UK, and the prevalence of infection may well be far higher. All these variables will affect the choice of the optimal test to employ-as will financial considerations and the expectations of the population involved. Problems of maintaining viability of the organism until cultured are unlikely to present problems in the usual UK clinic situation, whereas in field exercises or in rural surveys elsewhere in the world these may be of cardinal importance.