Enhancing the specificity of the COBAS AMPLICOR CT/NG test for Neisseria gonorrhoeae by retesting specimens with equivocal results

The COBAS AMPLICOR CT/NG test for Neisseria gonorrhoeae cross-reacts with c ertain strains of nonpathogenic Neisseria species. In some strains, the tar get sequence is identical to that of N. gonorrhoeae, whereas other strains have a small number of mismatches within the regions recognized by the prim ers or probe used in the COBAS AMPLICOR NG test. These cross-reactive strai ns are occasionally present in urogenital specimens, causing false-positive results in the COBAS AMPLICOR NG test. Analysis of the data generated in a large multicenter clinical trial showed that 2.9% of the specimens gave si gnals between A(660)s of 0.2 and 3.5 but that one-half of these equivocal s pecimens did not contain N. gonorrhoeae. Most of these equivocal specimens were correctly classified as true positive or true negative by retesting in duplicate and defining a PCR-positive result as two of three results with an A(660) of greater than or equal to2.0. If specimens had been classified as positive or negative based on a single test result using a cutoff of an A(660) of 0.2, specificity would have ranged from 96.2 to 98.9% depending o n specimen type, sex, and presence of symptoms. By employing the equivocal zone-retesting algorithm, specificity increased to 98.6 to 99.9% with littl e effect (0.1 to 4.9% decrease) on sensitivity in most specimen types, enab ling the test to achieve a positive predictive value of at least 90% in pop ulations with a prevalence of 4% or higher. In lower-prevalence populations , the test could be used to screen for presumptive infections that would ha ve to be confirmed by an independent test.