A cluster of culture positive gonococcal infections but with false negative cppB gene based PCR

Objectives: To describe the prevalence and characteristics of isolates of Neisseria gonorrhoeae grown from urine samples that produced negative results with nucleic acid amplification assays (NAA) targeting the cppB gene. Methods: An initial cluster of culture positive, but cppB gene based NAA negative, gonococcal infections was recognised. Urine samples and suspensions of gonococci isolated over 9 months in the Northern Territory of Australia were examined using cppB gene based and other non-cppB gene based NAA. The gonococcal isolates were phenotyped by determining the auxotype/ serovar (A/S) class and genotyped by pulsed field gel electrophoresis (PFGE). Results: 14 (9.8%) of 143 gonococci isolated were of A/S class Pro(-)/Brpyut, indistinguishable on PFGE and negative in cppB gene based, but not other, NAA. Conclusions: This cluster represents a temporal and geographic expansion of a gonococcal subtype lacking the cppB gene with consequent loss of sensitivity of NAA dependent on amplification of this target. Gonococci lacking the cppB gene have in the past been more commonly associated with the PAU(-)/PCU- auxotype, a gonococcal subtype hitherto infrequently encountered in Australia. NAA based on the cppB gene as a target may produce false positive as well as false negative NAA. This suggests that unless there is continuing comparison with culture to show their utility, cppB gene based NAA should be regarded as suboptimal for use either as a diagnostic or supplemental assay for diagnosis of gonorrhoea, and NAA with alternative amplification targets should be substituted.