Diagnostic implications of 16S ribosomal assay for gonorrhoea.

Objectives: In the absence of a single nucleic acid amplification test (NAAT) that is both highly specific and sensitive for gonorrhoea, many have put forward the 16S-based assay as a confirmatory test for Neisseria gonorrhoeae. This study was undertaken to evaluate the performance of PCR based on 16S ribosomal gene in comparison with a porA pseudogene-based assay.

Methods: The specificity of both the porA pseudogene-based PCR and 16S ribosomal gene PCR was checked against a panel of strains comprising of non N gonorrhoeae Neisseria sp (NgNS) and other gram-negative and gram-positive bacteria. The sensitivity studies were performed using different dilutions of N gonorrhoeae DNA. PCRs were also done on endocervical and urethral swab samples collected from a total of 100 female and 50 male patients presenting to sexually transmitted disease clinics, Dermatology OPD of AIIMS and Safdarjang Hospital, New Delhi, India, recruited as per inclusion criteria.

Results: PCR assay based on 16S ribosomal gene showed cross-reactivity with three of six strains of N sicca. The porA pseudogene-based PCR was highly specific. Analytical sensitivity of 16S-based ribosomal assay was more than that of porA pseudogene-based assay. In clinical samples, for female patients, sensitivity, specificity, positive predictive value and negative predictive value of 16S ribosomal assay was 100% (95% CI 51.7% to 100%), 91.5% (95% CI 83.4% to 96%), 42.9% (95% CI 18.8% to 70.4%) and 100% (95% CI 94.7% to 100%), respectively, while for the male patients it was 100% (95% CI 85% to 100%), 95.5% (95% CI 75.1% to 99.8%), 96.6% (95% CI 80.4% to 99.8%) and 100% (95% CI 80.8% to 100%), respectively.

Conclusions: The data presented in this report supports use of 16S ribosomal assay as a screening assay only. The porA pseudogene target is highly specific for N gonorrhoeae and may be used as a supplemental assay.