Diagnostic implications of 16S ribosomal assay for gonorrhoea

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.

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.

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.

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.