Is confirmatory testing of Roche cobas 4800 CT/NG test Neisseria gonorrhoeae positive samples required? Comparison of the Roche cobas 4800 CT/NG test with an opa/pap duplex assay for the detection of N gonorrhoeae.

Objectives: Recently marketed nucleic acid amplification tests (NAAT) for the detection of Neisseria gonorrhoeae (NG) have improved specificity over previous generation assays. A study to assess the necessity for confirmation of Roche cobas 4800 NG positive samples was undertaken by the Public Health Wales Microbiology Molecular Diagnostic Unit in Cardiff.

Methods: Classical NG culture identification was compared to cobas 4800 (DR-9), opacity (opa) gene and porA pseudogene (pap) results. Confirmatory NAATs (opa/pap) were performed prospectively for 120 cobas 4800 NG positive urogenital and extragenital samples. Retrospective supplementary NAAT and sequence analysis of additional cobas 4800 NG positive extragenital samples was also carried out.

Results: Of the 188 classically identified clinical NG isolates, 184 were identified as NG in all 3 molecular targets. Two isolates were only detected by 2 molecular targets. A further 2 isolates were culture false-positives. Combining the results from prospective and retrospective testing, the sensitivity and negative predictive value for cobas 4800 NG detection for urogenital, rectal and oropharyngeal samples was 100%. Specificity for all sample types was greater than 99.7%. Positive predictive value was 96.0% and 96.4% for urogenital and rectal specimens, respectively, and 88.6% for oropharyngeal samples.

Conclusions: Molecular tests could be used for culture confirmation where available. Roche cobas 4800 Chlamydia trachomatis/Neisseria gonorrhoeae (CT/NG)CT/NG gonorrhoea diagnosis is superior to culture with urogenital and rectal positives not requiring confirmation. Roche cobas 4800 oropharyngeal NG detection findings warrant further prospective study of routine confirmatory testing accounting for its cost and clinical usefulness.