Background Nucleic acid amplification testing (NAAT) is the optimal method for detection of Chlamydia trachomatis (CT) and Neisseria gonorrhoeae (GC), but no commercial tests are cleared by the US Food and Drug Administration for use with rectal swabs. The objective of this study was to evaluate the Xpert® CT/NG assay using residual ProbeTec ET rectal samples.
Methods 150 samples previously tested using the ProbeTec ET System and the APTIMA COMBO 2® Assay for the detection of CT and GC were tested using the Xpert® CT/NG. The rectal swabs were collected from volunteers aged 18–64 years, who reported having had at least one episode of anal receptive intercourse. APTIMA COMBO 2® Assay was used as the gold-standard for the present analysis as it was found to be superior to ProbeTec.
Results From the 150 samples, ProbeTec ET detected 23 (15%) CT positive and 16 (11%) GC positive. Xpert® CT/NG detected an additional 11 CT and 2 GC which were not detected by the ProbeTec ET System, for a total of 34 (23%) positive CT and 18 (12%) positive GC. All samples that were true negatives for CT or GC by ProbeTec ET System were also negative by Xpert® CT/NG. The diagnostic sensitivity and specificity of the CT test was 58% and 100% for ProbeTec and 85% and 100% for Xpert® CT/NG, respectively, compared to the Aptima COMBO 2®. The diagnostic sensitivity and specificity of the GC test was 70% and 100% for ProbeTec and 78% and 100% for Xpert® CT/NG, respectively, compared to the APTIMA COMBO 2®.
Conclusions Xpert® CT/NG is superior to the ProbeTec ET System for the detection of CT and GC from rectal swabs. Further studies to assess the sensitivity and specificity of the Xpert® CT/NG system using the swab collection kits designed for this system are warranted.