Prevalence of new variants of Chlamydia trachomatis escaping detection by the Aptima Combo 2 assay, England, June to August 2019

At the end of April 2019, Public Health England (PHE) was alerted, via an international Epidemic Intelligence System-Sexually Transmitted Infections (EPIS-STI) post from Finland, of false-negative Chlamydia trachomatis (CT) test results using the Aptima Combo 2 (AC2) assay (Hologic Inc., San Diego, California, United States (US)), a nucleic acid amplification test (NAAT) for CT (target: 23S rRNA) and Neisseria gonorrhoeae (GC) (target: 16S rRNA). Discrepant results between the AC2 assay and the Aptima Chlamydia trachomatis assay (ACT) (target: 16S rRNA) were reported to have occurred primarily in specimens that had AC2 relative light units (RLU) from 20 to 84 [1]. These false-negative AC2 results [2,3] were attributed to a C1515T mutation in the CT 23S rRNA gene. In early June 2019, Hologic Inc. issued a Field Safety Notice (FSN) to AC2-using laboratories, recommending ACT reflex retesting of AC2 CT-negative with RLU ≥ 15, CT-equivocal, or GC-equivocal/-positive (if CT-negative/equivocal) specimens to ensure detection of the Finnish new variant CT strain (F-nvCT) [3]. A European Centre for Disease Prevention and Control (ECDC) rapid risk assessment recommended countries estimate the spread of F-nvCT to inform the need for patient recall and retesting [4]. Here we report results from an investigation coordinated by a multiagency incident management team (IMT) to ascertain the prevalence of new variants of Chlamydia trachomatis escaping detection by the Aptima Combo 2 assay in England.