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Bacterial STIs
P76 Equivocal Aptima Combo 2 results: what do they mean in clinical practice?
  1. S Cassidy,
  2. R Artykov,
  3. J White
  1. Guy's and St Thomas' NHS Trust, London, UK

Abstract

Background Molecular diagnostic tests have become standard of care for detection of gonococcal (GC) and chlamydial (CT) infections. The Aptima Combo 2 (AC2) test is widely used and is highly sensitive and specific, even for non-genital specimens. Equivocal results occur when the initial AC2 assay detects target RNA but the confirmatory Aptima GC or CT assay fails to detect a different RNA sequence in the same specimen.

Aim To determine whether equivocal AC2 (EAC2) results were predictive of subsequently confirmed infection in our GUM/HIV clinic population.

Methods Retrospective review of all EAC2 results for GC or CT at 3 urban UK GUM/HIV clinics from January to December 2011. Patients with EAC2 results were routinely recalled for repeat testing unless treated at the initial visit.

Results From a total of approximately 38 000 AC2 tests performed in 2011, 3118 (8.2%) were confirmed positive: 1189 GC and 1929 CT infections. There were 222 EAC2 results in 2011 (0.6% of total AC2 tests); 45 (20.3%) occurred in women, the majority (73%) of which were equivocal genital CT results. Of the 177 EAC2 results in men (mostly MSM), 70% were non-genital specimens. Equivocal pharyngeal GC was common, comprising one-third of all male EAC2. Of 34 EAC2 patients analysed in more detail, 5 were GC/CT contacts and 24/34 reported unprotected sex at the site of the EAC2. None with equivocal GC had GC positive culture results, at the time or subsequently. Of 19 men with EAC2 GC results, 6 (all MSM) had confirmed GC at another mucosal site at that visit. 30/34 patients had the AC2 test repeated (range 7–24 days after initial test); 29 were AC2-negative and one remained equivocal (see abstract P76 table 1).

Abstract P76 Table 1

Rates of equivocal GC and CT results according to gender and site

Conclusions EAC2 results are uncommon but seem to occur in those at higher risk for infection; yet the vast majority does not have infection confirmed on subsequent testing. This suggests that these are spurious results, possibly from contamination, or low organism load infections that do not persist; thus routine treatment is not necessarily warranted.

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