Abstract
Sirs: We read with interest the findings of Killick et al. 1 and agree that Neisseria gonorrhoeae (GC) nucleic acid amplification testing (NAAT) can provide a valuable addition to culture for identifying GC in low prevalence settings. Following the introduction of GC NAAT into our integrated family planning (FP) and genitourinary (GU) medicine service, we evaluated whether the use of NAAT increased the rates of GC screening and of GC detection. 2 Prior to the introduction of GC NAAT, prevalence of diagnosed GC within the clinic as assessed by culture alone was 0.87%. We sought to investigate concordance between NAAT and culture when both were carried out, within this low prevalence population.
Dual-testing for GC and chlamydia (CT) using Aptima Combo 2 (AC2; Gen-Probe Inc., San Diego, CA, USA) was performed using self-taken vaginal swabs for asymptomatic women and urine for asymptomatic men. Those patients with clinical symptoms or signs, or contacts of GC underwent GC culture in addition to NAAT. When using AC2, all positive GC results were confirmed according to the manufacturer's instructions using the single analyte NAAT targeting a different RNA sequence: Aptima GC (Gen-Probe).
All patients attending the clinic and undergoing GC screening during a four-month period prior to, and following GC NAAT introduction were identified using laboratory databases; data were collected retrospectively. Follow-up attendances within an episode were excluded from analysis. Individuals diagnosed with GC were identified and case-notes were reviewed.
We found that in the four months prior to the introduction of NAAT, 2307 symptomatic and asymptomatic patients were screened using GC culture alone, of which 20 (0.87%) were positive.
Following the introduction of GC NAAT during a similar four-month period, a significant increase in screening was observed, with 3444 symptomatic and asymptomatic patients being screened with AC2. Of these, 43 (1.25%) were confirmed positive for GC. GC culture was also performed in 217 of the symptomatic individuals; of these 18 (8.3%) were culture positive for GC. GC culture and NAAT showed concordance of positivity in 20/43 (46.5%) of cases. Eleven patients were NAAT positive, culture negative; 10/11 of these individuals had symptoms, signs or were high risk for GC. One of 11 cultured Neisseria meningitidis although subsequent GC NAAT testing using a different platform was also positive at the Sexually Transmitted Bacteria Reference Laboratory in Colindale, making it unlikely this was a false-positive. Twelve patients who were AC2 positive had no culture performed. Similarly to the findings by Killick et al., all patients with positive GC cultures were also GC NAAT positive.
In conclusion, following the introduction of GC NAAT we observed increased rates of screening and a doubling in the number of GC diagnoses during a similar time period. While it is possible there was an actual rise in the local GC incidence, the increase in diagnoses might also be attributable to a more sensitive test, acceptability of self-screening and a less time-consuming approach for the clinicians. The use of GC NAAT identified more infections than if culture alone had been used. Despite concern over the positive predictive value of GC NAAT in low prevalence settings, of those patients with positive NAAT/negative cultures all had documented risk factors or symptoms of GC. We believe both these studies provide reassuring evidence for the use of GC NAAT in this setting, provided a confirmatory strategy is in place.