Successful Combination of Nucleic Acid Amplification Test Diagnostics and Targeted Deferred Neisseria gonorrhoeae Culture

Abstract

Nucleic acid amplification tests (NAATs) are recommended for the diagnosis of N. gonorrhoeae infections because of their superior sensitivity. Increasing NAAT use causes a decline in crucial antimicrobial resistance (AMR) surveillance data, which rely on culture. We analyzed the suitability of the ESwab system for NAAT diagnostics and deferred targeted N. gonorrhoeae culture to allow selective and efficient culture based on NAAT results. We included patients visiting the STI Clinic Amsterdam, The Netherlands, in 2013. Patient characteristics and urogenital and rectal samples for direct N. gonorrhoeae culture, standard NAAT, and ESwab were collected. Standard NAAT and NAAT on ESwab samples were performed using the Aptima Combo 2 assay for N. gonorrhoeae and C. trachomatis. Two deferred N. gonorrhoeae cultures were performed on NAAT-positive ESwab samples after storage at 4°C for 1 to 3 days. We included 2,452 samples from 1,893 patients. In the standard NAAT, 107 samples were N. gonorrhoeae positive and 284 were C. trachomatis positive. The sensitivities of NAAT on ESwab samples were 83% (95% confidence interval [CI], 75 to 90%) and 87% (95% CI, 82 to 90%), respectively. ESwab samples were available for 98 of the gonorrhea-positive samples. Of these, 82% were positive in direct culture and 69% and 56% were positive in the 1st and 2nd deferred cultures, respectively (median storage times, 27 and 48 h, respectively). Deferred culture was more often successful in urogenital samples or when the patient had symptoms at the sampling site. Deferred N. gonorrhoeae culture of stored ESwab samples is feasible and enables AMR surveillance. To limit the loss in NAAT sensitivity, we recommend obtaining separate samples for NAAT and deferred culture.

FIG 1

Flowchart of included patients, samples, and performed laboratory tests. All NAAT results were tested using the Aptima Combo 2 assay. Samples with equivocal results were retested using the Aptima GC single or CT single assay. NAAT on ESwab samples was performed by transferring ESwab liquid medium to an Aptima test tube. The 1st targeted deferred culture from ESwab was inoculated on the day the standard NAAT result became available. The 2nd targeted deferred culture was inoculated 1 day later but from the same ESwab tube as the 1st deferred culture. NAAT, nucleic acid amplification test; Ct, C. trachomatis; Ng, N. gonorrhoeae.

FIG 2

Results of targeted deferred cultures positive for N. gonorrhoeae according to calendar days since collection. Percentage of targeted deferred cultures positive for N. gonorrhoeae from urogenital samples (n = 48) and rectal samples (n = 50) compared to the gold standard (standard nucleic acid amplification test, at 100%) with the 95% CI. Only samples for which an ESwab was available were included in this analysis. Targeted deferred cultures were inoculated at ≤34 h (day 1), 35 to 58 h (day 2), and >58 h after collection (day 3).