Objectives Effective surveillance of antimicrobial-resistant Neisseria gonorrhoeae (Ng) is crucial, but culturing is labourious and costly. Focusing culturing efforts on high-yield subpopulations can enhance resource utilisation without compromising data quality or care. This cross-sectional retrospective study aims to pinpoint a nucleic acid amplification test (NAAT) cycle threshold (Ct) value for effective Ng surveillance culturing. Methods Surveillance and laboratory data from 3042 sexual health clinic clients in the Netherlands (December 2018 to October 2023) were analysed to determine correlations between Ng culture positivity and NAAT Ct value, culture timing and anatomical location. Fisher’s exact χ² test assessed associations between culture recovery and time intervals between NAAT and culture collection. Receiver operator curves and Youden’s J statistic were applied to determine an optimal Ct value cut-off. NAAT was performed on 6346 swabs from urogenital (urethra; 1389/vagina; 482) and extragenital (oropharynx; 2306/rectum; 2169) sites using the cobas CT/NG assay on the 6800 platform (Roche Molecular Systems). Culture plates were inoculated on the initial test day for clients treated presumptively (symptoms or notified for Ng) or during treatment consultation after positive NAAT results. Results Mean Ct values differed for positive and negative cultures (negative: Ct 33.0 (IQR 24.2–41.9); positive: Ct 25.4 (IQR 20.0–30.3); p<0.001). Oropharyngeal samples had the lowest culture positivity rate (22.0%). Culture positivity particularly declined when NAAT to culture intervals exceeded 14 days. Only 0.8% (11/1389) of urethral culture samples were positive above Ct 30. Between Ct 34 and 35, overall culture positivity dropped from 23.0% to 13.9%. A Ct value cut-off at 34 would reduce basic culturing costs by 25% while missing only 4.2% (108/2603) of positive cultures. Conclusions Establishing an NAAT Ct value cut-off can reduce both labour and costs without compromising vital surveillance data. Assay-specific validation is recommended prior to broader application.
Osbak et al. (Wed,) studied this question.