The widespread availability of highly sensitive nucleic acid amplification tests (NAATs) allows convenient and noninvasive screening for Chlamydia trachomatis. NAAT manufacturer instructions specify a minimum voiding interval of 1 or 2 hours before submitting urine for testing (Amplicor, Roche; Probetec, Becton and Dickinson Company; product information). However, these recommendations are not based on any real evidence and may constitute an additional barrier to screening uptake in men.1 Previous studies in predominantly symptomatic men have found no difference in NAAT sensitivity with voiding intervals less than the recommended minimum.2–4
The aim of this study was to estimate the sensitivity of 20-minute voiding intervals in men without symptomatic urethritis using the standard 1-hour post void as a gold standard.
MATERIALS AND METHODS
Men recalled to Sydney Sexual Health Centre during the period from July 15, 2009 to February 1, 2011 for treatment of laboratory-confirmed urethral C. trachomatis infection were invited to participate in the study if they attended at a time when a study nurse was on-site. Men who were diagnosed with nongonococcal urethritis (>5 polymorphonuclear leukocytes/high-power field in at least 3 fields) or with obvious clinical signs of urethritis and treated at the test visit were excluded from the study. Urethral Gram stain was only performed in the presence of urethral symptoms or signs.
A short questionnaire was used at the recall visit to determine whether participants had held their urine for at least 1 hour, had taken any antibiotics, or developed any minor urethral symptoms since their initial positive C. trachomatis test. Two first void urine samples were collected, the first at more than 1 hour since last void, and the second 20 minutes later. An electronic timer was used to alert men when 20 minutes had elapsed. Men were excluded if they had urinated in the last hour, had received chlamydia treatment, were unable to provide timely and adequate samples, or unable to read written instructions in English. All urine samples were stored and transported at 4°C. All samples reached the laboratory within 4 hours and were tested within 48 hours using the Roche COBAS Amplicor CT/NG PCR assay (Roche Molecular Diagnostics, Basel, Switzerland).
A power calculation was performed using OpenEpi version 2.3; a sample size of 30 would be required, with 80% likelihood, to detect a 10% difference in the proportion testing positive, assuming a sensitivity of 90% for the 20-minute post void sample. This interval was considered to fall within the reported sensitivity range of the assay.5 The sensitivity of 20-minute voiding intervals was assessed using the 1-hour post void sample as a gold standard, and 95% confidence intervals calculated using binomial distribution.
Informed consent was obtained from all participants, and the study was approved by the Human Research Ethics Committee of South Eastern Sydney Illawarra Area Health Service.
Over the study period, 366 men were diagnosed with urethral C. trachomatis at Sydney Sexual Health Centre. Of these, 113 were eligible men with no evidence of symptomatic urethritis and had not received any chlamydia treatment, and returned during the study period. Forty-seven (41.5%) men were invited to participate. A total of 37 (37 of 113 eligible, 32.7%) men consented to participate in the study. At the study visit, five participants tested negative for C. trachomatis on both specimens, and one was unable to produce a 20-minute sample. Subsequently, 31 pairs of first void urine samples were tested and included in data analysis.
The median age of participants included in the analysis was 26 years. Thirty-one men had positive initial samples. Of these 31, 29 (93.5%) were also positive at 20 minutes (95% CI: 78.6%–99.2%), whereas 2 men were negative. Of the 5 men who tested negative on the 1-hour sample, 2 had received antibiotics, 1 amoxicillin, and 1 ceftriaxone.
Previous studies have reported that NAAT test sensitivity for C. trachomatis is not affected by voiding intervals below the recommended minimum.2–4 Using the APTIMA Combo 2 assay, Matthew et al.4 demonstrated that samples obtained at least 20 minutes after recent urination were equally sensitive to standard 1-hour intervals. However, their sample consisted mostly of symptomatic men, whereas the current study recruited men without clinical or microbiologic evidence of urethritis. Asymptomatic men have a demonstrably lower organism load,6 and therefore a greater likelihood for reductions in test sensitivity with shorter voiding intervals. Also, comparisons of the Amplicor CT/NG assay and the APTIMA Combo 2 have shown the latter to be more sensitive in the detection of C. trachomatis.7 This may explain why in our study, 20-minute voiding intervals were associated with a high, although slightly reduced, sensitivity relative to standard intervals.
A limitation of the current study was the low number of eligible men recruited. The recruitment rate was fairly low (47 of 113; 41.5%), as this depended on the availability of a study nurse at the time eligible participants reattended. None of the men reported symptoms, nor were any symptoms documented in their medical records; however, the study questionnaire elicited minor urethral symptoms in ten of the men.
One screening study based in a family planning clinic found the most common reason for exclusion from chlamydia screening was having voided in the last hour, with as many as 14% of the survey population excluded for this reason.1 Using the reported prevalence of urethral chlamydia of 4.5% in heterosexual men and 8.7% in men who have sex with men,8,9 the negative predictive value of 20-minute samples can be calculated to exceed 99%. As the aim of an opportunistic screening program is the early detection of cases in the population, not total exclusion of infection in those tested, our findings suggest patients should not be excluded from opportunistic testing if they have recently voided. The greater uptake in opportunistic testing achieved by removing the 1 to 2 hour minimum voiding interval is likely to offset the small loss in sensitivity that may occur with shorter voiding intervals.
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