Asymptomatic infection with Chlamydia trachomatis (CT) and Neisseria gonorrhoeae (NG) is common among men who have sex with men (MSM) living in Asia and the Pacific.^1–4

We recently described CT and NG infections and associated factors among MSM in Bangkok, Thailand, and found that more than 80% of the CT and NG infections occurred in asymptomatic Thai MSM.⁴

Undiagnosed and untreated CT and NG may increase the risk of HIV transmission and other STI.^5,6 The U.S. Centers for Disease Control and Prevention (CDC) recommends that all sexually active MSM have (1) a test for urethral infection with NG and CT if there is a history of insertive intercourse in the preceding year and (2) a test for rectal infection with NG and CT if there is a history of receptive anal intercourse during the preceding year. In both sites, nucleic acid amplification test (NAAT) is the preferred approach.⁷ Thai national guidelines recommend that high risk (i.e. sexually active) MSM undergo testing for NG infection at the urethra, and at the rectum if they report any receptive anal sex in the past six months, and at the pharynx if they report any receptive oral sex in past six months (Gram stain and/or culture).⁸ Thai national guidelines contain no recommendation on screening for CT infection.

NAATs have been demonstrated to have improved sensitivity and specificity compared with culture for the detection of CT and NG infection.^9,10 The cost of NAAT testing for CT and NG infections is relatively high and these tests are not routinely available in most settings in Thailand.² An analysis including the number of infections missed when testing only one anatomic site, and NNS, the number needed to screen to detect one infection, is helpful to evaluate screening strategies and to support screening recommendations.^2,11

In this study, we assess the value of screening for CT and NG infection at the rectum, urethra, and pharynx (for CT only) using NAAT by addressing (1) the overall positivity rates of CT and NG infection; (2) factors associated with these infections at each site; (3) the NNS to detect one infection at each anatomic site (overall, and stratified by risk associated with CT and NG infection); and (4) the proportion of cases that would be missed if screening was not performed by each anatomic site.

We enrolled MSM into the Bangkok MSM Cohort Study (BMCS) at the Silom Community Clinic, located in central Bangkok, Thailand. BMCS was a 60-month longitudinal cohort study with four-monthly follow-up interval that aims to assess the prevalence and incidence of HIV infection, prevalence of STI, and risk factors for HIV infection. We conducted enrollment from April 2006 to January 2008 (Period 1) and September 2009 to November 2010 (Period 2). Inclusion criteria were as follows: Thai nationality, male sex at birth, age at least 18 years, residence in Bangkok or neighboring provinces, self-reported history of oral or anal sex with another man in the six months preceding study entry, ability and willingness to follow-up at four-month intervals for at least 36 months, and ability to provide written informed consent. The methods of this study have been previously described.^4,12,13

At enrollment, participants answered an audio computer-assisted self-interview (ACASI) to report their behaviors during the previous four months and underwent physical examination. Nurse collected rectal swabs and first-void urine specimens for CT and NG testing. Rectal swabs (Amplicor® STD Swab Specimen Collection and Transport set, Roche Diagnostics, Branchburg, NJ, USA) and urine were tested for CT and NG by NAAT (Roche Amplicor®, Roche Diagnostics, Branchburg, NJ, USA). Pharyngeal swabs were tested for CT by NAAT (Roche Amplicor®, Roche Diagnostics, Branchburg, NJ, USA). Initially, pharyngeal swabs were tested for NG by NAAT (Roche Amplicor®, Roche Diagnostics, Branchburg, NJ, USA) and also cultured for NG infection. Of the 623 specimens tested for NG at the pharynx by NAAT, 205/623 (32.9%) were positive and one (0.2%) was positive by culture. Subsequently, testing for NG at the pharynx was discontinued; however, culture for NG was continued.

Participants were tested for HIV infection using oral fluid by OraQuick® (OraSure Technologies Inc., Bethlehem, PA USA). If reactive, three other HIV rapid tests were performed on blood: (1) Determine™ HIV 1&2, Abbott, Japan; (2) DoubleCheck™ II HIV 1&2 Organics Ltd, Israel; and (3) Capillus™ HIV-1/HIV-2, Trinity Biotech, Jamestown, NY, USA, replaced after November 2008 by Core™ HIV-1/2, UK. HIV infection was confirmed if all three tests were reactive,⁴ and the result was provided to the participant during posttest counseling. If rapid tests were inconclusive, anti-HIV testing (both rapid tests and enzyme immunoassay) was performed on a second sample taken after 14 days from the initial blood draw. Treponema pallidum (TP) screening was performed using the rapid plasma reagin (RPR) assay (Macro-Vue™ RPR 18mm Circle Card Test, Becton Dickinson Microbiology Systems, Sparks, MD, USA). Specimens that were reactive at any titer were tested using a TP-specific antibody test (Determine™ Syphilis TP, Abbott Laboratories, Tokyo, Japan) to confirm a positive result. Prevalence of TP seropositivity in this cohort has been previously described.¹⁴ After the enrollment visit, participants returned for a follow-up visit every four months to assess HIV acquisition, during which they completed an ACASI, and underwent HIV counseling and testing. We referred all participants who tested positive for HIV for care and treatment, and treated all those diagnosed with a CT, NG, or syphilis infection on site, according to Thai national guidelines (NG: ceftriaxone 250 mg IM single dose and azithromycin 250 mg four capsules orally single dose, CT: azithromycin 250 mg four capsules orally single dose dual, syphilis: benzathine penicillin G 2.4 MU IM). At each visit, participants received 500 Thai baht (about US $16) as compensation for time and transportation costs.

In this analysis, we excluded participants who reported any symptoms of CT and NG infection (i.e. penile or anogenital pain, urethral discharge). We described asymptomatic CT and NG infection by site at the baseline visit. We used frequencies and proportions to describe baseline sociodemographic characteristics and CT and NG infections at each anatomic site using the exact binomial distribution to calculate 95% confidence intervals (CI). Because of the uncertainty regarding test specificity due to the presence of oropharyngeal commensal organisms including Neisseria subflava and Neisseria cinerae that may cross react with NG on NAAT,^15,16 we did not include pharyngeal NG test results obtained by NAAT in our results. Factors associated with site-specific CT and NG infection were separately evaluated using logistic regression, and those with a p ≤ 0.10 in bivariate analysis were included in a multivariable analysis. Likelihood ratio tests were used to determine factors to include in the final model (two-sided p<0.05). To determine the NNS to detect infection at relevant anatomic sites, we calculated the inverse of the proportion of asymptomatic MSM with unrecognized infection at the given site detected by NAAT, stratified by age, sexual role, and HIV status. We classified age into two groups using the standard WHO definition of youth.¹⁷ CIs for NNS were determined using the exact binomial method. We calculated the proportions of CT and NG infection that would have been missed by the following screening practices: urethral only, rectal only, and pharyngeal only (for CT only) for men who had all tests performed. We performed all analyses using STATA® (Version 12, 2011; Stata Corp., College Station, TX, USA).

The Ethical Review Committee for Research in Human Subjects of the Thailand Ministry of Public Health and the Institutional Review Board of the CDC reviewed and approved the study protocol. All participants were informed about the voluntary nature of participation. Written informed consent was obtained from all participants.

In this study, among MSM in Bangkok, Thailand, more than one in ten had asymptomatic CT infection, and about one in 20 had asymptomatic NG infection; asymptomatic infection was most commonly detected at the rectum. Risk factors associated with CT and NG infection were young age (18–24 years), HIV-1 infection, and insertive-only anal intercourse without a condom (for urethral CT infection only). With rectal and urethral specimens, the NNS to detect CT infection was significantly lower than the NNS to detect NG infection.

The overall positivity rate of CT infection among HIV-infected MSM reported in our study was higher than a previous study conducted in HIV-infected MSM in Bangkok that failed to include rectal sampling for the detection of CT infection² (this study: 22.6%, 95% CI: 18.3–27.3; and Sirivongrangson et al.: 10%, 95% CI: 5.9–16.4%). The NG positivity rates among HIV-infected MSM were comparable with the study by Sirivongrangson et al., because rectal screening was also included for the detection of NG infection (Sirivongrangson et al.: 13.0%, 95% CI: 8.2–19.6%; and this study: 12.4%, 95% CI: 9.0–16.5).² We found that the urethral CT positivity rate in our study was similar to the report from MSM attending a men’s sexual health clinic in Australia (this study 4.4%, 95% CI: 3.5–5.5; and Cornelisse et al.: 2.2%, 95% CI: 1.3–3.5).¹⁸However, the rectal CT positivity rate reported here was higher than the report from Cornelisse et al. (this study: 9.2%, 95% CI: 7.9–10.8; and Cornelisse et al.: 4.3%, 95% CI: 3.0–5.9).¹⁸ Australian MSM reported a higher rate of condom use for receptive anal intercourse with casual sexual partners than has been reported by Thai MSM (Cornelisse et al.: 82%, 95% CI: 78.8–83.1; and this study 52%, 95% CI: 49.5–54.6). The prevalence of rectal CT infection and pharyngeal CT infection reported in this study was similar to the literature review from Chan et al.¹⁹

We sought to describe the relative value of screening different anatomic sites using a NNS calculation. NNS can be used as a metric to use in models to assess potential or existing screening programs.¹² In our study, NNS values varied dramatically depending on the subpopulations examined (Tables 5 and 6), and these observations may be helpful in supporting STI screening programs in settings comparable to the one studied here. In particular, our data suggest that at least annual screening of rectal sites by NAAT, especially in asymptomatic HIV-infected MSM, may result in higher positivity rates of both CT and NG infection than screening solely at the urethral site. The overall NNS to detect CT and NG infections reported in this study was much lower than the NNS reported from the HIV-infected women attending hospitals in Thailand.¹² However, the NNS to detect CT infection among HIV-infected pregnant women was comparable to the NNS from the rectal site among HIV-infected MSM (HIV-infected women: 3, 95% CI: 4–24; HIV-infected MSM: 6, 95% CI: 5–8). HIV-infected pregnant women and HIV-infected MSM may be the subgroups that benefit most from CT screening using NAAT.

In Thailand, MSM with HIV or STI symptoms typically access STI clinics, anonymous Voluntary Counseling and Testing clinics, or general outpatient clinics for STI treatment, and HIV counseling and testing. Unpublished data from a survey at Silom Community Clinic Thailand, showed that 24% (46/191) of MSM visited the clinic because they had symptoms of an STI, and only 15% (29/191) of MSM visited the clinic for asymptomatic STI screening (p=0.03).²⁰ Based on our study, education about asymptomatic CT and NG infections and routine CT and NG screening regardless of symptoms are necessary for MSM in this setting, as well as potential other settings serving MSM.

Screening for CT and NG infections could be used as an entry point for inclusion in prevention interventions such as HIV pre-exposure prophylaxis (PrEP). Detection of CT and NG infection at the rectum could also be used as an opportunity to initiate sensitive discussions about HIV risk reduction, as many providers may not otherwise discuss sexual risk at each visit,^21,22 and self-reported sexual behavior, even when discussed, is often an inaccurate measure of true behaviors.²¹

Several limitations should be considered when interpreting these results. First, behaviors were self-reported and might not accurately reflect true behavioral risk. The definition of symptomatic infection used in the analysis included urethral pain but not specifically dysuria and may have failed to capture some cases of urethral infection. Because of uncertainty regarding test specificity, the analyses presented here did not include NAAT for pharyngeal NG infection.^15,16,23 We did not perform tissue culture for CT. In addition, the results presented here reflect the population of MSM participating in our cohort study and may not represent all MSM in Bangkok, Thailand or elsewhere. Lastly, the usefulness of the results presented here depends on the underlying prevalence of infection with each pathogen, which may vary over time.

Based on our findings, in settings serving MSM where screening for all specimens for CT and NG infection using NAAT is not feasible, rectal screening is important as including this specimen for testing may mean fewer cases of missed CT or NG infection. This study demonstrates the value of routine screening and especially the importance of rectal screening in any CT and NG screening program for sexually active MSM in Bangkok, Thailand.