Wang et al. BMC Public Health 2011, 11:955 http://www.biomedcentral.com/1471-2458/11/955 was provided to participants according to their sero- status. Study measures Structured questionnaire-based interviews provided socio-demographic characteristics, sexual health services accepted over the past year, and sexual behavior informa- tion. The questionnaires were confidential, but not anon- ymous, and subsequent diagnostic data was linked to the behavioral questionnaire. Socio-demographic variables of interest included age, ethnicity, educational level, marital status, residency, and living arrangement status. Sexual behavioral information mainly included sexual orientation, frequency of condom use with regular sex partner, whether condoms were used with regular sex partner(s) in the last sexual encounter, presence of recent encounters with casual sexual partner(s), and/or with FSW (and condom usage accordingly). Drug use status was also evaluated. Laboratory testing After completing the questionnaire, the participants were provided with information about HIV and other STD testing, and were requested to undergo testing for syphi- lis, HIV, Chlamydia trachomatis, and Neisseria gonor- rhoeae. Blood was drawn for syphilis testing in local hospitals. The study team provided identical testing reagents to local hospitals. The toluidine red unheated serum test (TRUST, Rongsheng Biotech Inc, Shanghai, China) was used for syphilis screening and quantitative analysis. Treponema pallidum particle agglutination test (TPPA; Fujirebio Inc., Tokyo, Japan) or Treponema palli- dum -enzyme-linked immunoassay (TP-ELISA, Wan- sheng Biotech Inc, Beijing, China) was used for syphilis test confirmation. HIV, Chlamydia trachomatis, and Neisseria gonorrhoeae testing were performed in the National Center for STD Control, China CDC. For HIV, a screening test algorithm using ELISA (Wansheng Bio- tech Inc, Beijing, China) and a confirmatory test using the Western blot assay (HIV Blot 2.2, Genelabs Diagnos- tics, Singapore) were used. First void urine specimens were tested for Chlamydia trachomatis and Neisseria gonorrhoeae by means of polymerase chain reaction (PCR) according to the manufacturer’s instructions (Amplicor Chlamydia trachomatis/Neisseria gonorrhoeae Test, Roche Diagnostic Corp, USA). This study was approved by the Ethics Committee of the National Center for STD Control, China CDC. Statistical analysis Data were recorded using Microsoft Excel™ (Edition 2003, Microsoft Corporation, Redmond, WA, USA) and validated through double entry. Univariate and multi- variable logistic regression analyses were performed Page 3 of 8 using Statistical Package for the Social Sciences (SPSS15.01™, SPSS Inc., Chicago, IL, USA) to test for evidence of an association between the categorical vari- ables. Odds ratios (ORs) and their corresponding 95% confidence intervals (CI) were produced to present the risk factors for different behaviors. Multiple regression analyses were performed to adjust ORs for potential confounders. Only variables that were significant in uni- variate analyses at P < 0.1 were included in multivariable logistic regression models for selecting the independent risk factors. Results Proportion of testing and STD/HIV prevalence During the period from July 2009 and September 2009, 2951(91.0%) of 3243 eligible attendees agreed to undergo HIV and syphilis testing, among which 2041 (69.2%) attendees also provided first void urine for C. trachomatis and N. gonorrhoeae testing. Prevalence of syphilis, N. gonorrhoeae, C. trachomatis, and HIV was 10.7%, 4.3%, 6.9% and 0.7% respectively. Among the 21 HIV-positive patients, 19%(4/21) were syphilis sero-posi- tive, 7.1% (1/14) were N. gonorrhoeae positive and 14.3% (2/14) were C. trachomatis positive. STDs and HIV prevalence rate by different areas are shown in Table 1. Syphilis and N. gonorrhoeae rates were highest in Jiangsu Province. Study population and sexual behavioral factors Demographic characteristics of the study population were shown in Table 2. The age of study patients ranged from 18 to 88 years and the median age was 34.8 years. The most-common ethnic group was Han (96.5%), and 68.6% of participants were married. The majority was composed of participants with education levels equal to or less than middle school (56.0%). A total of 77.5% of participants resided locally to the investigating city. It was shown that 45.5% of patients reported never using condoms, 48.9% sometimes used condoms, and only 5.5% consistently used condom with their steady sex partners within the last year. 24.8% of participants used condoms at their last sexual encounter with their steady sex partners. 46.0% of participants acknowledged that they had visited female sex workers (FSWs) within the last 3 months. The proportion of condom use with FSWs at last sexual encounter was 31.8%. At same time, up to 25.7% acknowledged having a non-FSW casual sex part- ner(s) within the last 3 months, of which only 30.1% used condoms at their last sexual encounter. Twenty-six (0.9%) patients were homosexual/bisexual (Table 3). 3.9% (116/2941) of participants reported drug abuse in the last year. Fifteen patients reported injection drug use (IDU) in last 6 months, of whom 3 patients reported sharing needles for drug injection.