Prevalence and associated factors of chlamydial infection among patients seeking clinic-based STI services in Shenzhen, China

Prevalence assessment is one of the core components of WHO-recommended STI surveillance programming. This study aimed to provide prevalence and associated factors data for estimating the disease burden of CT, developing intervention programmes, and planning for resource allocations. We carried out a cross-sectional survey among patients attending clinics for STI services. We collected background information from patients (aged 18-49) and determined positivity of CT by nucleic acid amplification test (NAAT) with self-collected urine specimens. Associated factors identified through logistic regression.

Chlamydia caused by Chlamydia trachomatis (CT) is a major sexually transmitted infection (STI) [1] which can cause significant morbidity, particularly in women. Because up to 70% of cases in women and 50% in men are asymptomatic [2], CT infection often remains undiagnosed and untreated. Untreated CT infection can lead to pelvic inflammatory disease (PID), chronic pelvic pain, ectopic pregnancy, and tubal factor infertility in women [3] and urethritis, epididymitis and other complications including infertility in men [4]. CT infection has become a major public health concern globally. Based on the estimation of the World Health Organization (WHO) in 2012, there were an estimated 131 million new cases and 127 million prevalent cases of CT infection per year [5]. Data from the surveillance programmes in the United States [6], the United Kingdom (UK) [7] and Canada [8] had indicated an increasing trend of the infection in recent years. Based on the national case-reporting system in China, the reported incidence of CT infection has increased from 35.8/100,000 in 2011 to 37.1/100,000 in 2015 [9]. However, the true burden of the infections in many parts of China including in Shenzhen is unknown because of significant under-reporting of the infections, particularly asymptomatic infections, in health facilities. In order to provide baseline data for estimating the disease burden, developing intervention programmes, and planning for resource allocations, we carried out a cross-sectional survey to estimate the prevalence of the infection and explored the factors associated with the infection among patients attending clinics for STI services in Shenzhen City, China.

Study setting and population
The survey was a cross-sectional study in Shenzhen, a "special economic zone" located in south coastal China and adjacent to Hong Kong. The city has witnessed an alarming increase in its economy, migration of population, and the spread of syphilis and other STIs [10]. The survey was conducted using a stratified sampling strategy to recruit potential participants. Of the 10 administrative districts in the city, 6 were purposively selected and 4 hospitals reporting a high number of STI cases in the previous year were included into the study in each of 5 districts and 2 hospitals were included in 1 district. In each day during the period of the survey (from April 15 to May 16, 2018), the first 15 patients attending clinics at departments of dermatology, gynecology, urology, and andrology for seeking STI services were invited to participate in the study according to the eligibility criteria. The criteria included being a patient aged 18-49 years old, seeking STIrelated services, and having not any antibiotic use in the last 2 weeks. A written informed consent should be obtained before the survey can be conducted and each participant did provide written, informed consent.

Questionnaire interview and specimen collection
Interview with a structured questionnaire was conducted by the physician to collect demographic and behavioral data as well as clinical findings and the details of the questionnaire were kept anonymous. After completing the interview, participants were asked to provide a self-administered 15-30 mL first-catch urine specimen. A research nurse was assigned to check the integrity of questionnaire information and instruct participants on specimen collection. Urine specimens were collected using the Cobas1 urine specimen collection kit (Roche P/N 05170486190) according to the manufacturer's instructions. The specimens were temporarily stored at 4°C at the local laboratory for a maximum of 10 days before being transported to a central laboratory for testing.

Laboratory assays
At the central laboratory, DNA was extracted and purified from the urine specimens by an automated magnetism nucleic acid isolation method using the MagNA Pure 96 System (Roche, Switzerland) according to the manufacturer's instructions. The extracted DNA was further evaluated for CT and Neisseria Gonorrhoeae (NG) based on polymerase chain reaction (PCR) of the Cobas® 4800 System (Roche, Switzerland) using Cobas® 4800 CT/NG Amplification/Detection Kit. Diagnosis reagent and supplies were preserved under requested condition. Laboratory performance was run according to standard operating procedures (SOPs). CT or NG infection was defined as having a positive PCR for CT or NG.

Statistical analyses
All data from questionnaires and laboratory tests were double entered into computer to establish a database using Epidata software (V.3.1, Denmark). The Epidata 3.1 dataset was subsequently transferred to the IBM SPSS Statistics for Windows Version 23.0 (IBM Corp., Armonk, NY) for statistical analyses. Univariate analysis was used to determine the association between variables and CT infection and odds ratio (OR) and 95% confidence interval (CI) were calculated. To adjust for potential confounders, all factors associated with the infection at P < 0.2 in univariate analysis were included in multivariable logistic regression analysis using a backward stepwise procedure. Variables significant at P < 0.05 were considered the factors independently associated with the infections.

Ethics consideration
The study was approved by the Ethical Review Committee of Shenzhen Centre for Chronic Disease Control (Approval No.20180301). Participation in this study was voluntary and the questionnaire was anonymous. Confidentiality of the study data was ensured to protect the privacies of the participants. Participants who tested positive for CT and/or NG were contacted privately by the research team members for further diagnosis, treatment, and other interventions at the STI clinic in Shenzhen Centre for Chronic Disease Control.
Partner notification was conducted according to the routine process in the clinic.

Participant characteristics
Out of the 8444 patients who provided urine specimens, 120 did not participate in a questionnaire interview. Therefore, a total of 8324 (98.6%) participants were included in the final analyses. The mean age was 32. In the univariate analyses, 11 variables were associated with CT infection at P < 0.20 (Table 2). In the multivariate analyses using these 11 variables as independent variables and potential interactions between these variables, the following factors were found to be  Table 2. * The sum of participants in some items may be less than the total number of 8324 because some participants did not respond to these items.

Discussion
The findings from our study indicated a high prevalence of CT infection among either male or female patients attending clinics in departments of dermatology, gynecology, urology, and andrology in Shenzhen and highlight the risk factors associated with the infection in this population. STI surveillance plays an important role on measuring the magnitude of the STI burden in the general and target populations to assist in programme planning, monitoring trends over time and identify emerging infections and outbreaks, providing data to advocate for mobilization of resources, and assisting in evaluating the effectiveness of the response [11]. Prevalence assessment is one of the core components of WHO-recommended STI surveillance programming.
Our study was a cross-sectional study on genital CT infection with the largest sample size of patients seeking clinic-based STI services in China. Among 8324 patients, the total prevalence of CT infections was 9.0% with 10.7% for male and 8.3% for female respectively, which was higher than the national rates of 2.1% for male and 2.6% for female (1999-2000) [12]. This CT prevalence was also higher than that reported in many high-income countries [13], such as 1.7% in the US [14], 1.5% in the UK [15], and 1.7% in France [16]. Compare with high-risk populations, such as 8.5% among cross-border truck drivers in Hong Kong [17], 6.5% among MSM in Jiangsu Province [18], this CT prevalence still remained at a high level. The relatively high prevalence of CT infection further emphasized the importance of urgently implementing comprehensive interventions among patients seeking clinic-based STI services.
In our study, the highest prevalence (15.4%) was detected among patients seeking clinicbased STI services aged younger than 24. After controlling for confounding factors, aged younger than 24 was still significantly associated with an increased risk of CT infection (P < 0.05). This finding was similar to the findings of the US CDC, which reported that in 2013, the highest incidence of CT in the US was among people aged between 14 and 24 years [19]. These may indicate that patients of this age group had more exposure to new infection due to greater sexual activity and less knowledge about STI prevention.
Opportunistic screening for CT among young sexually active adults had been recommended in many high-income countries including the USA, the UK, Australia, Sweden, Denmark and Norway [20][21][22]. The UK had run the nationwide programme-the National Chlamydia Screening Programme (NCSP) [23], which targeted all sexually active men and women under 25 years of age for annual chlamydia screening through various clinical and nonclinical settings. As an economically developed region, Shenzhen has a good ability to undertake economic health. It is a feasible measure to develop a CT screening strategy for young people in reference to developed countries.
In addition, participants that were single/divorced/widowed were almost 50% more likely to have a CT infection than those that were married. This was consistent with the results Moreover, our results showed that positive for NG was significantly associated with an increased risk of CT infection. This was consistent with the findings of Guangdong Province [26]. Therefore, it would be better to suggest patients have a CT infections test when they need to have an NG detection.
Regarding educational level, we found that participants with lower educational level were at higher risk of incident CT infection, and this association remained after adjusted for age and occupation. Similar results were obtained in other studies [27][28][29]. Also, we found that a higher prevalence in patients who had no hold of health insurance. Poor healthcareseeking behavior associated with higher infection rates, lower partner referral or inadequate care had been reported for people with lower socioeconomic status in many countries [30][31][32]. A German study showed that, because a quarterly fee had to be paid by persons, posing a possible barrier for people with a small income, the higher prevalence in groups with low or medium social status could be due to a lower healthcare use and subsequently a lower chance to be tested for CT [33][34][35]. It might be useful to reduce the CT infection of this population with strengthening the publicity and education of CT among the low-education population, and reducing the cost of CT screening and treatment.
Our study had limitations to be highlighted. First, the study was conducted in one city with a special economic background in China and convenient sampling might shift the results so that any generalization of the results from this study should be made with caution.
Second, using a retrospective questionnaire may lead to recall bias. Although these limitations should be addressed in future studies, our findings could have significant implications for policy, practice, and research.

Conclusion
CT infection is prevalent among patients seeking clinic-based STI services, particularly aged younger than 24 in Shenzhen, China. Furthermore, patients that were single/divorced/widowed, positive for NG, low educational level and having no hold of health insurance were significantly associated with an increased risk of CT infection. It is strongly recommended that clinicians perform key screening and interventions for CT in these populations. As well, these findings suggest that the CT integrated prevention and control projects could be considered as routine public health services by the government.

Funding
This study was supported by Sanming Project of Medicine in Shenzhen (SZSM201611077).
The funders of the study had no role in study design, data collection, data analysis, data interpretation, or writing of the report.

Availability of data and materials
The results section, figures and tables contain all data on which the conclusions of this paper rely. The raw data used for this study are available from the corresponding author upon request.

Ethics approval and consent to participate
The study was approved by the Ethical Review Committee of Shenzhen Centre for Chronic Disease Control (Approval No.20180206). We informed all study participants of the study details prior to their recruitment. Each study participant gave his written consent prior to enrollment.

Consent for publication
Not Applicable.