Because of the large burden of disease and risks associated with infection, the USPSTF recommends screening for C. trachomatis infection for all sexually active non-pregnant young women aged 24 or younger and older non-pregnant women who are at increased risk . The threshold population prevalence of C. trachomatis infection over which the evaluations were cost-effective varied from 3.1 to 10%. Screening can be cost-effective at a prevalence as low as 1.1%, when age is used to select women and nucleic acid amplification based tests are carried out on urine samples, although the large numbers of unnecessary tests diminishes effectiveness .
There are scarce data regarding the prevalence of C. trachomatis infection in Spain, all of them from risk population attending in STI clinics. Corbeto et al. studied people under 35 years-old attending in sexual health clinic and concluded that overall C. trachomatis prevalence was 4% , but it was higher in those under 25 years-old (5.8%) and higher than in our non-selected population (4.1%) . Nogales et al. studied young and adults attending in a clinic for STI, 50% of them belonged to groups engaged in high risk sexual practices as commercial sex workers, MSM, users of prostitution, and the prevalence was 6% (4.3% in women and 7.8% in men) . Similar prevalence (5.9%) was found by Folch et al. in immigrant female sex workers, but these data cannot be compared with our study due to differences in population characteristics .
Comparisons with prevalence data previously reported by other countries are not simple, owing to the use of different age groups to calculate rates. In our survey, the C. trachomatis prevalence in women (4%) resembles those found in other countries in Europe. In a sexual survey in France in 2006 , the prevalence of C. trachomatis in the national population in women in the 18–24 year group was 3.6% (CI 95%: 1.9-6.8) and in men 2.4% (CI 95%: 1–5.7). In the UK, in the second National Survey of Sexual Attitudes and Lifestyles  the prevalence of C. trachomatis in women in the 18–24 years group was 3% (CI 95%: 1.7-5) and in men 2.7% (CI 95%: 1.2-5.8). In Slovenia, considering the prevalence in the captured general population (1999–2011), the prevalence in women and men in the 18–24 years group was found to be 4.7% (CI 95%: 2.5-8.5), and was the same in both genders . Confidence intervals for these prevalence estimates were wide, with overlapping 95% confidence intervals, and the differences between countries were not statistically significant.
In the USA, where substantial racial/ethnic disparities in C. trachomatis infection exist, the prevalence in women is higher for 15–19 year olds and lower for 20–24 year olds when compared to the results in our survey. According to data from the National Health and Nutrition Examination Survey 1999–2008 (NHANES) , the C. trachomatis prevalence among sexually active women in the 14–19 age range was 6.8% (4.4% among non-Hispanic Caucasians and 16.2% among non-Hispanic Afro-Americans) and in the 20–24 age range it was 3.2% (1.3% among non-Hispanic Caucasians and 12.1% among non-Hispanic Afro-Americans). In our study the prevalence of C. trachomatis in the 20–24 years group (women 4.8% and men 4.5%) is higher than that seen in the 15–19 years group (women 2.3% and men 3.9%). In the 15–19 years group the level of sexual activity was especially low (women 58.1% and men 56.7%) and in the 20–24 years group it was women 97.9% and men 97% (Table 1). The prevalence of C. trachomatis infections among young adults (aged 18–26 years) in the USA who participated in the nationally representative National Longitudinal Study of Adolescent Health during 2001–2002, was: global prevalence 4.19%; women (4.74%) and men (3.67%) .
Our prevalence in men (4.3%) was similar to that in women (4%). Increasing efforts in partner notification and their treatment may contribute at least as much to the control of C. trachomatis infection as increasing screening coverage rates .
Our results, in both men and women, suggest that prevention of C. trachomatis with routine condom use could lead to a reduction in the observed C. trachomatis infection rates. However, as non-compliance with safe sex practices is commonly seen in young age groups and complications of a C. trachomatis infection are sometimes severe, with infertility and major sequels, universal free screening for young sexually active people in the health sector would aid the immediate recognition of the infection and thus facilitate action towards implementing the available effective therapy.
A response rate of 60% has been used as the threshold of acceptability by some and is valid as a measure of survey quality; however 60% is only a “rule of thumb” that masks a more complex issue. Non-response bias is more useful for understanding survey limitations . This non-response can potentially bias the prevalence estimates under 2 conditions: if the response rate varies according to an observed attribute, such as age, race, gender or sex, which is associated with prevalence or if the non-respondents have a different pattern of prevalence from respondents with similar observed attributes . But once the variable of age was adjusted according to sexual activity by means of a stratified analysis, it showed us that there was no significant difference with regards to participation related to age (Table 1). All this, being considered with the analysis of the motives for non-participation, with a response rate of 59.8% in our prevalence determination study, showed us that the distribution of the captured population was not distorted and comparable to the total reference population.
There are some limitations in our survey. Only 93 of the 467 sexually active young people with a negative urine test answered the survey on risk factors for C. trachomatis infection. Due to the fact that the young people had to give their names in the prevalence survey, we were worried that the numbers participating in that prevalence survey could have been reduced, as the young people would not want to be identified on account of their reluctance to report their sexual activity and preferences. Therefore, we made the second test, the sexual risk factor survey, anonymous and at a later date. This second survey had low numbers of participants showing the young people’s reluctance to report their sexual activity and preferences. Another limitation is the target population (the sexually active population), an unknown quantity, was determined by an initial survey, and therefore it must be considered as an estimation. Other limitations are the potentially inaccurate reporting of sexual behaviour data (social desirability bias), response rate and lack of information about non-responders, and lack of generalisability of these results.
The generalization of these prevalence values to other places in Spain could be possible but considerations about sample size, geographical limitations, lack of socioeconomic stratification and other factors, should be considered. We think that this is the first study in Spain to determine the communitary prevalence of C. trachomatis genital infection in young people.