METHODS

We used stratified two stage probability sampling of the general population 18–49 years old. Data were collected between November 1999 and February 2001 at respondents’ homes by a combination of face to face interviews and anonymous self administered pencil and paper questionnaires adapted from the British National Survey of Sexual Attitudes and Lifestyles conducted in 1990.⁵ Respondents were invited to provide a urine specimen for testing for genital C trachomatis infection. Following signed informed consent, participants provided approximately 10 ml of first void urine (FVU) in a plastic, sterile container. Specimens were frozen and stored at −20°C at interviewers’ homes on the day of collection and transported to the laboratory in cold boxes every 2 weeks. The Amplicor PCR test (Roche Diagnostic) for C trachomatis was performed on pool sizes of five thawed specimens.^6,7 Internal controls to identify inhibitory specimens were used.⁸ Specimens from reactive pools were retested individually. Cases of C trachomatis infection were referred for treatment. The linkage of testing results with the information reported by respondents is described elsewhere.⁹

Weights were computed to adjust for differences between the sample and Slovenian population estimates according to sex, age, region, and size and type of community. Statistical methods for complex survey data (Stata statistical software, Release 7.0) were used to account for stratification, two stage sampling, and weighting. Univariate and multivariate analyses of association between C trachomatis infection and explanatory variables were performed by logistic regression. C trachomatis prevalence and selected indicators of high risk sexual behaviour in sexually experienced individuals aged 18–24 years were compared to the results from the British National Survey of Sexual Attitudes and Lifestyles conducted during the same period.^10,11

Ethical approval was obtained from the medical ethics committees of the Ministry of Health of the Republic of Slovenia and the London School of Hygiene and Tropical Medicine.

RESULTS

In all, 849 men and 903 women aged 18–49 years were interviewed. Survey response was 67.0%, 63.3% among men and 70.9% among women. A total of 683 men and 764 women, 82.6% of survey respondents, were tested for C trachomatis infection. Taking into account survey non-response, the participation rate in FVU specimen collection among eligible individuals was 55.3%, 50.9% among men and 60.0% among women.

Genital C trachomatis infection prevalence rates in men and women by age are shown in table 1. Age specific prevalence was highest among 18–24 year olds, 4.1% (95% CI 2.2 to 7.4) in both men and women. Excluding individuals who had never had sexual intercourse (vaginal, oral, or anal), the prevalence was 4.7% (CI 2.5 to 8.5) in both men and women.

Prevalences and odds ratios for C trachomatis infection among 18–24 year old individuals, according to selected sexual behaviours, are shown in table 2. Prevalence was higher in those reporting first heterosexual intercourse before the age of 16, unprotected sexual intercourse with at least one heterosexual partner during the preceding year, concurrent heterosexual relationships during the preceding year, and five or more lifetime heterosexual partners. However, the association was statistically significant only for five or more lifetime partners (age and sex adjusted OR 3.0; CI 1.3 to 6.9; p = 0.01).

Sexually experienced Slovenians aged 18–24 years have lower risk reported sexual behaviour than the corresponding groups in Britain: mean annual sexual partner acquisition rates among Slovenian men and women were 1.1 (SD 3.6) and 0.4 (SD 0.9) and among British 1.7 (SD 3.5) and 0.8 (SD 1.7). Proportions of those with at least five lifetime heterosexual partners among Slovenian men and women were 36.9% (CI 31.6 to 42.4) and 17.7% (CI 13.8 to 22.4), and among British 48.0% (CI 44.1 to 51.8) and 43.6% (CI 40.2 to 47.0). Finally, proportions of those with concurrent sexual relationships during the preceding year among Slovenian men and women were 13.3% (CI 9.4 to 18.5) and 8.1% (CI 5.2 to 12.2) and among British 22.6% (CI 19.2 to 26.5) and 16.1% (CI 13.4 to 19.4).

DISCUSSION

The prevalence of C trachomatis infection among sexually experienced Slovenians aged 18–24 is substantial, at 4.7% (CI 2.5 to 8.5) in both sexes. The corresponding estimates for Britain are appreciably lower, 2.7% (CI 1.2 to 5.8) among men and 3.0% (CI 1.7 to 5.0) among women,¹⁰ in spite of the higher reported prevalence of risk behaviours in Britain. However, confidence intervals for these prevalence estimates were wide and the differences between countries were not statistically significant. In Slovenia, methods for the transport, storage, and laboratory testing of FVU were robust, whereas the British researchers indicated that they may have underestimated the prevalence.¹⁰ Neither this, nor participation biases can be excluded as accounting for part of the observed difference in prevalence.

Slovenian men and women aged 18–24 reported lower risk sexual behaviour than their British counterparts. There is no evidence that Slovenians are more likely to under-report high risk sexual behaviour than the British, in spite of differences in the cultural context and slight differences in survey methodology. This cannot however be excluded as accounting for part of the apparent differences in behaviour. Improved reporting accuracy was assumed for some sensitive behaviours in the 2000 British survey, compared with the 1999 survey, in line with greater social tolerance and improved methodology.¹²

Why is the estimated prevalence of genital C trachomatis infection among 18–24 year old Slovenians higher than among the British, if there is lower risk sexual behaviour in Slovenia? Together with low C trachomatis testing rates and low reported infection rates, our results suggest serious gaps in the diagnosis and treatment of C trachomatis infection in Slovenia. This implies that a large proportion of infections remain untreated, and provides strong support for the introduction of C trachomatis infection screening.