Sonnenberg et al previously reported an overview of STI prevalence and service use using data from Natsal-3. 17 Chlamydia prevalence in 16–44 year olds was 1.5% in women and 1.1% in men and was higher among 16–24 year olds (women: 3.1%; men: 2.3%). Among 16–24 year olds, 54.2% of women and 34.6% of men reported testing in the last year. Although prevalence was reported by age group, factors associated with prevalent infection were assessed among all 16–44 year olds. Only a limited number of factors associated with chlamydia prevalence and testing were explored (age group, area-level deprivation, sexual partners in the last year, sexual partners in the last year without a condom (investigated for prevalence only), age at first sex and any same-sex experience). In this paper, we report a detailed analysis among 16–24 year olds in Britain as this is the age group targeted by the NCSP in England. We describe and compare factors associated with prevalent chlamydia infection, previous chlamydia diagnosis and chlamydia testing to assess the extent to which opportunistic chlamydia screening is reaching young adults at risk of chlamydia.

The third National Survey of Sexual Attitudes and Lifestyles (Natsal-3) is a stratified cross-sectional probability sample survey of adults resident in Britain (England, Scotland and Wales; Northern Ireland was not included). 16 Conducted from 2010 to 2012, Natsal-3 included anonymous testing of urine specimens for STI, including chlamydia, and asked questions on chlamydia testing and diagnosis history. The survey provides a unique opportunity to investigate patterns of chlamydia infection and testing within a nationally representative sample of the British population.

Chlamydia testing of young adults increased substantially in the UK over the last decade. Increases in testing occurred in GUM clinics as a result of improved access to sexual health services 9–11 and availability of diagnostic testing using non-invasive samples. 12 In England, a major increase was driven by the national scale-up of the NCSP. After a phased roll-out from 2003 to 2008, a step change in screening activity outside of GUM clinics was seen from 2008 to 2010 as local areas responded to national targets for testing coverage. 13 Testing coverage (number of tests divided by total 15-year-old to 24-year-old population) peaked at 34% in 2010 and fell slightly to 30% and 26% in 2011 and 2012, respectively. 14 , 15

Chlamydia trachomatis (‘chlamydia’) is the most commonly diagnosed sexually transmitted infection (STI) in the UK. 1 Most chlamydia infections are asymptomatic, and untreated infections can cause serious complications including pelvic inflammatory disease, ectopic pregnancy and tubal factor infertility in women. 2 By diagnosing and treating asymptomatic infections, chlamydia screening potentially reduces the risk of complications 3 and is expected to reduce chlamydia prevalence and transmission. 4 In England, the National Chlamydia Screening Programme (NCSP) recommends that sexually active under 25 year olds are tested annually and on change of sexual partner. 5 Chlamydia screening is offered opportunistically in clinical and non-clinical settings in England. Scotland and Wales do not have an organised screening programme; guidelines recommend asymptomatic testing of young adults 6–8 with a focus on those at high risk (eg, those reporting multiple sexual partners in the last year, those with a previous diagnosis or patients attending genitourinary medicine (GUM) clinics).

With two exceptions, all variables included in univariable models were included in multivariable models: number of sexual partners in the last year was not included due to collinearity with other sexual partnership variables; age left school was not included as data were unavailable for 16 year olds.

Factors associated with prevalent infection, recent diagnosis and recent testing were investigated using univariable and multivariable logistic regression, for women and men separately. Although the overall percentage diagnosed with chlamydia (ever or in the last year) was estimated among the sexually experienced population, risk factors for recent diagnosis were investigated among those with a recent test to investigate associations with being infected at the time of testing rather than with testing per se. Socio-demographic and behavioural factors previously demonstrated to be associated with STI risk were included as predictor variables. 22–24 Associations with deprivation were explored using both residence-based (quintile of Index of Multiple Deprivation (IMD) for the lower layer super output area (LSOA) of residence (a geographical area of around 1500 people 25 )) and individual-based (age left school) measures. Sexual behaviours investigated included numbers of sexual partners in the last year (total, new, without a condom), number of sexual partners by the time of the interview (hereafter ‘lifetime sexual partners’) and condom use at last sex. Frequency of binge drinking was included as a proxy for sexual risk behaviour that may not be captured in reported numbers of sexual partners.

Analyses were carried out using Stata V.12.1, accounting for weighting, clustering and stratification of the data. Survey weights were applied to adjust for unequal probability of selection and non-response to make the sample data broadly representative of the British general population, according to the 2011 Census, in terms of sex, age group and Government Office Region. 16 Willingness to provide a urine sample varied by demographic and behavioural variables, including age, number of sexual partners (by the time of the interview/without a condom in the last year), same-sex experience and sexual health clinic attendance. Estimates of prevalent infection were therefore given an additional weight to reduce bias in the profile of urine sample respondents. 16 , 21

A flow chart of participants included in our analyses is presented in the online supplementary material . Analyses of recent testing and recent diagnosis were based on sexually experienced 16–24 year olds (n=3115). Analyses of prevalent infection were among those who provided a urine sample for STI testing and for whom a valid chlamydia test result is available (n=1832, 62 of whom had a prevalent infection).

We estimated the prevalence of chlamydia detected in urine (hereafter termed ‘prevalent infection’), self-reported chlamydia test in the last year (‘recent testing’), self-reported chlamydia diagnosis in the last year (‘recent diagnosis’) and self-reported chlamydia diagnosis ever.

In Natsal-3, participants were interviewed using computer-assisted face-to-face and computer-assisted self-interview for the most sensitive questions. The overall response rate was 57.7%, in line with other major social surveys conducted in Britain around the same time, 18 , 19 achieving a sample of 15 162 16–74 year olds. 16 A subset of participants, including all 16–17 year olds (regardless of reported sexual activity) and 18–24 year olds who reported at least one sexual partner by the time of the interview (hereafter termed ‘sexually experienced’) were invited to provide a urine sample for anonymous STI testing. 16 , 17 Participants did not receive their test results. 20 Of all Natsal-3 respondents eligible for the urine study, 57% provided a sample. Urine samples were posted to Public Health England where they were batch-tested for chlamydia using the Aptima Combo 2 assay (Hologic Gen-Probe); positive and equivocal results were confirmed with the Aptima chlamydia monospecific assay. 17 Details of the survey methods and questionnaire are available elsewhere. 16

Results

Table 1 shows chlamydia prevalence and self-reported chlamydia testing and diagnosis in the last year among sexually experienced 16–24 year olds. Around two-thirds (62.5%) of women and 43.2% of men had either been tested or offered a test in the last year. A total of 12.3% of women and 5.3% men had ever been diagnosed with chlamydia.

Table 1 Prevalence of chlamydia infection detected in urine and of self-reported testing and diagnosis by sex (sexually experienced 16–24 year olds)

Among those recently tested, <10% reported a clinical indication (symptoms; a partner with chlamydia/symptoms; check-up after a previous diagnosis) for their last test. Around three-quarters of women and half of men had last been tested in a sexual health clinic, general practice (GP) surgery or family planning clinic. Almost all (95.4%) individuals recently diagnosed had most recently been tested in one of these settings. Half of those recently diagnosed had last been tested due to symptoms or having a partner with chlamydia/symptoms (table 2).

Table 2 Reason and location of most recent chlamydia test, among those tested for chlamydia in the last year, by sex and by whether diagnosed in last year (sexually experienced 16–24 year olds)

Tables 3 and 4 explore the associations between socio-demographic and behavioural variables and prevalent infection, recent testing and recent diagnosis. In univariable analyses, higher numbers of sexual partners (total/new/without a condom) in the last year were significantly (p<0.05) associated with prevalent infection among women and men. In women, area-level deprivation (measured at LSOA level) and frequency of binge drinking were also associated with prevalent infection. Among men, number of lifetime sexual partners, age group, age left school, age at first sex and condom non-use at last sex were significantly associated with prevalent infection. Similar factors were associated with recent diagnosis among those tested. In multivariable analyses, living in more deprived areas and more frequent binge drinking remained significantly associated with having a prevalent infection in women. Older age group, living in more deprived areas and higher numbers of lifetime sexual partners remained significantly associated with prevalent infection in men.

Table 3 Percentage, unadjusted and adjusted ORs for prevalent chlamydia infection, self-reported diagnosis in the last year and self-reported testing by socio-demographic and behavioural factors (sexually experienced 16–24 year old women)

Table 4 Percentage, unadjusted and adjusted ORs for prevalent chlamydia infection, self-reported diagnosis in the last year and self-reported testing by socio-demographic and behavioural factors (sexually experienced 16–24 year old men)

Figure 1 shows unadjusted ORs for prevalent infection and recent testing by socio-demographic and behavioural factors. Groups in the upper right hand quadrant are those where both the odds of prevalent infection and of testing were higher than the reference group. Groups in the upper-left-hand quadrant had higher odds of prevalent infection, but lower odds of testing than the reference group. Factors associated with recent testing were similar to those associated with prevalent infection, with some exceptions. Whereas women living in one of the two most deprived IMD quintiles had almost four times higher odds of prevalent infection versus those living in less deprived areas (OR 3.82, 95% CI 1.35 to 10.79), the odds of recent testing did not differ by deprivation (OR 0.99, 0.77 to 1.27). Among men, the odds of prevalent infection were higher among 20–24 vs 16–19 year olds (OR 10.6, 2.40 to 46.3), but odds of recent testing were lower in the older age group (OR 0.67, 0.44 to 0.84). In men, not having used a condom at last sex was associated with a sixfold increase in the odds of prevalent infection (OR 6.03, 1.87 to 19.42), but was not associated with recent testing (OR 1.22, 0.95 to 1.56). Similar patterns were seen when comparing adjusted ORs from multivariable models (tables 3 and 4).

Figure 1 Bubble plot showing unadjusted ORs for prevalent chlamydia infection compared with recent testing by socio-demographic and behavioural factors, and proportion of prevalent infections in each group (16-year-old to 24-year-old sexually experienced women (A) and men (B)). Factors in the upper-right-hand quadrant are those where both the odds of prevalent infection and of testing were higher than the reference group. Factors in the upper-left-hand quadrant show those where the odds of prevalent infection were higher, but odds of testing were lower than the reference group (for ORs, 95% CIs and denominators, see tables 3 and 4). The area of the bubble and percentage in parentheses represents the proportion of individuals with a prevalent infection who reported the specified characteristic (for 95% CIs, see online supplementary table S1). Letters indicate reference groups: (a) 16–19 years old; (b) resident in lower super output area in the two least deprived quintiles, as measured by the Index of Multiple Deprivation; (c) left school at 17+ (among those aged ≥16); (d) 17+ years at first heterosexual sex; (e) 0 or 1 sexual partners in the last year; (f) 0 new sexual partners in the last year; (g) 0 sexual partners in the last year without a condom; (h) 1–9 lifetime sexual partners; (i) condom used at last sex; (j) no concurrent partnership in last year (among those with 1+ more sexual partners in last year); (k) reports binge drinking never or less than monthly; and (l) never had same sex contact/experience.

Although the proportion recently tested was generally higher in those reporting risk factors for chlamydia, recent testing remained well below 100% in all socio-demographic and behavioural subgroups. For example, 30.0% of women and 53.7% of men with ≥2 new sexual partners in the last year and 25.8% of women and 51.2% of men reporting ≥2 sexual partners without a condom in the last year had not been recently tested (tables 3 and 4).

Among individuals with a prevalent chlamydia infection, 14% (95% CI 7% to 14%) had ever been diagnosed with chlamydia and 5% (2% to 17%) reported a diagnosis in the last year (indicating either repeat or persistent infections). Fifty per cent (35–64%) of those with a prevalent infection reported a recent chlamydia test (89% of whom did not report a recent diagnosis, thus indicating incident infections within the last year). Over two-thirds of prevalent infections were among individuals resident in one of the 40% most deprived LSOA. Infections in women were more evenly distributed by numbers of sexual partners than in men. For example, among men, 80% of those with a prevalent infection and 77% of those recently diagnosed reported ≥10 lifetime sexual partners versus only 25% of the population. In women, 35% of those with a prevalent infection reported ≥10 lifetime sexual partners versus 21% of the population (see online supplementary table S1).