Demographic and behavioural characteristics
Compared with Natsal-2, the NCSP tested larger proportions of women (67.2%, 95% CI 66.8% to 67.2% vs 49.3%, 95% CI 46.5% to 52.1%) (data not shown in tables). compares the demographic and behavioural characteristics of NCSP participants relative to the general population aged 16–24 years, according to Natsal-2, by gender. Relative to Natsal-2 respondents, men and women participating in the NCSP were more likely to be younger, specifically aged 16–17 years: 29% of men and women in the NCSP versus 16% of men and 15% of women in Natsal-2. NCSP participants were also more likely to be from ethnic minority groups: 17% of men and 14% of women versus 8% and 6%, respectively, in Natsal-2, and particularly from black ethnic groups: 8% of men and 7% of women in contrast to 2% and 1%, respectively, among Natsal-2 participants. NCSP participants were also more likely to report not having used condoms at last sex than their counterparts in the general population: 66% of men and 68% of women versus 48% and 63%, respectively in Natsal-2. NCSP participants were also more likely to report multiple partners in the last year: 62% of men and 47% of women versus 47% and 30%, respectively, in Natsal-2.
A comparison of the demographic and behavioural characteristics of National Chlamydia Screening Programme (NCSP) participants relative to the general population aged 16–24 years, according to Natsal-2, by gender
Due to the large proportion of NCSP participants for whom data on condom use were not available (see above), we compared the characteristics of NCSP participants with and without data for this variable to see whether there were any differences. While statistically significant differences do exist, reflecting the size of these two groups, the magnitude of these differences are slim, typically within four percentage points (see online table 1). One notable exception though is that the proportion of women from ethnic minority groups is larger among those for whom data on condom use were not collected: 15% in contrast to 5% of women for whom these data were collected. Given these differences, we also compared the NCSP and Natsal-2 participants limited to those with data for all four factors (see online table 2). We found that the differences between the two data sources shown in persisted, although there were smaller differences in the proportions of participants of non-white ethnicity.
Factors associated with chlamydia positivity
Overall, both men and women (aged 18–24 years) who tested in the NCSP had higher chlamydia positivity than those tested in the general population sample: among men: 9.4% (95% CI 9.2% to 9.5%) versus 2.9% (95% CI 1.3% to 5.2%) and among women: 8.5% (95% CI 8.4% to 8.6%) versus 3.1% (95% CI 1.6% to 5.7%). The corresponding numbers of positive chlamydia cases were 11 454 men and 21 316 women in the NCSP; 8 men and 16 women in the Natsal-2 sample.
shows the factors associated with increased risk of chlamydia infection in each data source for men and women separately. Age was only significantly associated with having chlamydia among women in the NCSP with lower odds for those aged 20–24 years relative to those aged 18–19 years (AOR 0.84, 95% CI 0.74 to 0.96). Ethnicity appeared significantly associated for women in the NCSP (only) with lower odds for black women relative to white women (AOR 0.34, 95% CI 0.15 to 0.76). However, NCSP women of black ethnicity who responded to the question regarding condom use (and were therefore included in our analysis) had a lower prevalence of chlamydia infection than all NCSP women of black ethnicity (3% vs 9.5%), which will have introduced bias into our analysis with respect to ethnicity. When the multivariate model was run for those participants with data for the other factors, excluding condom use (online tables 3A and 3B), the AOR for these women increased to 1.19 (95% CI 1.11 to 1.27) and a similar relationship was observed for men in the NCSP of black ethnicity: AOR 1.42 (95% CI 1.30 to 1.54).
Percentages, crude and adjusted ORs for demographic and behavioural risk factors associated with testing positive for chlamydia, by data source: men and women
In terms of sexual risk behaviours, for both genders in both data sources the AOR was greater than one for participants who reported not using condoms at last sex: among men, AORs of 1.53 for NCSP participants and 10.3 for men in Natsal-2; among women, AORs of 1.23 and 1.31, respectively. Similarly, NCSP participants who reported multiple partners in the year prior to testing for chlamydia were significantly more likely to test positive: AORs of 1.62 and 1.81 for men and women, respectively. This relationship was also observed among Natsal-2 women but was not statistically significant (AOR 2.85, 95% CI 0.88 to 9.22). Among men in Natsal-2 sample, none of those reporting at most one partner in this time-frame tested positive for chlamydia, while prevalence was 6.7% (95% CI 3.0% to 14.1%) among those reported multiple partners.