The National Chlamydia Screening Programme (NCSP) appears to be reaching young people at increased risk of chlamydia. A study examined the differences in the demographic and behaviour profiles of participants in the NCSP compared with the general population sample in the second National Survey of Sexual Attitudes and Lifestyles (Natsal-2).

Natsal-2 is deemed a representative sample of the British population. It used a stratified probability sample of 11,161 men and women aged 16-44 interviewed between 1999 and 2001.

Data on 538,119 men and women aged 16-24 years who were tested for chlamydia through the NCSP in 2008 were compared with data from 2,180 interviewees in Natasal-2, of whom 644 were tested for chlamydia.

Two analyses were done to compare the two samples. The demographic and behavioural characteristics of those in the NCSP were compared with respondents aged 16-24 years in Natsal-2 with respect to age, ethnicity, gender, condom use at last sexual encounter, and having two or more partners in the past year. Another analysis looked at factors associated with testing positive for chlamydia.

Only people for whom all five variables were available were included in the study; this was 627 of the eligible Natsal-2 population and 17,259 from the NCSP.

The NCSP tested larger proportions of women (67%) compared with Natsal-2 (49%). NCSP participants were also more likely to be younger (29% aged 16-17 compared with 16% of men and 15% of women in Natsal-2); from ethnic minority groups (17% of men and 14% of women vs 8% and 6% respectively) and to report not using condoms at the last sexual encounter (66% of men and 68% of women vs 48% and 63% respectively) and more than one sexual partner in the past year (62% of men and 47% of women vs 47% and 30% respectively).

Both men and women who were tested in the NCSP had higher chlamydia positivity than those tested in Natsal-2; 9.4% vs 2.9% for men and 8.5% vs 3.1% for women. Age was significantly associated with chlamydia positivity among women in the NCSP with lower odds for the 20-24 age group compared with the 18-19 age group. Ethnicity was significantly associated with positivity in the NCSP with lower odds for black compared with white women.

For both genders in both populations, the risk of chlamydia increased for those who reported not using condoms at the last sexual encounter. NCSP participants who reported greater numbers of sexual partners in the year before testing were significantly more likely to test positive.

Despite both reporting similar behavioural risk factors for testing positive for chlamydia, this paper suggests that the NCSP was reaching out to those who might be more at risk of chlamydia compared with the general population.

Those participating in screening appear to be at higher risk than the general population. Whether this is due to selection by attendance at screening venues or by health professionals offering the test or by acceptance of the offer of a test, or a combination of these factors, is unclear.

While it might be difficult to imagine GPs and nurses offering chlamydia screening to any 18-24 year old they see, they might be more likely to offer the test if they perceive that the young person is at higher risk of sexually transmitted infection. This may explain the differences in positivities between the NCSP and the general population.

 

Dr Richard Ma

 

• Rhia J, Mercer CH, Soldan K et al. Who is being tested by the English National Chlamydia Screening Programme? A comparison with national probability survey data. Sex Transm Infect 2011;87:306-311