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BMJ. 2005 April 23; 330(7497): 940.
PMCID: PMC556339

Coverage and uptake of systematic postal screening for genital Chlamydia trachomatis and prevalence of infection in the United Kingdom general population: cross sectional study

John Macleod, senior lecturer in primary care,1 Chris Salisbury, professor of primary health care,2 Nicola Low, senior lecturer in epidemiology and public health,3 Anne McCarthy, project manager,3 Jonathan A C Sterne, reader in medical statistics and epidemiology,3 Aisha Holloway, research fellow,1 Rita Patel, research associate,2 Emma Sanford, research associate,3 Andrea Morcom, research fellow,1 Paddy Horner, consultant in genitourinary medicine,5 George Davey Smith, professor of clinical epidemiology,3 Susan Skidmore, consultant clinical scientist,6 Alan Herring, head, Genitourinary Infections Reference Laboratory,7 Owen Caul, consultant virologist,7 F D Richard Hobbs, professor of primary care and general practice,1 and Matthias Egger, professor of epidemiology and public health4


Objective To measure the coverage and uptake of systematic postal screening for genital Chlamydia trachomatis and the prevalence of infection in the general population in the United Kingdom. To investigate factors associated with these measures.

Design Cross sectional survey of people randomly selected from general practice registers. Invitation to provide a specimen collected at home.

Setting England.

Participants 19 773 men and women aged 16-39 years invited to participate in screening.

Main outcome measures Coverage and uptake of screening; prevalence of chlamydia.

Results Coverage of chlamydia screening was 73% and was lower in areas with a higher proportion of non-white residents. Uptake in 16-24 year olds was 31.5% and was lower in men, younger adults, and practices in disadvantaged areas. Overall prevalence of chlamydia was 2.8% (95%confidence interval 2.2% to 3.4%) in men and 3.6% (3.1% to 4.9%) in women, but it was higher in people younger than 25 years (men 5.1%; 4.0% to 6.3%; women 6.2%; 5.2% to 7.8%). Prevalence was higher in the subgroup of younger women who were harder to engage in screening. The strongest determinant of chlamydial infection was having one or more new sexual partners in the past year.

Conclusions Postal chlamydia screening was feasible, but coverage was incomplete and uptake was modest. Lower coverage of postal screening in areas with more non-white residents along with poorer uptake in more deprived areas and among women at higher risk of infection could mean that screening leads to wider inequalities in sexual health.


Randomised trials show that systematic screening for genital Chlamydia trachomatis might reduce the incidence of pelvic inflammatory disease by about 50%.1 We investigated the coverage and uptake of systematic chlamydia screening by post, estimated prevalence of chlamydia, and explored factors associated with these measures.


The chlamydia screening studies (ClaSS) project has been described previously,2 and the full protocol is available at

We invited men and women aged 16-39, randomly selected from 27 general practices in the West Midlands and Avon, to collect their own specimens (urine in men, urine and vulvovaginal swab in women) and post them to us, and to complete a questionnaire on risk factors. Invitations and testing kits went to patients' registered address, and we confirmed residence at this address. We used postal reminders, telephone calls, home visits, and “flagging” of records at study practices to encourage participation.

Our colleagues in the reference laboratory (SS and AHe) used nucleic acid amplification tests to detect C trachomatis and confirmed positive results. Postal delays did not affect positivity rates (data not shown). Participants with chlamydia received results and treatment at their practice. Notification of partners was undertaken at either the practice or a genitourinary clinic. Statistical analyses of coverage of chlamydia screening (proportion receiving an invitation to be screened), uptake (proportion returning a specimen), and prevalence took into account clustering at the practice level and sampling probability.


Of 19 773 people aged 16-39 who had been invited, we contacted 73% (14 382, see figure on successfully. Uptake of screening was 34.5% (95% confidence interval 31.2% to 38.0%) overall, and 31.5% (28.6% to 34.6%) in 16-24 year olds (table 1). After a single postal invitation, uptake was 22.2% (18.6% to 26.2%) in 16-24 year olds and was higher in women than men (table 1). A postal reminder and face to face contact (in a home visit, or “flagged” patients being invited to participate when they attended their practice) each increased uptake by around 5%. Screening coverage and uptake varied by practice (see table A on Coverage was lower in areas with higher proportions of residents from minority ethnic groups, and uptake was lower in practices with higher deprivation scores (see table B on

Table 1
Uptake of screening and prevalence of chlamydia according to effort needed to secure participation

The overall prevalence of chlamydia was 2.8% (2.2% to 3.4%) in men and 3.6% (3.1% to 4.9%) in women, but it was higher in people who were younger than 25 (men 5.1%, 4.0% to 6.3%), women 6.2%, 5.2% to 7.8%). Prevalence was below 1% in men older than 24 and women older than 29 years. Prevalence was higher among the subgroup of 16-24 year old women who only participated after repeated contacts (table 1). Having one or more new partners in the past 12 months was the strongest predictor of infection (table 2). The prevalence of chlamydia was also higher in single compared with married women and in men aged 20-24 than in people aged 16-19 years (tables C and D on

Table 2
Prevalence of infection according to number of sexual partners, and association between prevalence and sexual behaviour, in men and women

Prevalence did not vary substantially between practices (I2 = 34% of variation attributable to heterogeneity between practices, tables A, C, and D on We found weak evidence that chlamydia was more common in practices in more deprived areas (adjusted odds ratio for a 10 point increase in deprivation score, 1.2, 1.0 to 1.4, P = 0.077).


A single round of systematic, postal screening for chlamydia was feasible, but coverage was incomplete and uptake was modest, particularly in more deprived areas. The prevalence of chlamydia was 5-6% in men and women younger than 25 years. Having one or more new sexual partners in the past year was the strongest determinant of infection. In young women the risk of chlamydia was higher among those who were harder to engage in screening.

Strengths and limitations of the study

The strengths of this study are that it was large, population based, included both men and women, and tested a screening strategy shown to be feasible and effective in other settings.3 Limitations were that screening coverage was incomplete, reflecting inaccuracies in practice lists, and uptake was lower than expected,4 so the power of the study was reduced and prevalence estimates might be affected by selection bias.5 Repeated reminders had little impact on increasing uptake and would be unfeasible in routine practice.

Comparison with other studies

Our estimates of chlamydia prevalence were broadly comparable with other population based studies showing that 2-6% of men and women under the age of 25 are infected.1,3,6 The variability might reflect differences in study populations or selection biases since participation in all studies was incomplete.

Meaning of the study

Chlamydia risk was most strongly associated with having a new sexual partner in the past year, rather than the total number of partners. Although being from a black minority ethnic group has been found to be associated with chlamydia,7 we did not find a strong association with ethnicity. Our findings indicate that chlamydia screening should be offered to all men and women under 25 years because selection based on sexual behaviour could be difficult.

Effectiveness of chlamydia screening

Opportunistic chlamydia screening in selected settings, focusing on women younger than 25, is now being introduced in England,8 even though this approach has not been evaluated in randomised trials1 and has not controlled transmission of chlamydia in Sweden.1 In contrast, randomised trials in Denmark have found postal screening, with uptake similar to this study, to be effective and cost effective.9 The cost effectiveness of postal chlamydia screening in the ClaSS project will be reported separately. If postal screening were introduced, publicity and increasing familiarity may lead to higher uptake. Postal screening could also have a role as an adjunct to opportunistic screening since we also found that we reached a proportion of individuals who had not visited their practice in the past year.10

Effective health technology can contribute to widening health inequalities if access to care varies with social position. We found lower coverage of chlamydia screening in areas with more residents from minority ethnic groups, lower uptake in more deprived areas, and a trend towards higher prevalence in more deprived areas. Additionally, young women with the highest prevalence of chlamydia participated only after repeated invitations. Further research is needed to clarify the impact of chlamydia screening on sexual health inequalities.


Evidence for the long term effectiveness and impact of chlamydia screening programmes remains limited.1 Randomised trials to determine the most effective strategy are required. In England these could be incorporated within the phased introduction of the national screening programme.

What is already known

Rates of infection with genital Chlamydia trachomatis are increasing across Europe and in the United States

Opportunistic chlamydia screening is being introduced in England, although there is no high quality evidence of its effectiveness

Systematic population screening for chlamydia has been shown to be effective but factors affecting the coverage and uptake are poorly understood

What this study adds

Postal screening was feasible, but coverage and uptake were incomplete

Young women at higher risk of infection were harder to engage in screening

Undiagnosed chlamydia is common in both men and women under the age of 25 in the general population

An age younger than 25 and having a recent new sexual partner were the only important risk factors for chlamydia identified

Supplementary Material

[extra: Additional tables and figure]


An external file that holds a picture, illustration, etc.
Object name is webplus.f1.gifAdditional tables and a figure are on

Contributors: JM wrote the first draft of the paper, which was revised by JM and NL following comments from CS, JACS, AMcC, GDS, and ME. All authors commented on the final draft. ME, JM, CS, PH, OC, GDS, NL, and RH devised the ClaSS prevalence survey and helped obtain funding. JM, CS, NL, AMcC, JS, PH, ME, and the ClaSS study group undertook strategic management of ClaSS. AMcC was also responsible for day to day project management. AHo, RP, and AM recruited practices and undertook the survey under the supervision of JM and CS. SS and AHe supervised sample testing. JS devised the analysis strategy with JM, CS, and NL. ES undertook statistical analyses under the supervision of JS and NL. ME is lead investigator of the ClaSS study and is the guarantor.

Funding: ClaSS is funded through the NHS R&D HTA programme. Participating individuals and institutions can be viewed at JM and NL are funded by NHS Career Scientist Awards.

Competing interests: None declared.

Ethical approval: South West Multicentre Research Ethics Committee and all relevant local research ethics committees.


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