Uptake, positivity, and equity of online postal self-sampling for chlamydia testing in England: a retrospective cohort study.

Background: Chlamydia is the most commonly reported sexually transmitted infection (STI) in Europe and untreated chlamydia is associated with poor health outcomes. Online postal self-sampling enables people to test for STIs including chlamydia without having to visit a health-care provider, but the extent to which the addition of this mode of testing in England has impacted access to testing in different populations is unclear. In England, there is national-level surveillance data enabling identification of the factors associated with use of online postal self-sampling (OPSS) for chlamydia testing. The aim of this analysis was to determine the change in chlamydia testing, chlamydia positivity, and test location as a result of the introduction of OPSS, and to determine socio-demographic factors associated with uptake of OPSS services compared to testing in-person. Methods: We conducted a retrospective cohort study analysing data on all publicly funded chlamydia tests between 01/01/2015 and 31/12/2022 in England using two pseudonymised national surveillance systems (GUMCAD STI Surveillance System and CTAD Chlamydia Surveillance System) for STIs. We conducted a descriptive analysis of 25,171,919 chlamydia tests to establish the uptake and positivity of chlamydia tests by testing mode and gender over time. We used bivariate and multivariable logistic regression to examine associations of uptake of testing and positivity of tests with sociodemographic characteristics and testing by OPSS or a different mode. Findings: The overall number of chlamydia tests/quarter (OPSS and in-person) gradually increased over time until 2019 (884,843 tests/quarter in quarter 1) and then declined in early 2020 (376,118 in quarter 2) and had not returned to 2019 levels by the end of 2022 (715,166 in quarter 4). During this time, the proportion of OPSS testing completed through OPSS increased from 2.6% (88,144/3,433,987) in 2015 to 38.4% (1,168,828/2,972,226) in 2022. Women were less likely than men to use OPSS compared to all available in-person testing (aOR = 0.75, 95% CI 0.75-0.75)). Those aged 20-24 were more likely to use OPSS than 15-19-year-olds (aOR = 1.55, 95% CI 1.55-1.56) and use of OPSS then decreased with increasing age. People in the most deprived areas were the least likely to use OPSS (aORs 1.18-1.28 for index of multiple deprivation quintile groups 2-5 vs 1). People were less likely to test positive using OPSS compared to in-person testing (aOR = 0.87, 95% CI 0.87-0.88). Between 2015 and 2022 OPSS chlamydia test-positivity decreased from 9.3% (2551/27,557) to 7.5% (34,050/454,596) in men and 7.4% (4458/60,367) to 6.1% (43,088/702,867) in women. During the same period, chlamydia test-positivity in sexual health services increased from 8.2% (57,139/692,873) to 10.6% (43,061/406,161) in men and 6.4% (51,080/797,143) to 7.9% (33,292/420,760) in women. Interpretation: We have found changes in access to care, with a shift towards testing via OPSS, and variations in who tests where and differences in positivity by testing mode and gender. Further research is needed to ensure available testing pathways meet the needs of all populations. Funding: National Institute for Health and Care Research.