Background The National Chlamydia Screening Programme (NCSP) updated its guidelines in 2013 to recommend retesting for all chlamydia positive individuals around three months after treatment, due to the risk of reinfection.

Objectives Investigate the impact of implementing new retesting guidance on chlamydia screening activities and the economic cost of updating current testing practice.

Methods We developed a spreadsheet tool to calculate the additional costs of implementing new retesting guidance. We collected data from pilot evaluations of retesting to estimate the number of tests performed and the cost of administering retesting within existing services. We used these to estimate the national impact of the new guidelines, and to inform future updates to guidelines.

Results The baseline scenario is based on findings from pilot evaluations: for every 10,000 chlamydia tests, this will generate 750 positives (assuming 7.5% positivity), of whom 40% (300) would be retested within 6 months. This would identify an additional 30 positives (10% positivity at retest). In this scenario, only 3% of all tests performed are retests, which would have minimal impact on the overall cost of the screening programme. The slight increased cost of retesting, associated with active recall of positive individuals is offset by the higher positivity observed at retest.

Conclusions The new guidelines to retest chlamydia positive individuals within 6 months appear feasible within the context of current programmes and will identify individuals at continued risk of infection with relatively low resource and time input.