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Background: The number of chlamydia diagnoses made in genitourinary medicine (GUM) clinics in England has increased over the last decade. It is often assumed that rising diagnoses are caused by increases in transmission. However factors other than transmission that affect diagnoses have also changed over the same period; more tests have been performed, and nucleic acid amplification tests (NAATs) gradually replaced less accurate enzyme immunoassay (EIA) tests in the mid-2000s. We investigated the potential effect of changes in testing rates and test technologies on chlamydia trends.
Methods: Numbers of chlamydia tests and diagnoses reported from GUM clinics in England from 2003 to 2012 were analysed. The proportion of laboratories in England using NAATs and EIA tests each year between 2003 and 2012 was obtained from the National External Quality Assessment Service. Estimates of sensitivity and specificity of EIAs compared to NAATs were derived from literature. Trends in the number of diagnoses, crude positivity (diagnoses/tests) and positivity adjusted for test sensitivity/specificity were compared.
Results: Between 2003 and 2012, numbers of tests increased from 633,289 to 1,288,443 and diagnoses from 86,595 to 97,166. Crude positivity decreased from 13.7% to 8.1%. After adjusting for average sensitivity (80%) and specificity (95%) estimates for EIAs, positivity declined less steeply, from 11.7% to 8.1%. Sensitivity analyses using a range of plausible sensitivity and specificity estimates had a substantial impact on the range of positivity trends over the analysis period.
Conclusions: Although diagnoses increased between 2003 and 2012, trends in chlamydia diagnoses alone do not provide a full picture of the burden of infection. Diagnosis trends should be interpreted with caution in the absence of information on test volume. Changes in sensitivity and specificity of tests in use should be taken into account when interpreting trends in diagnoses or positivity.