Introduction Nucleic Acid Amplification Tests (NAATs) are the recommended test type for diagnosing Chlamydia trachomatis (chlamydia). However poorer performing methods remain in use. We compared the risk of pelvic inflammatory disease (PID) in women tested for chlamydia by diagnostic test type.

Methods We used a sub-set of the Danish Chlamydia study that included all female residents of Denmark who were tested for chlamydia (1998–2001) when aged 15–34 years. Chlamydia tests performed on urinary or genital samples with a definitive positive or negative result were categorised as non-NAAT (ELISA; IF; “antigen”) or NAAT (PCR; SDA; TMA; LCR; DNA/RNA) and limited to each woman’s first test. Test records were linked to hospital presentations for PID within 12 months. Women with previous PID or PID diagnosed on the same date as the test were excluded. We used logistic regression to compare the risk of PID by test type adjusted for age, test year and test result.

Results Of the 2 72 105 women in the study, 44.78% were tested using NAAT, 6.38% tested positive for chlamydia and 0.64% were diagnosed with PID within 12 months. Overall, the adjusted risk of PID within 12 months of a chlamydia test was higher following a positive test (AOR 1.40 (95%CI 1.18–1.67) and in older women (25–34 years 1.36 (1.23–1.49)) and lower in women tested using a NAAT (0.87 (0.78–0.96)) and in the more recent time interval (2000/2001 0.89 (0.80–0.99)). In women with a positive test, and presumably treated infection, the risk of PID did not differ by test type (1.25 (0.87–1.79)). In women with a negative test, the risk of PID was lower following a NAAT (0.84 (0.75–0.93)).

Conclusion Women with a negative result from a non-NAAT chlamydia test have a 16% higher risk of PID by 12 months compared to women with a negative result from a NAAT. This is presumably due to the increased proportion of false negative tests with the less sensitive non-NAATs. This study quantifies the health impact of using poorer performing chlamydia diagnostic tests and provides further evidence for phasing them out.