Repeat chlamydia testing across a New Zealand district: 3 years of laboratory data

To investigate the impact of repeat chlamydia testing on annual population coverage estimates and to examine repeat testing patterns in a New Zealand district with high chlamydia testing rates.

Chlamydia testing data for 15-44-year-old men and women in a single New Zealand district during February 2008 to January 2011 were analysed. Annual coverage of testing was estimated in two ways, using the number of tests and the number of individuals as the numerator. Rates of repeat testing were calculated using survival analysis.

There were 73 879 tests (12 251 men, 61 628 women) from 41 342 individuals (8437 men, 32 905 women) during 3 years. Coverage estimates in 2010 using the number of individuals as the numerator were 17% lower for men and 26% lower for women than when the number of tests was used (5.9% vs 4.9% for men and 28.7% vs 21.2% for women). The rate of repeat testing was 16.9 per 100 person-years among men (95% CI 16.2 to 17.7) and 31.6 among women (95% CI 31.1 to 32.2). Rates of repeat testing were higher among women, in younger age groups and following a positive rather than a negative baseline test (p<0.001).

Relatively high rates of repeat testing were observed among young women in Waikato district during 2008 to 2010. Estimates of population coverage by test for this group therefore considerably overestimate individual coverage. The findings will inform discussions about improving surveillance to capture more accurate chlamydia testing coverage rates in New Zealand.