Comparison of STD prevalences in the Mwanza, Rakai, and Masaka trial populations: the role of selection bias and diagnostic errors

Abstract

Objectives: To assess bias in estimates of STD prevalence in population based surveys resulting from diagnostic error and selection bias. To evaluate the effects of such biases on STD prevalence estimates from three community randomised trials of STD treatment for HIV prevention in Masaka and Rakai, Uganda and Mwanza, Tanzania. Methods: Age and sex stratified prevalences of gonorrhoea, chlamydia, syphilis, HSV-2 infection, and trichomoniasis observed at baseline in the three trials were adjusted for sensitivity and specificity of diagnostic tests and for sample selection criteria. Results: STD prevalences were underestimated in all three populations because of diagnostic errors and selection bias. After adjustment, gonorrhoea prevalence was higher in men and women in Mwanza (2.8% and 2.3%) compared to Rakai (1.1% and 1.9%) and Masaka (0.9% and 1.8%). Chlamydia prevalence was higher in women in Mwanza (13.0%) compared to Rakai (3.2%) and Masaka (1.6%) but similar in men (2.3% in Mwanza, 2.7% in Rakai, and 2.2% in Masaka). Prevalence of trichomoniasis was higher in women in Mwanza compared to women in Rakai (41.9% versus 30.8%). Herpes simplex virus type 2 (HSV-2) seroprevalence and prevalence of serological syphilis (TPHA+/RPR+) were similar in the three populations but the prevalence of high titre syphilis (TPHA+/RPR >1:8) in men and women was higher in Mwanza (5.6% and 6.3%) than in Rakai (2.3% and 1.4%) and Masaka (1.2% and 0.7%). Conclusions: Limited sensitivity of diagnostic and screening tests led to underestimation of STD prevalence in all three trials but especially in Mwanza. Adjusted prevalences of curable STD were higher in Mwanza than in Rakai and Masaka. P revalences of sexually transmitted diseases (STD) are often compared between populations in order to gain a better understanding of STD and HIV epidemiology or to determine which control strategy may be most effective in a given epidemiological situation. Three community randomised trials of STD treatment as HIV prevention strategies have been conducted in east Africa in Mwanza, Tanzania, and Masaka and Rakai, Uganda.1-3 In the Mwanza trial, improved STD case management was associated with a 38% reduction in HIV incidence. 4 In contrast, in Uganda a trial of information, education, and communication coupled with improved STD case management had no impact on HIV incidence in Masaka district 5 and STD mass treatment had no impact on HIV incidence in Rakai district.3 Baseline prevalence of gonorrhoea and chlamydia was about 5% or less in the populations while that for serological syphilis was 10-15%. Various hypotheses have been suggested to explain the apparently contrasting results, including that the Mwanza and Rakai trials differed with respect to the stage of the HIV epidemic, that the proportion of ulcers due to herpes simplex virus type 2 (HSV-2) was higher in Rakai than Mwanza, and that differences in the interventions-that is, continuous treatment of symptomatic STD (as in Mwanza) may have a larger effect than treating all STD periodically (a...

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