Influence of study population on the identification of risk factors for sexually transmitted diseases using a case-control design: the example of gonorrhea.

The population prevalence of many sexually transmitted diseases (STDs) is low. Thus, most epidemiologic studies of STDs are conducted among STD clinic populations to maximize efficiency. However, STD clinic patients have unique sociobehavioral characteristics. To examine the potential effect of study population on identification of risk factors, the authors compared 1) STD clinic patients with a random digit dialing telephone sample, 2) general population cases with random digit dialing controls, and 3) STD clinic cases with STD clinic controls (Seattle, Washington, 1992-1995). Risk factors for gonorrhea identified among STD clinic patients formed a subset of those identified in the general population. In both populations, risk decreased with age (odds ratio for the general population (OR(GP)) = 0.4, 95% confidence interval (CI): 0.22, 0.59; odds ratio for the clinic population (OR(clinic)) = 0.5, 95% CI: 0.30, 0.81) and was increased among Blacks (OR(GP) = 15.5, 95% CI: 4.93, 49.0; OR(clinic) = 10.5, 95% CI: 4.51, 24.68) and persons whose partner had been jailed (OR(GP) = 5.4, 95% CI: 2.07, 13.9; OR(clinic )= 3.1, 95% CI: 1.32, 7.30). Additional factors associated with gonorrhea in the general population included secondary education (OR = 0.3, 95% CI: 0.11, 0.70), anal intercourse (OR = 10.5, 95% CI: 2.01, 54.7, STD history (OR = 5.9, 95% CI: 1.76, 19.5), meeting partners in structured settings (OR = 0.2, 95% CI: 0.09, 0.50), no condom use (OR = 3.2, 95% CI: 1.30, 7.89), and divorce (OR = 3.6, 95% CI: 1.07, 11.9). Risk factors identified in STD clinics will probably be confirmed in a general population sample, despite overcontrolling for shared behaviors; however, factors associated with both disease and STD clinic attendance may be missed.