Potential for Selection Bias in Studies of the Association of Hormonal Contraception and Chronic Vulvar Pain.

Abstract

Background: Hormonal contraceptive use is common among reproductive-aged women, but research evaluating its etiological relationship to vulvodynia remains mixed. We sought to evaluate this association and examine the potential for bias due to care-seeking behavior. Materials and Methods: We conducted a case-control study of women recruited from a large health care network database from 2008 to 2011. Of 26,455 eligible respondents, 1168 met the case definition for chronic vulvar pain (CVP). We matched each case to three controls by age and used conditional logistic regression to calculate odds ratios (ORs) for prior hormonal contraception (HC) use and CVP, stratifying cases by whether or not they sought care for their vulvar pain. We also simulated the influence of potential biases due to care seeking, using parameters based on this dataset. Results: HC users had higher odds of CVP (adjusted OR = 2.6, 95% confidence interval [CI]: 2.2-3.2). Effect estimates were stronger when cases were restricted to care seekers (adjusted OR = 2.9, 95% CI: 2.2-3.7). Effect estimates decreased slightly as time increased between HC initiation and pain onset. Our simulations suggested that effect estimates may be spuriously strengthened when cases are restricted to care-seeking women, but controls are recruited from the general population. Conclusions: Our results suggest an association between antecedent HC use and CVP that is potentially spuriously strengthened in case-control studies when cases are restricted to care seekers but controls are not.