Demographic and health factors associated with enrollment in posttrial studies: The women's health initiative hormone therapy trials

Background After clinical trials end, continued follow-up of the assembled cohort often is desirable for additional research. Factors influencing participants decisions to consent to additional follow-up and how these shape posttrial cohorts have not been broadly studied. Purpose We examined how two re-enrollment campaigns and the passage of time altered features of the posttrial cohorts compared with the original Womens Health Initiative (WHI) Hormone Therapy clinical trials. Methods We examined associations that markers of sociodemography, health, lifestyle, and on-trial experiences had with re-enrollment and contrasted the characteristics of successive posttrial cohorts with those of the original enrollees. Results The posttrial enrollment campaigns re-enrolled 81.1% and 82.5% of available women, respectively. Women who re-enrolled tended to have better health characteristics than those not re-enrolled. Compared to women of comparable age in the original cohort, women retained for the second posttrial follow-up less often had a history of cardiovascular disease (odds ratio (OR) = 0.36), hypertension (OR = 0.57), diabetes (OR = 0.59), or measured cognitive deficit (OR = 0.40). These women more often had graduated from high school (OR = 1.72) and had participated in other WHI trials (OR = 1.76). Limitations We have examined experience with creating follow-up cohorts from participants in a single study. Thus, our findings may not apply to other cohorts and protocols. Conclusions Posttrial enrollment in follow-up studies can be successful; however, the characteristics of the resulting cohort may differ substantially from the originally assembled group of trial participants. Collection during the original trial of potential predictors of differential re-enrollment may strengthen interpretation of findings.