Donation frequency of blood donors participating in a prospective cohort study of iron status

BACKGROUND: Blood centers are interested in understanding determinants of frequent blood donation. We hypothesized that participation in uncompensated research could result in higher donation rates. STUDY DESIGN AND METHODS: Donation rates for 2425 subjects from six US blood centers enrolled in the Retrovirus Epidemiology Donor Study‐II Donor Iron Status Evaluation Study were compared to those of nonenrolled donors (n = 202,383). Over 15 months, we compared mean donation rates and adjusted rate ratios (RRs) between enrolled and nonenrolled for three subgroups, first‐time, reactivated, and frequent donors, and donation rates before and after the study enrollment period for frequent donors only. RESULTS: Enrolled donors had higher 15‐month mean donation rates than nonenrolled donors (first‐time, 1.21 [RR = 1.91]; reactivated, 1.68 [RR = 1.83]; frequent, 3.40 [RR = 1.12]). However, frequent donors donated at approximately the same rate after enrollment as they did before enrollment in the study (3.62 per 15 months [RR = 1.12]). CONCLUSION: Donors enrolled in the study donated at a higher rate than nonenrolled donors, but frequent donors remained consistent in their donation frequency both before and after enrollment. Although increased donation rates could have been causally related to study enrollment, we cannot rule out an enrollment bias whereby more committed donors were more likely to enroll in the study.