Seasonal temperature variation and the rate of donor deferral for low hematocrit in the American Red Cross

BACKGROUND: Hematocrit (Hct) values in healthy adult populations exhibit seasonal variation, with the lowest values occurring in the summer. The extent to which environmental temperature contributes to the seasonal trend in deferral rates for unacceptable Hct in the American Red Cross was further analyzed. STUDY DESIGN AND METHODS: A centralized database of donations during 2002 to 2004, constituting 24.3 million donor presentations, was further characterized. Data on mean monthly temperature in the United States were obtained for the same period from a government agency. Multivariate regression analyses were performed to determine the relationship between Hct deferral rates among blood donors and environmental temperature and donor characteristics. RESULTS: Hct deferral rates were associated with mean monthly temperature in the United States (R(2) = 0.77). The relationship between the Hct deferral rate and environmental temperature was strongest in the region of the country with the highest seasonal variation in temperature, followed by regions with intermediate and low seasonal variation in temperature, respectively. The seasonal pattern in Hct deferral rates occurred in both sexes and across all age groups, with significantly higher Hct deferral rates occurring in June through August compared to other quarters (p < 0.0007). CONCLUSION: There is a significant seasonal pattern in Hct deferral rates that is associated with environmental temperature. The relationship between Hct deferral rates and temperature is strongest in areas of the country with greater temperature variability. The effect of seasonality on Hct deferrals should be taken into account for donor counseling, recruitment, and retention efforts.