The original INTERVAL trial randomly assigned more than 45,000 blood donors in the United Kingdom to different donation frequencies and found that frequent donors give more blood but are more likely to be iron deficient over a two year period. The trial was extended another two years in order to evaluate the long-term effects of frequent donations on donor health and the effectiveness of more intensive donation reminders. In the trial extension, 10,843 men and 9,914 women were randomly assigned to receive either routine or more intensive reminders to donate blood based on their original inter-donation intervals (men 12, 10 and 8 weeks; women 16, 14 and 12 weeks). More intensive donation reminders significantly increased blood collected by a mean of 0.11 units per year in men and 0.06 units per year in women—which approximates to an extra 75,000 units of blood per year with the current donor base of approximately 900,000 in the U.K. For each week the inter-donation interval was shortened, blood collection increased by a mean of 0.23 units and 0.14 units per year in men and women, respectively (p<0.001 for both), but donors were more likely to be deferred for low hemoglobin and had lower levels of hemoglobin and ferritin. Blood collection services may be able to increase the blood supply by shortening donation intervals and/or more frequent reminders as long as donors maintain sufficient iron stores and hemoglobin levels.
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