INTERVAL, a large pragmatic randomised trial of blood donors in England, is. studies (especially RCTs) is a lack of consideration for external validity and. The 8-week interval between donations is more than enough time to replace your donated red cells. Your Body – Plasma and Platelets: The body replaces plasma within 24 hours and platelets within 72 hours. As blood donation may cause iron deficiency, the donation interval is a…