The FIND’EM study evaluates, by means of a nationwide stepped wedge cluster randomised trial, the effects of the ferritin-guided donation interval policy for whole blood donors on (1) Hb and ferritin levels, (2) low-Hb deferral, (3) donor return rates and (4) donor health (i.e. physical and mental wellbeing and iron deficiency- and donation-related symptoms) [14]. The gene discussed is GSTM1; the disease is Iron deficiency anemia.