When applied in a screen-and-treat strategy to control iron deficiency in paediatric populations with a prevalence of iron deficiency of <37.4% (which covers most settings in developing countries), our data suggest that a diagnostic rule of 0.038689 [Hb] + 0.00694 [whole blood ZPP] > 5.93120 can correctly identify 90% of children with iron deficiency, and correctly rule out iron deficiency in 37.4%–53.7% of children who are tested, depending on the true prevalence. Here, GSTM1 is linked to nutritional disorder.