Erythrocyte zinc protoporphyrin.

In iron deficiency and lead poisoning, the enzyme ferrochelatase catalyzes the incorporation of zinc, instead of iron, into protoporphyrin IX, resulting in the formation of zinc protoporphyrin (ZPP). In healthy blood donors, there is a good inverse correlation between serum ferritin and ZPP levels. In renal failure patients and in patients with anemia caused by a variety of chronic disorders, two different types of iron deficiency are found: (a) absolute iron deficiency and (b) relative, or functional, iron deficiency. The latter occurs when iron, despite adequate stores, is not delivered rapidly enough to the erythroblasts. ZPP is not only indicative of absolute iron deficiency, but it is also, for now, the best indicator of iron-deficient erythropoiesis, along with the percentage of hypochromic red blood cells. By contrast, serum ferritin and transferrin saturation may not adequately assess functional iron deficiency. Elevated ZPP levels in renal failure patients can be caused by different pathogenetic mechanisms, such as chronic inflammatory disease, lead poisoning, and the presence of uremic factors, all of which could potentially inhibit heme biosynthesis. However, ZPP levels do not consistently predict an erythropoietic response to iron supplementation in maintenance hemodialysis patients, and thus, iron overload during i.v. iron supplementation cannot be detected by measuring ZPP.