Zinc protoporphyrin and iron-deficient erythropoiesis

Iron-deficient erythropoiesis may occur in patients with adequate levels of storage iron as well as those with tissue iron deficiency. Here we compare two methods of detecting iron-deficient erythropoiesis. The measurement of percent hypochromic cells in the full blood count provides a direct indicator of iron-deficient erythropoiesis. The zinc protoporphyrin (ZPP) determination is simple, precise and reproducible, and also appears to provide a sensitive index of iron-deficient erythropoiesis. There was a significant correlation between ZPP levels and percent hypochromic cells in patients with iron deficiency anaemia, rheumatoid arthritis and with patients with renal failure undergoing dialysis and receiving erythropoietin. However in the latter group ZPP levels were raised in almost all patients, suggesting that there may be interference by other metabolites in the assay. This may be overcome by washing the red cells before assay, but the procedure becomes cumbersome. If the laboratory is equipped to determine percent hypochromic cells during the blood count this direct measure of iron-deficient erythropoiesis dispenses with the need to determine ZPP. Otherwise ZPP determinations on washed cells may be of diagnostic use.