1 Department of Pediatrics, Baltimore City Hospitals, 4940 Eastern Avenue, Baltimore, Md. 21224
I am glad to have the opportunity to respond to Dr. Metcalf's letter to you concerning erythrocyte protoporphyrin tests. A clear distinction should be made between blood lead measurements and measurements such as erythrocyte protoporphyrin tests. Whole blood lead concentration primarily reflects current and recent absorption of lead, but is not a measure of toxicity, per se. Protoporphyrin is one of the heme metabolites which accumulates when heme synthesis is inhibited by lead and so provides a sensitive index of increasing adverse metabolic effect, as the concentration of lead in the tissues increases.