Protoporphyrin is an inter- mediate in the pathway to heme synthesis. Under conditions in which heme synthesis is impaired, protoporphyrin accumulates within the red cell. This reflects an inadequate iron supply to erythroid precursors to support hemoglobin synthesis. Normal values are <30 μg/dL="" of="" red="" cells.="" In="" iron="" deficiency,="" values="">100 μg/dL are seen. The most common causes of increased red cell protoporphyrin levels are absolute or rela- tive iron deficiency and lead poisoning.