Longitudinal observations of the relationship between free erythrocyte porphyrins and whole blood lead.

Short- and long-term comparison of the variations with time of whole blood lead and free erythrocyte porphyrins (FEP) suggests that changes in FEP are slow and predictable whereas blood lead changes are quite unpredictable. However, when FEP suggests a different clinical category from blood lead, the blood lead is likely to change in the direction predicted by the FEP. Comparison of FEP to blood lead at first contact in 349 children with mild elevation of blood lead reliably predicted which children would still have elevated blood lead six weeks later and which would fall or be normal. The observation of long-term follow-up in four groups of children with various combinations of FEP and blood lead indicated that the follow-up blood lead could be predicted to change in the direction indicated by the FEP measurement. The implications for screening for lead poisoning are discussed.