Children with moderately elevated blood lead levels: a role for other diagnostic tests?

In this study we examined potential limitations of relying exclusively on blood lead (BPb) levels to evaluate children with moderately elevated BPb levels (1.21-2.12 micromol/l, or 25-44 microg/dl). We tested the following hypotheses: 1) such children without elevated erythrocyte protoporphyrin (EP) levels (>=0.62 micromol/l or >/= 35 microg/dl) are unlikely to respond to a chelating agent with a brisk urinary Pb diuresis; 2) those with elevated EP levels, but low hematologic indices consistent with iron deficiency, are also unlikely to respond to a chelating agent with a robust urinary Pb diuresis; and 3) those with elevated EP levels and iron sufficiency are more likely to respond to a chelating agent. To test these hypotheses, we performed retrospective analyses of the relationships between EP concentrations, hematologic indices, and urinary Pb excretion ratios (uPbr) in moderately Pb-poisoned children undergoing the CaNa2EDTA lead mobilization test (Pb-MT). Data from 122 children were available. Urinary Pb excretion was limited in children with an EP <0.62 micromol/l (<35 microg/dl); only 5% (1/21) of Pb-MTs were positive (uPbr >=0.6). In children with an EP >=0.62 micromol/l, low hematologic indices, such as a mean corpuscular hemoglobin (MCH) <23 pg, were associated with relatively little Pb excretion (0/14 positive Pb-MTs). In contrast, 32% (28/87) of Pb-MTs were positive in children with an EP >/= 0.62 micromol/l and iron sufficiency (p<0.01 by chi-square comparison between groups with EP >/= 0.62 micromol/l and either MCH <23 pg or MCH >/= 23 pg). We conclude that only a minority of moderately Pb-poisoned children will demonstrate enhanced urinary Pb excretion in response to chelation therapy. Some of the predicted nonresponders can be readily identified by adding the EP and complete blood count to the panel of tests performed.