Use of urinary lead concentration in interpretation of the EDTA mobilization test

In young children with plumbism who receive an EDTA mobilization test (EMT), the need exists for a means of interpreting inadequate or incomplete urine collections. We evaluated the ability of urinary lead concentration (UPbC), whole blood lead (PbB) and erythrocyte protoporphyrin (EP) to predict a positive EMT ratio (greater than or equal to 0.05% of EDTA dose excreted as urine lead). The results of 58 satisfactory EMT's in a group of young (aged 1-3.5 yr) lead-poisoned children were reviewed. A strong and significant correlation was found between UPbC and mobilization ratio (r = .63, p less than 0.0001). Weaker but significant correlation was also found between PbB versus mobilization ratio (r = .45, p = 0.0007) and EP versus ratio (r = .48, p = 0.0003). These data were placed into receiver-operator curves (ROC) to determine their optimal use as discriminants between children with positive versus negative mobilization tests. Ideal ROC characteristics (optimal sensitivity and specificity) were found in the use of a UPbC greater than or equal to 1.0 mcg/ml; at this cutoff a sensitivity of .80, specificity of .89 and positive predictive value of .90 were found. ROC's for PbB and EP demonstrated poor discriminating abilities. Our results suggest that in the absence of a satisfactory urine collection, urinary lead concentration serves as an acceptable indirect indicator of mobilization ratio. A urinary lead concentration of greater than or equal to 1.0 micrograms/ml most accurately identifies children who will have a positive mobilization ratio.