Specific gravity, early measurement

PbU as a measure of spontaneous, i.e., unstimulated, Pb excretion has not typically been a Pb-exposure biomarker with wide laboratory or diagnostic popularity for various reasons. First, early efforts at analytical measurement were plagued by various technical problems of matrix complexity, sensitivity, and specificity along with questions about reliable sample collection protocols and the most reliable way to index a PbU measurement 24-hour collections in outpatients, 24-hour collections adjusted for specific gravity, PbU level/liter urine, and PbU excretion rate per unit mass of creatinine. PbU as an adjunct measure in chelation therapy or provocative Pb chelation for evaluating the toxicologic ally active fraction has been less of a problem, inasmuch as the amounts of PbU were much higher than encountered in spontaneous excretions. 

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