Urinary Metabolite Testing

Urine Testing · Urine Testing overview

Urinary metabolite testing measures the products of phase I and phase II biotransformation as proxies for the parent substance's absorbed dose. Most BMGVs and BEIs are expressed in terms of metabolites rather than parent compounds.

Phase I oxidation products

Cytochrome P450 oxidation produces hydroxylated metabolites such as o-cresol from toluene, mandelic acid from styrene and trichloroacetic acid from trichloroethylene. These appear in urine within hours of exposure and clear over a shift to overnight window.

Phase II conjugates

Glucuronide, sulphate and mercapturate conjugates make lipophilic metabolites water-soluble for excretion. S-PMA (from benzene via glutathione conjugation) and methylhippuric acids (from xylenes via glycine conjugation) are clinically important examples.

Inter-individual variability

Genetic polymorphism in CYP2E1, GSTM1 and NAT2 produces measurable variability in metabolite ratios between individuals. This is one reason cohort statistics — geometric mean, 90th percentile — are more informative than single results.