This assay allows for the quantitative determination of uroporphyrin, heptacarboxyporphyrin, hexacarboxyporphyrin, pentacarboxyporphyrin, coproporphyrin I and coproporphyrin III in human urine samples. It is intended to be used primarily as an aid to diagnosis of suspected acute intermittent porphyria, porphyria variegata, aminolevulinic acid dehydratase deficiency porphyria, hereditary coproporphyria, porphyria cutanea tarda or congenital erythropoietic porphyria and/or for their differential diagnosis.