Porphyrias are inherited metabolic diseases caused by a defect in a specific enzyme in the heme synthetic pathway. Three autosomal dominant acute hepatic porphyrias (AHP) are defined by the occurrence of acute neurovisceral attacks: acute intermittent porphyria (AIP), the most frequent, variegata porphyria (VP), and hereditary coproporphyria (HCP). The precursors, 5-aminolevulinic acid (ALA) and porphobilinogen (PBG), are involved in the heme biosynthetic pathway. ALA and PBG concentrations are elevated in urine and plasma during an attack of acute porphyrias, such as acute intermittent porphyria (AIP), or porphyria variegata (VP). They are used as markers to screen for these conditions. Urinary ALA can also be elevated in cases of lead poisoning.