ALP isoenzymes are different molecular forms of alkaline phosphatase originating from specific tissues — primarily the liver, bone, intestine, and placenta. Because total ALP can be elevated by any of these sources, identifying which isoenzyme is elevated is essential for pinpointing the organ responsible and guiding further investigation.

Liver ALP elevation points toward hepatobiliary disease; bone ALP elevation suggests conditions of increased bone turnover such as Paget's disease, bone metastases, or hyperparathyroidism; and intestinal ALP can be elevated in certain blood groups and after fatty meals. Placental ALP (including the Regan isoenzyme) is normally found in pregnancy but can indicate certain cancers in non-pregnant individuals.