Alkaline Phosphatase

The Alkaline Phosphatase test, also known as ALP, ALK PHOS, or ALKP, is used to assess hepatobiliary disease and bone disorders characterized by increased osteoblast activity. Elevated results occur with cholestasis, including biliary obstruction, primary biliary cholangitis, and primary sclerosing cholangitis; with infiltrative or metastatic liver disease; and with bone conditions such as Paget disease, healing fractures, rickets/osteomalacia, and metastases to bone. When increased, concurrent gamma‑glutamyltransferase testing can support a hepatic source. Serial ALP measurements can be used to monitor disease activity or response to therapy when values are elevated. Low ALP is uncommon but may be seen in hypophosphatasia, or rarely with hypothyroidism or pernicious anemia.

Alkaline phosphatase is expressed by biliary epithelium and hepatocellular canalicular membranes and enters the circulation with cellular injury or cholestasis; obstruction of bile flow is associated with marked increases. In the skeleton, ALP is produced by osteoblasts and reflects bone formation and remodeling, leading to higher activity in childhood and after fractures. Placental and intestinal isoenzymes may contribute to serum activity in defined clinical settings. Because ALP is not tissue‑specific, interpretation in isolation can be misleading. Results should be integrated with other laboratory findings and clinical information to determine the source and significance.