130 Alkaline phosphatase (ALP)
Alkaline phosphatase (ALP) is an enzyme that removes phosphate groups from molecules and works best in an alkaline environment. The ALP in adult blood comes mainly from the liver and bile ducts and from bone, where bone-forming cells (osteoblasts) produce it. Smaller amounts come from the intestine and, in pregnancy, the placenta. A raised ALP therefore points either to impaired bile flow (cholestasis) or to increased bone turnover.
Liver or bone?
If ALP is raised together with GGT (gamma-glutamyl transferase) and often bilirubin, the source is the liver or bile ducts. If GGT is normal, the bone is more likely, and bone-specific ALP or isoenzyme tests can confirm it. Vitamin D, calcium, phosphate and parathyroid hormone help evaluate bone causes.
Causes of raised ALP
| Origin | Causes |
|---|---|
| Liver and bile ducts | Gallstones in the common bile duct, tumours of the pancreas or bile ducts, primary biliary cholangitis, primary sclerosing cholangitis, liver metastases, infiltrative diseases (sarcoidosis), drug-induced cholestasis, fatty liver |
| Bone | Growth in children and adolescents, healing fractures, Paget disease of bone, vitamin D deficiency and osteomalacia, hyperparathyroidism, bone metastases |
| Other | Pregnancy (third trimester), after a fatty meal in some people (intestinal ALP), hyperthyroidism |
Low ALP
Low ALP is uncommon. It can occur with zinc or magnesium deficiency, malnutrition, hypothyroidism, after cardiac surgery, with some medicines, and in the rare inherited disorder hypophosphatasia, which causes weak bones and teeth.
if a raised ALP comes with jaundice, fever and chills, or severe pain in the upper right abdomen. These may be signs of bile duct obstruction or infection (cholangitis).
A persistently raised ALP in an adult should be explained: GGT, bilirubin, liver ultrasound and, if bone is suspected, calcium, phosphate, vitamin D and PTH. Itching, unexplained bone pain or weight loss are reasons not to delay.
References
- Kwo PY, Cohen SM, Lim JK. ACG clinical guideline: evaluation of abnormal liver chemistries. Am J Gastroenterol. 2017;112(1):18–35. doi:10.1038/ajg.2016.517
- MedlinePlus. ALP — blood test. U.S. National Library of Medicine. medlineplus.gov/ency/article/003470.htm
This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.
Published on 12 May 2024