Alkaline phosphatase (ALP) is an enzyme most abundant in the bile ducts of the liver and in the bones. So a rise in it is almost always a signal from one of two systems: hepatobiliary (bile stasis) or skeletal.
The key trick: to tell where a high ALP "came from", it is looked at together with GGT. If both are elevated, the source is hepatic (cholestasis); if GGT is normal, it is more likely bone. ALP is often assessed alongside bilirubin.
Reference ranges
The reference depends on the lab, age and sex; a typical range for adults:
| Adults (typical reference) | 40 – 130 U/L |
| Children and adolescents | 2–3× higher — active bone growth |
| Pregnancy (3rd trimester) | raised by placental ALP |
| Hepatic source | ALP ↑ together with GGT (cholestasis) |
| Bone source | ALP ↑ with a normal GGT |
What an elevated level means
An elevated ALP is the most common reason for further investigation. GGT helps distinguish the source.
Hepatic (cholestasis), GGT also elevated: stones or narrowing of the bile ducts, primary biliary cholangitis, drug-induced injury, tumors, bile stasis. Often accompanied by a rise in bilirubin.
Bone (GGT normal): Paget's disease, fracture healing, osteomalacia and vitamin D deficiency, hyperparathyroidism, bone metastases; in children and adolescents — normal growth.
Physiological: the third trimester of pregnancy (placental ALP).
What a low level means
A low ALP is rare but should not be ignored.
Causes: hypophosphatasia (a rare hereditary disease due to mutations in the ALPL gene), marked protein-energy malnutrition, zinc and magnesium deficiency (the enzyme's cofactors), hypothyroidism, severe anemia. A persistently low ALP alongside fractures or bone problems is a reason for targeted evaluation.
How the test is done
ALP is part of a venous blood biochemistry panel. It is best tested fasting: after a fatty meal the intestinal ALP fraction can rise temporarily (especially in people with blood groups I and III).
ALP in isolation is not very informative. It is interpreted in context: with GGT (liver or bone), bilirubin and the liver enzymes ALT/AST. For a bone source, the bone ALP fraction is measured.
What affects the value
- Food intake (a fatty meal) — temporarily raises the intestinal fraction; test fasting.
- Age — in children and adolescents ALP is physiologically higher (bone growth).
- Pregnancy — raised by the placental fraction.
- Deficiency of vitamin D, zinc, magnesium — affects the level.
- Medications: some antibiotics, anticonvulsants, hormone therapy.
- Blood group (I, III) — higher intestinal ALP after eating.
Assess liver status
If ALP is elevated together with liver markers, assess the liver (FibroTest accounts for GGT and bilirubin):
Frequently asked questions
How do I tell if ALP is high from the liver or from bone?
The main guide is GGT. If GGT is elevated together with ALP, the source is hepatic (bile stasis, cholestasis). If GGT is normal, it is most likely bone (growth, fracture, Paget's disease). In doubtful cases the bone ALP fraction is measured.
ALP is high but the liver doesn't hurt — is it dangerous?
Not necessarily. A moderate rise can be physiological: in children and adolescents (bone growth) and in the third trimester of pregnancy (placental ALP). The significance is determined by GGT, bilirubin, ALT/AST and the clinical picture, not by a single number.
What does a low alkaline phosphatase indicate?
A low ALP is rare and often goes unnoticed. Causes include zinc and magnesium deficiency, poor nutrition, hypothyroidism, and the rare hereditary disease hypophosphatasia. A persistently low ALP with bone problems is a reason for targeted evaluation.
Related terms
Sources
- MedlinePlus (NIH) — ALP (Alkaline Phosphatase) Blood Test
- Clinical Methods (NCBI) — Alkaline Phosphatase and Gamma-Glutamyltransferase
- MedlinePlus (NIH) — ALP Isoenzyme Test