Alkaline Phosphatase
ALP· also: Alk Phos, Alkaline Phosphatase
Clinical Overview
ALP is an enzyme found in the liver (bile ducts), bone, intestine, kidneys, and placenta. Elevated ALP indicates either liver cholestasis (bile duct obstruction) or increased bone turnover. Distinguishing between these sources requires GGT (elevated in liver disease but not bone disease) and other tests.
Why This Test Matters
ALP is the primary marker of cholestatic liver disease (bile duct obstruction, primary biliary cholangitis, primary sclerosing cholangitis). In bone disease, ALP rises due to increased osteoblast activity — useful for monitoring Paget's disease and bone metastases. A markedly elevated ALP with normal ALT/AST suggests bone rather than liver origin.
Reference RangesWHO/IFCC standards
| Age Group | Sex | Reference Range | Unit | Notes |
|---|---|---|---|---|
| Adults (18–64) | All | 44 – 147 | U/L | — |
| Children / Pediatric | All | 100 – 400 | U/L | Physiologically higher during growth spurts |
| All ages | Female | 100 – 500 | U/L | Third trimester of pregnancy (placental ALP) |
Also reported in: IU/L.
What Causes Abnormal Results?
High ALP Causes
- Liver: cholestasis, primary biliary cholangitis, PSC, drug-induced cholestasis, liver metastases
- Bone: Paget's disease, bone fractures healing, bone metastases, hyperparathyroidism, rickets
- Physiological: pregnancy (placental ALP), growth in children and adolescents
- Hyperthyroidism
Low ALP Causes
- Hypothyroidism
- Pernicious anemia
- Zinc deficiency
- Rare: hypophosphatasia (genetic ALP deficiency)
Signs & Symptoms to Watch For
How to Prepare for This Test
Fasting for 2–4 hours is preferred (eating a fatty meal raises intestinal ALP). A morning specimen is ideal. Inform your doctor if you are pregnant or recently had a bone fracture.
Factors That Can Affect Results
- Age and sex (adolescents have physiologically high ALP during bone growth)
- Pregnancy (placental ALP elevates total ALP)
- Eating (intestinal ALP temporarily rises after meals)
- Blood group B and O (higher intestinal ALP)
- Medications: anticonvulsants (phenytoin, carbamazepine) increase ALP
Related Topics
Frequently Asked Questions
How do I know if a high ALP is from bone or liver?
The easiest way is to check GGT: GGT is elevated in liver disease but normal in bone disease. ALP isoenzyme electrophoresis can specifically identify the liver versus bone fraction. If ALP is elevated with elevated GGT, the source is the liver. If ALP is elevated with normal GGT in a patient with bone pain or known cancer, bone metastases or Paget's disease should be considered.
Is a high ALP always abnormal?
No. Physiologically elevated ALP is normal in growing children and adolescents (due to active bone growth), in the third trimester of pregnancy (placental ALP), and transiently after eating (intestinal ALP). These sources should always be considered before pursuing liver or bone investigations.