Increased alkaline phosphatase indicates ?
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Increased Alkaline Phosphatase (ALP) - Causes & Significance
Sources of ALP
ALP activity is concentrated in bone, liver, intestine, and placenta. Elevated serum ALP most commonly originates from the liver or bone.
Physiological (Non-pathological) Causes
| Condition | Mechanism |
|---|---|
| Normal childhood growth | Active bone formation by osteoblasts |
| Pregnancy | Placental isoenzyme production |
| Perimenopausal period | Rising levels in women approaching male reference range |
| Post-meal (non-fasting) | Intestinal ALP - can elevate by ~30% for up to 12 hours (especially in Lewis+ blood group B or O secretors) |
| Male sex | Slightly higher than females at baseline |
Pathological Causes
Liver / Biliary (Hepatobiliary isoenzyme)
- Cholestasis - ALP is the most sensitive marker of hepatic metastases among hepatic chemistry tests
- Biliary obstruction (intrahepatic or extrahepatic)
- Primary biliary cholangitis
- Infiltrative liver disease
Bone (Bone isoenzyme - produced by osteoblasts, reflects bone-forming activity)
- Paget disease - one of the most common causes of raised bone ALP in adults
- Bone metastases (osteoblastic)
- Hyperparathyroidism
- Healing fractures
- Osteomalacia / rickets
- Osteogenic sarcoma
Malignancy
- Regan isoenzyme (placental-like) - observed in a small proportion of individuals with malignant disease
Medications
- Oral contraceptives can elevate ALP
Isoenzyme Differentiation by Heat/Inhibition
(Quick Compendium of Clinical Pathology, p. 4)
| Source | Heat/Urea Inhibition | L-phenylalanine Inhibition |
|---|---|---|
| Biliary | + (50%) | - |
| Bone | +++ (90%, "bone burns") | - |
| Placenta | - (0%) | +++ |
| Intestinal | + | +++ |
Key memory aid: "Bone burns" - bone ALP is most sensitive to heat inactivation (90%).
How to Determine the Source of Elevated ALP
If GGT and 5' nucleotidase are within normal limits, the excess ALP is most likely of bone origin. If they are elevated alongside ALP, a hepatobiliary source is implicated.
Decreased ALP (for contrast)
- Hypophosphatasia (inborn deficiency) - classic cause
- Malnutrition
- Wilson disease
- Theophylline or estrogen therapy
- Hemolysis (falsely lowers measured ALP)
Sources: Quick Compendium of Clinical Pathology, 5th edition - sections 1.1.1.3.2 and Liver Function Tests
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