Alkaline Phosphatase

Alkaline Phosphatase (ALP) is an enzyme that hydrolyzes phosphate esters at an alkaline pH. It is mainly found in:

• Liver (bile canalicular membrane)
• Bone (osteoblasts)
• Placenta
• Intestine

1️⃣ Hepatobiliary Disease (Most Important)

ALP rises markedly in cholestasis.

Increased in:

• Obstructive jaundice
• Intrahepatic cholestasis
• Gallstones
• Carcinoma head of pancreas
• Hepatic tumors

🔎 Significance:

• ALP is more elevated in obstructive (cholestatic) liver disease than in hepatocellular injury.
• Often rises along with GGT and 5′-nucleotidase.

2️⃣ Bone Disorders

ALP reflects osteoblastic activity (bone formation).

Increased in:

• Rickets
• Osteomalacia
• Paget’s disease
• Hyperparathyroidism
• Healing fractures
• Bone metastasis

🔎 Significance:

• High ALP with normal liver markers suggests bone origin.

3️⃣ Physiological Elevation

ALP can be increased in:

• Growing children (active bone growth)
• Pregnancy (placental ALP)
• Third trimester of pregnancy

4️⃣ Decreased ALP (Rare)

Seen in:

• Hypophosphatasia
• Severe malnutrition
• Hypothyroidism
• Pernicious anemia

Differentiating Liver vs Bone Cause:
Condition ALP GGT 5′-NT
Liver disease ↑ ↑ ↑
Bone disease ↑ Normal Normal
Normal Value

Adults: 40–125 IU/L (may vary by lab)
Higher in children and adolescents.