Alkaline Phosphatase (ALP)
Alkaline phosphatases (ALP) are group of enzymes that hydrolyze organic phosphates at high pH (9 – 10.5)
1- Main Physilogical Sources of ALP:
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Cells of hepatobiliary tract (hepatocytes adjacent to the biliary canalculi) Activity in the liver is localized on those parts of the cell membrane of the parenchymal cell adjoining the biliary canalculi
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Osteoblasts of bone: The function of ALP is important for bone calcification.
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placenta
Clinical significance of increased serum ALP activities:
1-Physiological increase of ALP:
During periods of active bone growth in infancy and at puberty.
- In children under 3 years, total ALP activity is increased up to 2.5 times the upper limit
- Increased twice, during the third trimester of pregnancy (placental ALP).
2- Pathological increase of ALP:
A- Bone causes (due to increased osteoblastic activity):
- Paget’s Disease of bone Marked increase , may be 10 – 25 folds
- Primary osteogenic tumors
- Secondary malignant deposits in bone e.g. cancer prostate causing deposits in bones (osteoblastic tumor)
- Rickets & osteomalacia (vitamin D deficiency)
- Primary & secondary hyperparathyroidism (increased PTH)
- Healing of bone fractures
B-Hepatobiliray tract: liver diseases with involvement of biliary tract
- Obstructive jaundice (intrahepatic or extrahepatic)
- Extrahepatic cholestiasis : (Marked increase, up to 10 – 12 folds)
- Intrahepatic cholestiasis: (Moderate increase , ~ 3 -5 folds)
- hepatitis: Viral, toxic & alcoholic
- Mild to moderate increase, less than 3 folds (ALP may be may be normal)
Note
Clinical Hint: Compare to what degree the ALT and ALP are raised. A greater than 10-fold increase in ALT and a less than 3-fold increase in ALP suggests a predominantly hepatocellular injury.
A less than 10-fold increase in ALT and a more than 3-fold increase in ALP suggests cholestasis.
If ALP is elevated due to a bone disease: In this case, GGT is normal i.e. GGT is used to differentiate increased ALP is due to bone or hepatobiliary disease