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Refer case study-Elevated serum Alkaline Phosphatase level
Serum alkaline phosphatase (ALP) is a member of a family of zinc metalloprotein enzymes that function to split off a terminal phosphate group from an organic phosphate ester. Many conditions can cause increases of ALP activity in serum,the most common being obstructive liver disease and metabolic bone disease. An increase of the liver or particularly the bone isoform in serum can provide valuable diagnostic information. 
Bone specific alkaline phosphatase isoenzyme is elevated as a result of increased osteoblastic activity. The normal range for serum ALP is three to five times higher in children than in adults owing to increased osteoblastic activity in active bone growth. 
The highest total ALP values have been attributed to an increased bone isoenzyme level due to Paget disease or rickets/osteomalacia.Causes of high bone ALP include bone growth, healing fracture, acromegaly,osteogenic sarcoma, or bone metastases, leukemia, myelofibrosis, and rarely myeloma; so ALP is  also used as a tumor marker. Hyperthyroidism, by its effects upon bone, may also elevate ALP.
In the given case, since the serum GGT is normal, the serum ALP is not of liver origin. Serum ALP and GGT are indices of cholestasis (blockage to bile flow) in liver disease; hence both should be elevated if the ALP is of liver origin.
The elevated ALP over normal for age in this patient  is due to healing of the bone fracture.
A hemolyzed serum sample does not significantly alter the alkaline phosphatase activity; however, it would falsely increase the serum potassium, serum lactate dehydrogenase, and serum aspartate Transaminase (AST; SGOT) because these enzymes are located within red blood cells.
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