SERUM ALKALINE PHOSPHATASE- ALP
Pre-test preparation or requirements
• Sample can be drawn at any time. No fasting or special preparation required.
• Sample should taken in plain vial.
Children 0-5 Year: 47- 406 U/l
Adults: 30- 120 U/l
Significance of test
ALP Alkaline phosphatase refers to a family of enzymes that catalyze hydrolysis of phosphate esters at an alkaline pH. ALP is present (in decreasing order of abundance) in placenta, intestine, kidney, bone and liver. In adults, more than 80% of serum ALP activity derives from liver and bone.
In late pregnancy, placental ALP is increased.
In children and adolescents most serum ALP activity originates in osteoblasts and correlates with the rate of bone growth. The serum half life is seven days.
ALP is most useful in diagnosing cholestatic liver diseases. Bile duct obstruction results in increased synthesis of ALP by bile duct epithelial cells and release of ALP into the serum. Alkaline phosphatase may be increased even if only a few small bile ducts are obstructed and serum bilirubin is normal. Serum ALP often exceeds four times the upper limit of normal in extrahepatic and intrahepatic cholestasis. The most common causes of extrahepatic cholestasis are pancreatic cancer, common duct stones and strictures, and primary sclerosing cholangitis.
Intrahepatic cholestasis is usually due to primary biliary cirrhosis or drug reactions (erythromycin, chlorpromazine, estrogens, and methyltestosterone).
Patients with primary sclerosing cholangitis and primary biliary cirrhosis initially have elevated ALP and normal bilirubin levels. Sometimes it is useful to look at the relationship of ALP to bilirubin and lactate dehydrogenase (LD) levels. Osteoblastic bone disease can also increase serum ALP.
The most common bone disorders associated with elevated ALP are; Paget's disease, osteomalacia, hyperparathyroidism, osteogenic sarcoma, and bone metastases.
Low alkaline phosphatase levels have been reported in patients with magnesium deficiency, hypothyroidism, malnutrition, hemolytic anemia, Wilson's disease, post coronary bypass surgery, estrogen replacement therapy, and congenital hypophosphatasia.
Blood transfusion causes transient decreases in ALP, due to chelation of cations by citrate.