LIVER FUNCTIONS TESTS
Alkaline phosphatase (ALP):The ALPs are a group of enzymes that hydrolyze organic phosphate at high pH (10). They are present in most tissues but are in particularly high concentration in the osteoblasts of bone and the cells of the hepatobiliary tract (produced in the membranes of cell lining bile duct and canaliculi), intestinal wall, renal tubules and placenta. In adults, plasma ALP is derived mainly from bone and liver in approximately equal proportions; plasma activities rise in cholestatic liver disease because ALP synthesis is increased, the stimulus for this increased synthesis in patient with liver disease has been attributed to bile duct obstruction (cholestasis) by stone, tumors, intrahepatically by infiltrative disorders or space-occupying lesions.
It is used for many years in differential diagnosis of jaundice. It is increased in both infectious hepatitis (viral hepatitis) and post hepatic jaundice in latter the plasma ALP activity may be up to 10 times the upper normal value, but the rise is usually much greater in case of obstructive jaundice. Serum ALP is found to be normal in haemolytic jaundice.
A raised ALP concentration in the presence of a raised g-glutamyl transferase (GGT) concentration implies that the ALP is of hepatic origin.
Causes of raised plasma alkaline phosphatase activity
Physiological:
During the last trimester of pregnancy, the plasma total ALP activity rises due to the contribution of the placental isoenzyme. Plasma ALP concentration may increase by up to five times and usually returns to normal levels by 1 month postpartum.
In preterm infant plasma total ALP activity is up to 10 times the level in adults, and consists predominantly of the bone isoenzyme.
In children, the total activity increases by about two to five times during the pubertal bone growth shot.
In the elderly, the plasma bone isoenzyme activity may increase slightly.
Pathological:
Bone diseaseRickets and osteomalacia.
Paget's disease of bone.
Secondary malignant deposits in bone.
Osteogenic sarcoma.
Primary hyperparathyroidism with extensive bone disease .
Secondary hyperparathyroidism.
Healing fractures, Rheumatoid Arthritis.
Liver disease
Intrahepatic or extrahepatic cholestasis.
Space-occupying lesions tumors, granulomas and other causes of hepatic infiltration.
Inflammatory bowel disease: the gut ALP isoenzymes can be increased in ulcerative colitis.
Malignancy: bone or liver involvement or direct tumors production.Possible causes of low plasma alkaline phosphatase activity
A low plasma ALP concentration may be cause by the following:Arrested bone growth (Achondroplasia), Hypothyroidism, Sever vitamin C and vitamin B, Magnesium and zinc deficiency.
How are the isoenzymes of ALP used in diagnosis?
Isoenzymes of ALP are used to distinguish between liver and bone disease.
Isoenzymes are most easily differentiated by heat stability test and by electrophoresis.
ALP isoenzyme produced in liver (ALP1) is heat stable.
ALP isoenzyme produced in bone (ALP2) is inactivated by heat.
Detection of isoenzymes helps differentiate the source of pathologic condition associated with elevated total ALP.
ALP 1 is expected to be higher in liver.