Alkaline Phosphatase (ALP)

General Information

Alkaline phosphatase (ALP) is an enzyme present in many tissues throughout the body, with particularly high activity in the liver, bone, intestine and placenta. In clinical practice, an elevated ALP most commonly reflects either hepatobiliary disease (due to cholestasis or biliary obstruction) or increased osteoblastic activity in bone disorders such as growth, healing fractures or metabolic bone disease. Measurement of gamma-glutamyl transferase (GGT) is recommended as a frontline test to help differentiate a liver source from a bone source, as GGT is typically raised in hepatobiliary disease but normal in isolated bone disorders. ALP isoenzyme analysis can further identify the tissue origin but is rarely required in routine practice and is usually reserved for cases where the clinical picture and initial liver biochemistry are inconclusive.

ALP activity changes markedly throughout childhood and rises rapidly during the adolescent growth spurt, particularly in boys. Increases in bone ALP of 3.4-4.0 times the adult range can be seen during this time. Values of over 1000 U/L in teenage boys or 800 U/L in teenage girls require investigation.

ALP is part of the liver function test.