Alkaline phosphatase (ALP) is found in a wide variety of tissues, including the liver, bone, intestine, and placenta. The reference value for ALP depends on age and gender, with higher levels in childhood, adolescence, and pregnancy. A typical reference range in an adult is 25 to 100 U/L. In adults, the source of an elevated ALP is the liver, bone, or medication (Table 15-5). Typically, hepatic elevations of ALP are suggestive of cholestatic liver disease or biliary tract dysfunction. Mild ALP elevations (one to two times above reference range) can occur with parenchymal liver disease, such as hepatitis or cirrhosis. Marked ALP elevations occur with infiltrative liver disease or biliary obstruction, intrahepatic or extrahepatic. A persistently elevated ALP level can be an early sign of primary biliary cirrhosis. In cholestatic liver disease, bilirubin and gamma-glutamyltransferase (GGT) levels are increased as well, with less prominent elevations in aminotransferase levels. To confirm a hepatic source of an elevated ALP level, one can simultaneously measure GGT, which is elevated in obstructive liver disease but not with bone disease. Imaging studies of the liver, by sonography or computed tomography (CT), can define an anatomic basis for obstruction in the setting of an elevated ALP level of hepatic origin.
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