Why are alkaline phosphatase levels elevated in cholestasis and how can isoenzyme testing assist in interpretation?

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Cholestasis raises alkaline phosphatase because the bile ducts and canalicular membranes respond to impaired bile flow by upregulating and releasing more ALP into the serum. Since ALP comes from multiple tissues, a high total ALP can also reflect increased bone turnover, so the source isn’t obvious from the total level alone. Isoenzyme testing separates the different ALP forms, allowing you to tell whether the elevated enzyme is primarily hepatic (from liver/biliary tissue) or bone in origin. In cholestasis, the hepatic isoenzyme is disproportionately elevated, whereas the bone isoenzyme would predominate in bone diseases or high bone turnover. Various methods—such as electrophoresis, substrate-specific assays with selective heat inactivation, or immunoassays—can distinguish the hepatic from the bone form. Using this distinction alongside markers like GGT can further clarify the liver versus bone origin, since GGT tends to be elevated with hepatobiliary disease but not with most bone conditions.