If serum alkaline phosphatase (ALP) levels are elevated but the clinical picture does not provide enough information to determine the origin of the excess, ALP isoenzymes are evaluated. The major ALP isoenzymes derive from liver, bone, intestine, and placenta.
ALP isoenzymes may be partitioned by electrophoresis or by exploitation of differences in physical properties on optimal substrates. Electrophoresis has been applied with only modest success. Hepatic and intestinal isoenzymes are easier to differentiate with this method than are hepatic and bone enzymes. Because hepatic ALP is more heat resistant than bone ALP, the most common way to differentiate between these two isoenzymes is by heating the serum to 132.8°F (56°C).
Evaluation of ALP isoenzymes usually focuses on measuring those of hepatic origin not affected by bone growth or pregnancy. These are 5'-nucleotidase, leucine aminopeptidase, and 
-glutamyl transpeptidase.
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