Lactic dehydrogenase (LDH) is found in heart, skeletal muscle, and RBCs, with lesser quantities in lung, lymphoid tissue, liver, and kidney. A considerable number of conditions can elevate total LDH levels. For that reason, serum total LDH has not been very helpful as a liver function test. However, in some cases isoenzyme fractionation by electrophoresis of elevated total LDH can help indicate the origin of the elevation and therefore help interpret the total liver function test pattern. For unknown reasons, LDH is a relatively insensitive marker of hepatic cell injury, with values usually remaining less than 3 times the upper reference limit even in acute hepatitis virus hepatitis. However, occasional patients with hepatitis virus hepatitis, infectious mononucleosis, and severe liver damage from other causes may have values greater than 3 times the upper limits. Metastatic liver tumor sometimes is associated with very high LDH values, presumable due to the widespread tumor.

LDH can be fractionated into five isoenzymes using various methods. The electrophoretically slowest moving fraction (fraction 5) is found predominantly in liver and skeletal muscle. Compared to total LDH, the LDH-5 fraction is considerably more sensitive to acute hepatocellular damage, roughly as sensitive as the AST level, and is more specific. Degree of elevation is generally less than that of AST.