When checking LFTs, it is important to know that abnormalities may reflect a problem in a different organ system. “Liver function” is actually a misnomer, as only prothrombin time (PT) and albumin are actual measures of the liver’s synthetic function. The other parameters are indirect measures of liver function. Deranged LFTs may represent muscle injury rather than liver damage, and checking a CK level is essential to determining the correct etiology.

The aminotransferases include aspartate transaminase (AST) and alanine transaminase (ALT). These enzymes are found within hepatocytes, are released during cell necrosis, and are the most sensitive tests of he- patocyte injury. Common insults include viral hepatitis, toxic injury (i.e., acetaminophen toxicity), and ischemic injury. The degree of elevation of the aminotransferases does not necessarily correlate with the severity of liver disease. The aminotransferases may rise slightly as a result of cholestatic jaundice because bile is hepatotoxic.

In addition to being present in the liver, AST is also found in cardiac and skeletal muscle, erythrocytes, and kidney and pancreatic tissue. Elevations of AST and ALT may represent muscle injury, with AST rising to a greater degree than ALT. Even strenuous exercise can result in mild elevations in the transaminases.

A CK level may be useful to help distinguish muscle disease from liver disease when aminotransferase levels are elevated. CK is a sensitive marker of muscle injury, as it is found predominately in skeletal and cardiac muscle. CK is composed of three types of dimers: MM is found mostly in skeletal muscle, MB in cardiac muscle, and BB is predominately in the brain and intestine. An elevated CK level is highly sensitive for muscle injury.

Other enzymes may also be helpful when interpreting LFTs. Lactate dehydrogenase (LDH) is present in blood cells and is elevated in hemolytic conditions. It is also released from the liver early in the course of viral
hepatitis, and with space-occupying lesions of the liver. In addition, LDH is found in the lungs, kidney, brain, and skeletal and cardiac muscle.