After a careful history and physical exam, concern for liver disease prompts many health care providers to order laboratory tests. LFTs are a commonly ordered panel when evaluating for liver disease. LFTs usually consist of aspartate aminotransferase (AST, serum glutamic-oxaloacetic transaminase-SGOT), alanine aminotransferase (ALT, serum glutamic-pyruvic transaminase-SGPT), alkaline phosphatase (AP), albumin, total and conjugated bilirubin, and total protein. However, all of these lab values are not always the best measure of liver function.

LFTs and other enzyme levels can help differentiate between two types of liver injury: (a) cholestasis or obstructive bile duct injury, and (b) he- patocellular injury. Therefore, if bile duct injury is suspected, it is valuable to measure the serum levels of substances that are normally excreted in bile such as alkaline phosphatase, bilirubin and gamma glutamyl transpeptidase. When there is toxic injury to the liver, enzymes that are normally within hepatocytes (i.e., aminotransferases) will be the most telling. However, to evaluate the liver’s synthetic function, it is best to measure serum albumin and prothrombin time (PT), rather than LFTs.

The aminotransaminases include alanine and aspartate. ALT and AST facilitate the conversion of ketogluatarate to pyruvate and oxaloacetate, respectively. However, ALT is found primarily in the liver, whereas AST is found in the liver, erythrocytes, cardiac and skeletal muscle, the kidney, and pancreas. Serum levels of ALT and AST rise when there is either damage to the hepatocytes and other tissues containing transaminases, resulting in the enzymes being released or when there is an increased permeability of tissue, permitting leakage of the enzymes into the circulation. Given the concentration of ALT in the liver, it is more specific to liver damage than AST. The coenzyme for both aminotransaminases is vitamin B₆, so low levels of both enzymes may suggest vitamin B deficiency.

AP is primarily located in the canalicular membrane of the liver cell, so elevated AP levels often indicate obstructive liver disease. The highest
concentrations of AP are in liver and bone (osteoblasts), and lower levels are present in the kidney, gastrointestinal tract, leukocytes, and the placenta. Sepsis and drugs can also affect the levels of AP. Other laboratory values should be considered in conjunction with AP when evaluating for liver function. For example, 5-nucleotidase and gamma-glutamyl transferase increase in liver disease, so coexistent elevations with AP assist in identifying the liver as the source of AP elevation.