Types of liver function tests

Liver function tests, also referred to as "LFTs" are blood tests that assess the state of the liver and the biliary system. Liver function tests are among the most commonly used tests in medicine. LFTs are divided into true tests of liver function, such as serum albumin, bilirubin, and prothrombin time, and tests that are indicators of liver injury or biliary tract disease.

The liver is a storehouse for many enzymes and injury or disease affecting the liver causes release of these enzymes into the bloodstream. Enzyme tests that may indicate liver damage include the aspartate aminotransferase (AST), and alanine aminotransferase (ALT). Other frequently tested liver enzymes that indicate biliary tract obstruction, whether in the liver or in bile channels outside the liver, are alkaline phosphatase (ALP) and gamma-glutamyltranspeptidase (GGT). These enzymes are also produced in other organs; therefore, enzyme measurements are not specific for liver disease alone. In addition to LFTs obtained on routine chemistry tests, the physician may order more specific tests such as viral serologic tests or autoimmune tests, that can determine the specific etiology of a liver disease.

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Normal values for liver function tests are:

• AST (Aspartate aminotransferase): 5 - 40 IU/L
• ALT (Alanine aminotransferase): 5 - 35 IU/L
• ALP (Alkaline phosphatase): 30 - 85 ImU/ml
• GGT (Gamma-glutamyltranspeptidase): 5 - 27 U/L
• Total serum protein: 6 - 8 g/dl *
• Albumin: 3.2 - 4.5.g/dl
• Total serum bilirubin: 0.1 - 1.0 mg/dl
• Indirect bilirubin: 0.2 - 0.8mg/dl
• Direct bilirubin: 0.1 - 0.3mg/dl
• Urine bilirubin: negative
• Total bilirubin in newborns: 1 - 12 mg/dl

(Note: These values may vary with the laboratory performing them. Check with your agency's Laboratory Manual for normal values and for any test preparation requirements.)

*Note also that the term "serum" refers to the blood plasma that remains after the blood sample has clotted and been removed.