Liver function is assessed by a battery of tests which generally include a minimum of plasma bilirubin, albumin, and the enzymes alanine transaminase, aspartate transaminase and alkaline phosphatase. Individual tests have less value than consideration of the results as a whole.
Biochemical measures of liver function commonly assess:
- hepatic anion transport - principally, serum bilirubin; less than 5% of serum bilirubin is normally conjugated.
- abnormal protein synthesis:
- serum albumin - hypoalbuminaemia in chronic liver injury
- prothrombin time - may be increased because of failure to absorb fat-soluble vitamin K in cholestasis - Factors II (prothrombin), VII, IX and X are vitamin K dependent; or, from impaired synthesis of coagulation factors - as above plus Factor V and fibrinogen. A raised PT due to cholestasis can generally be corrected by the addition of parenteral vitamin K
- serum immunoglobulins - usually increased in chronic liver disease. IgM predominantly increased in primary biliary cirrhosis; IgG in chronic autoimmune hepatitis.
- liver enzyme tests:
- cytoplasmic and mitochondrial enzymes - raised in hepatocellular damage. ALT is more liver specific than AST and rises more than AST in early hepatocellular injury. AST is raised more in chronic injury.
- membrane-associated enzymes - alkaline phosphatase and gamma glutamyl transferase are anchored to the biliary canaliculus. They are raised in biliary outflow obstruction rather than hepatocellular damage.
- miscellaneous - anti-mitochondrial antibodies in primary biliary cirrhosis; increased plasma lipids in cholestasis; serum urea may be reduced in severe hepatic disease