Topic: Organ function tests
Question: Discuss in detail about liver function tests to identify hepatic dysfunction, hepatocellular injury, and cholestasis.
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Liver function tests (LFTs) measure enzymes and proteins synthesized by the liver. They are used to detect hepatic dysfunction, determine the underlying etiology, and monitor disease progression or treatment response.
Tests for hepatocellular injury include:
– Alanine aminotransferase (ALT): Sensitive marker of hepatocyte injury. Elevated in hepatitis, cirrhosis, drug-induced liver injury. Normal <40 U/L.
– Aspartate aminotransferase (AST): Also indicates hepatocyte damage. May be mildly-moderately elevated in liver disease. AST > ALT suggests alcoholic or ischemic hepatitis. Normal <40 U/L.
– Alkaline phosphatase (ALP): Markedly elevated in bile duct obstruction (cholestasis) from tumours or gallstones. Mild-moderate elevation in hepatitis. Usually normal <135 U/L.
– Gamma-glutamyl transferase (GGT): Also indicates cholestasis or bile duct disease. More specific than ALP, useful in determining if ALP elevation is liver-derived. Normal <65 U/L.
– Bilirubin: Indirect (unconjugated) bilirubin is elevated in hemolytic anemia or Gilbert’s syndrome. Direct (conjugated) bilirubin rises in cholestasis or hepatocellular disease. Total bilirubin >17 μmol/L is abnormal.
– Albumin: Synthesized only in the liver. Decreased in liver failure, advanced cirrhosis or severe acute hepatitis. Normal 35-50 g/L.
– Prothrombin time (PT)/INR: Assesses liver’s ability to produce clotting factors. Prolonged in advanced liver disease with decreased protein synthesis. Normal PT 11-15 sec; INR 0.8-1.2.
Other tests: Ferritin and iron studies may show excess in hemochromatosis. Viral hepatitis serology determines if hepatitis is present. Autoimmune markers like ANA detect autoimmune hepatitis.
Patterns of LFT abnormalities can indicate the likely etiology:
– Hepatocellular: Marked ALT/AST elevation with mild or normal bilirubin/ALP. Suggests active hepatitis or toxin exposure.
– Cholestatic: Prominent ALP/bilirubin elevation with mild or normal ALT/AST. Indicates bile duct obstruction or primary biliary disease.
– Mixed: Moderate elevation of ALT/AST and ALP/bilirubin. Can be seen in chronic liver disease, cirrhosis, liver metastases or during hepatic decompensation.
Liver function tests provide a window into the structural and functional integrity of the liver. They are frequently used as a screening tool, to monitor known liver disease, or evaluate non-specific symptoms that could reflect underlying hepatic impairment or failure. Interpretation requires correlation with history, risk factors and other clinical information to determine the likely source of any abnormalities detected on testing or significant trends over serial measurements. They remain a crucial means of assessing liver health and directing appropriate management.
