Liver Function Tests Overview

Liver Function Tests (LFTs)

• LFTs do not directly measure liver function, particularly ALT and AST.
• Abnormalities in LFTs often appear late in the progression of hepatic disease.
• LFT abnormalities primarily reflect hepatocellular injury rather than liver function.

LFTs and Liver Function Assessment

• LFTs help define the type of liver pathology when combined with clinical assessment.

Synthetic Function

• Prothrombin Time (PT): Measures the activity of clotting factors (fibrinogen, prothrombin, and factors V, VII, and X).
  • A normal PT range is 11-14 seconds.
  • A prolonged PT usually indicates liver dysfunction, affecting the balance between coagulation and anticoagulation factors.
• Albumin: Evaluates chronic liver disease and hepatocellular function (protein synthesis).
  • Albumin levels below 2.5-3 g/dL generally suggest a chronic problem.
  • Low albumin levels can be seen in malnutrition, renal losses, and third-spacing.
• International Normalized Ratio (INR), Serum Cholesterol, Plasma Pseudocholinesterase:
  • INR: A prolonged INR indicates impaired liver function.
  • These tests contribute to a more comprehensive evaluation of liver function.

Hepatocellular Injury

• Aminotransferases (AST and ALT):
  • Alanine Aminotransferase (ALT): Primarily found in the liver.
  • Aspartate Aminotransferase (AST): Found in both liver and non-hepatic tissues.
    • Isolated ALT elevation can originate from non-liver sources (heart, muscle, brain, kidney, pancreas, adipose tissue, blood).
  • Mild aminotransferase elevation: Indicates any hepatocyte injury, including fatty liver or infection.
  • Large elevations: Suggest massive hepatic necrosis, fulminant viral hepatitis, drug-induced liver injury, or shock liver.
  • AST/ALT Ratio: May differentiate alcoholic liver disease (AST 2-3 times greater than ALT).
• Lactate Dehydrogenase (LDH):
  • A nonspecific marker of hepatic injury.
  • Extreme elevations are seen in ischemia or drug-induced hepatotoxicity, especially acetaminophen overdose, often accompanied by elevated AST/ALT.
  • Extrahepatic disorders like hemolysis, rhabdomyolysis, tumor necrosis, renal infarction, and acute stroke can also increase LDH.
• Glutathione-S-Transferase (GST):
  • A sensitive marker for liver injury.
  • Short half-life and rapid increase with liver injury.
  • Can be used to monitor improvement in liver function.

Cholestatic Disorders

• Bilirubin:
  • Breakdown product of hemoglobin.
  • Indicates liver's excretory function.
  • Normal total bilirubin is less than 1.5 mg/dL.
  • Jaundice is usually visible when total bilirubin persistently exceeds 3 mg/dL.
  • Direct Bilirubin (Conjugated): Water-soluble, elevated in bile duct blockage or obstruction.
  • Indirect Bilirubin (Unconjugated): Lipid-soluble, elevated in hemolysis or cirrhosis.
• Hepatic Clearance:
  • Alkaline Phosphatase (AP):
    • Mild elevations may be within normal range.
    • A nonspecific marker found in extrahepatic tissues.
    • Values 2-4 times normal are seen in cholestatic disease.
    • Can indicate intrahepatic or extrahepatic biliary obstruction.
  • Y Glutamyl Transpeptidase (GTTP):
  • 5' Nucleotidase (5'-NT):

Interpretation of Liver Function Tests

• In liver failure, improving LFT values may not indicate actual improvement. It could suggest that there are no more functional hepatocytes.

This quiz covers key concepts related to Liver Function Tests (LFTs), including their role in assessing liver pathology and synthetic function. It also discusses the implications of measurement parameters like Prothrombin Time and Albumin levels in diagnosing liver conditions.