Liver Function Tests (LFTs): Decoding ALT, AST, and Bilirubin

 

The Enzymes: ALT vs. AST (Location Matters)

When liver cells are damaged, they leak enzymes into the bloodstream. As biochemists, we look at two main players

  • ALT (Alanine Transaminase)

    • Function: Transfers amino groups from Alanine

    • Location: Found primarily in the Cytoplasm of liver cells

    • Specificity: It is highly specific to the liver. If ALT is high, the liver is almost certainly the culprit

  • AST (Aspartate Transaminase)

    • Function: Transfers amino groups from Aspartate

    • Location: Found in both the Cytoplasm and the Mitochondria. It exists in the liver, heart, and skeletal muscles

    • Specificity: Less specific; a high AST alone could mean muscle injury or heart issues

Clinical Interpretation: Reading the Patterns

  • Liver Injury: High ALT & High AST

  • Muscle/Heart Injury: High AST with Normal ALT

  • The "Alcoholic" Signature (De Ritis Ratio)

    • In alcoholic liver disease, AST is typically double the ALT (2:1 ratio)

    • Biochemical Reason: Alcohol specifically destroys the Mitochondria of liver cells, releasing the massive stores of mitochondrial AST into the blood, while ALT (cytoplasmic) rises less dramatically

Bilirubin: The Journey from Spleen to Bile

Bilirubin is the yellow breakdown product of old Red Blood Cells (RBCs). Its journey tells us where the problem lies:

  1. Indirect Bilirubin (Unconjugated): Produced in the Spleen from RBC breakdown. It is water-insoluble and travels in the blood bound to Albumin

  2. In the Liver: The liver detaches it from Albumin and links it with Glucuronic Acid. This process (Conjugation) makes it water-soluble

  3. Direct Bilirubin (Conjugated): Now soluble, it is excreted into the bile and stored in the Gallbladder

 
GGT and Albumin: The Duct and The Factory

  • GGT (Gamma-Glutamyl Transferase): Located in the lining of the bile ducts. High GGT often points to biliary obstruction or alcohol use

  • Albumin: The liver is the sole factory for Albumin synthesis

    • Low Albumin indicates chronic liver failure (Cirrhosis), implying the factory has lost its manufacturing capacity over a long period

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