Liver Function Tests (LFTs) – Notes

Definition
Liver function tests (hepatic panel) are a group of blood tests that measure enzymes and proteins produced or excreted by the liver. Abnormal levels (higher or lower than normal) suggest liver dysfunction or systemic issues.

Main Parameters in LFTs

1. Bilirubin (Conjugated & Unconjugated)

   • Source: Breakdown product of hemoglobin from aged RBCs (~120 days lifespan).

   • Pathway:

     • Hemoglobin → Heme + Globin

     • Heme → Biliverdin → Unconjugated (indirect) Bilirubin

     • Unconjugated bilirubin (water-insoluble) binds to albumin → transported to liver → converted to conjugated (direct) bilirubin (water-soluble) → excreted in urine/feces.

   • Causes of Elevated Bilirubin:

     • Hemolysis (excess RBC breakdown)

     • Impaired hepatic conjugation

     • Biliary obstruction

2. Aspartate Aminotransferase (AST / SGOT)

   • Distribution: High in heart, liver, skeletal muscle; low in kidney, pancreas, RBCs.

   • Pattern:

     • Released after tissue damage.

     • Rises 8 hrs post-injury → peaks 24–36 hrs → normal in 3–7 days.

   • Causes of Elevated AST:

     • Hepatocellular disease (acute/chronic hepatitis, cirrhosis)

     • Extra-hepatic obstruction

     • Muscle injury, myocardial infarction

   • Interferences:

     • ↓ in pregnancy

     • ↑ with exercise, certain drugs (antihypertensives, OCPs)

3. Alanine Aminotransferase (ALT / SGPT)

   • Distribution: Mainly in liver; smaller amounts in kidney, heart, skeletal muscle.

   • Clinical Value:

     • More specific indicator of liver injury than AST.

     • ALT > AST → usually liver disease.

   • Causes of Elevated ALT:

     • Alcoholic liver disease

     • Hepatic cancer

     • Chronic cirrhosis

4. Alkaline Phosphatase (ALP)

   • Distribution: Liver, bone, placenta.

   • Low specificity (can be elevated in multiple conditions).

   • Causes of Elevated ALP:

     • Cholestasis (bile duct obstruction)

     • Pregnancy

     • Bone disease (Paget’s, osteoblastic cancer, new bone growth)

   • Problem: Cannot distinguish liver vs. bone source alone → use GGT.

5. Gamma Glutamyl Transpeptidase (GGT / GGTP)

   • Distribution: High in liver; low in kidney, heart, intestine, brain, prostate.

   • Clinical Value:

     • Highly sensitive for cholestasis, biliary obstruction, cholangitis, cholecystitis.

     • Parallels ALP elevation in liver disease.

     • Key Differentiator:

       • ALP ↑ + GGT ↑ → Liver origin

       • ALP ↑ + GGT normal → Bone origin

Exam Pearls / Mnemonics:

• AST = Skeletal + heart + liver (“Systemic”)

• ALT = Liver (specific)

• ALP = Alkaline (think “bones grow in alkaline soil” → bone & liver)

• GGT = Gallbladder (biliary tree marker)