Liver Function Tests

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LFT Basics - Liver's Check‑Up Kit

  • LFTs (Liver Function Tests): Blood tests assessing liver health & function.
  • Purpose:
    • Screening for liver damage.
    • Diagnosing liver diseases.
    • Monitoring disease progression/treatment.
  • Categories:
    • Hepatocellular injury (e.g., ALT, AST).
    • Cholestasis (e.g., ALP, GGT, Bilirubin).
    • Synthetic function (e.g., Albumin, Prothrombin Time). When is a Liver Function Test Used?

⭐ LFTs can be normal in significant liver disease like compensated cirrhosis.

Injury Markers - Hepatocyte SOS Signals

FeatureALT (Alanine Aminotransferase / SGPT)AST (Aspartate Aminotransferase / SGOT)
📌 Mnemonic / LocationLiver (primarily cytoplasm)Skeletal muscle, heart, liver (mito & cyto)
Liver SpecificityHigh (more liver-specific)Moderate (found in other tissues)
Significance of ↑Acute/chronic hepatocellular injuryHepatocellular injury, muscle damage (MI, rhabdo)

De Ritis Ratio ($AST/ALT$):

  • Viral hepatitis, NAFLD: Usually < 1
  • Alcoholic liver disease: Typically > 2
  • Cirrhosis (non-viral): Often ≥ 1

⭐ An AST/ALT ratio > 2, especially with elevated GGT, strongly suggests alcoholic hepatitis.

Bile Flow - Plumbing Check

  • Cholestatic Markers:
    • ALP (Alkaline Phosphatase): Liver (biliary epithelium), bone, placenta. Elevation > 4x ULN suggests cholestasis.
    • GGT (Gamma-Glutamyl Transferase): Liver, biliary. Sensitive for cholestasis, alcohol. 📌 GGT: Get Good Toddy (alcohol).
    • 5'-Nucleotidase (5'-NT): Specific for hepatobiliary cholestasis.
  • Cholestatic Pattern:
    EnzymeCholestasisIsolated ALP ↑Isolated GGT ↑
    ALP↑↑↑Bone, Placenta
    GGT↑↑↑Alcohol, Drugs
    5'-NT↑↑↑
  • Bilirubin: Total < 1.2 mg/dL; Direct < 0.3 mg/dL.
    • Conjugation: Bilirubin + UDPGA $\xrightarrow{\text{UGT1A1}}$ Bilirubin diglucuronide. Bilirubin metabolism pathway in hepatocyte

⭐ Isolated GGT elevation can be due to enzyme induction by alcohol or drugs (e.g., phenytoin).

Synthesis Power - Liver's Factory Output

  • Serum Albumin (Normal: 3.5-5.5 g/dL)
    • ↓ indicates: Chronic Liver Disease (CLD), nephrotic syndrome, malnutrition.
    • Long half-life (~20 days) → reflects chronic synthetic dysfunction.
  • Prothrombin Time (PT) / INR
    • Normal PT: 11-14 seconds; Normal INR: 0.8-1.2.
    • ↑ PT/INR indicates: Liver failure (acute/chronic), Vitamin K deficiency.
    • PT prolonged by > 3-4 seconds or INR > 1.5 suggests significant dysfunction.

    ⭐ PT/INR is a sensitive indicator of acute liver synthetic dysfunction due to the short half-life of clotting factors (especially Factor VII).

Pattern Puzzles - Decoding LFTs

Interpreting LFTs involves pattern recognition.

  • Hepatocellular Injury:
    • Acute Viral Hepatitis: ALT/AST ↑↑↑ (>10-15x ULN), ALT > AST.
    • Alcoholic Liver Disease: AST > ALT (ratio often >2:1), GGT ↑↑.
    • Chronic Hepatitis: Mild-moderate ↑ ALT/AST.
    • NAFLD/NASH: Mild ↑ ALT/AST, often ALT > AST.
  • Cholestatic Pattern: ALP ↑↑↑, GGT ↑↑, Bilirubin ↑. Differentiate intra- vs. extrahepatic.
  • Specific Clues:
    • Autoimmune Hepatitis: ANA, ASMA, ↑IgG.
    • Wilson's Disease: ↓ Ceruloplasmin, low ALP.
    • Hemochromatosis: ↑ Ferritin, ↑ Transferrin Saturation.
    • Hepatocellular Carcinoma (HCC): AFP > 400 ng/mL highly suggestive.

⭐ Markedly elevated aminotransferases (>1000 IU/L) are typical of acute viral hepatitis, ischemic hepatitis, or drug/toxin‑induced liver injury.

High‑Yield Points - ⚡ Biggest Takeaways

  • ALT is more liver-specific than AST for detecting acute hepatocellular injury.
  • An AST/ALT ratio > 2 is highly suggestive of alcoholic liver disease, especially with ↑ GGT.
  • Markedly ↑ ALP with ↑ GGT indicates cholestasis or biliary obstruction.
  • Bilirubin (total, direct, indirect) patterns help differentiate pre-hepatic, hepatic, and post-hepatic jaundice.
  • Serum albumin & ↑ Prothrombin Time (PT/INR) reflect impaired liver synthetic function.
  • Isolated ↑ unconjugated bilirubin often points to Gilbert's syndrome or hemolysis.

Practice Questions: Liver Function Tests

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Discriminant score is used for management of?

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Flashcards: Liver Function Tests

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_____ is used to identify occult alcohol use

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_____ is used to identify occult alcohol use

GGT

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