Liver Function Tests (LFTs): A Complete Guide for Medical Exams

LFT- Liver function test: Indication, Interpretatin, Normal Range

LFT- Liver function test: Indication, Interpretatin, Normal Range

Table of Contents(toc)

🧾 Introduction

Liver Function Tests (LFTs) are a panel of blood tests used to evaluate the overall health and function of the liver. These tests help detect liver inflammation (hepatitis), infection, damage, obstruction, and synthetic dysfunction. They are crucial in diagnosing liver diseases such as hepatitis, cirrhosis, alcoholic liver disease, drug-induced liver injury, and biliary obstruction.

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📊 Normal Values of Key Liver Function Tests

Test	Normal Range	Clinical Significance
ALT (Alanine Aminotransferase)	10–40 U/L	Sensitive marker of hepatocellular injury
AST (Aspartate Aminotransferase)	12–38 U/L	In liver and other tissues (heart, muscle)
ALP (Alkaline Phosphatase)	25–100 U/L	Elevated in cholestasis and bone disorders
Amylase	25–125 U/L	Mainly a pancreatic marker (not a core LFT)
Bilirubin (Total / Direct)	0.1–1.0 mg/dL / 0.0–0.3 mg/dL	Reflects hepatic conjugation and excretion
GGT (Gamma-Glutamyl Transferase)	0–30 U/L (varies)	Elevated in alcohol use and biliary obstruction
Albumin	3.5–5.0 g/dL	Reflects liver’s synthetic function
Prothrombin Time (PT/INR)	PT: 11–13.5 sec	Liver’s synthesis of clotting factors

🔬 Interpreting Abnormal Liver Function Tests

1. Hepatocellular Pattern (↑ALT, ↑AST)

• Causes:

  • Viral Hepatitis (A, B, C)

  • Drug-induced liver injury (e.g., acetaminophen)

  • Autoimmune hepatitis

  • Ischemic hepatitis

• AST/ALT Ratio:

  • 2:1 → suggestive of alcoholic hepatitis

  • ALT > AST → typical of viral hepatitis

2. Cholestatic Pattern (↑ALP, ↑GGT)

• Causes:

  • Biliary obstruction (e.g., gallstones, tumors)

  • Primary biliary cholangitis

  • Primary sclerosing cholangitis

  • Drug-induced cholestasis

• GGT is often used to confirm hepatic origin of ALP elevation.

3. Mixed Pattern (↑ALT, ↑ALP)

• Seen in:

  • Drug-induced liver injury

  • Viral hepatitis

  • Autoimmune disorders

4. Synthetic Dysfunction (↓Albumin, ↑PT/INR)

• Seen in:

  • Chronic liver disease (cirrhosis)

  • Acute liver failure

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🧠 Clinical Clues from Patterns

Pattern	Key Indicators	Common Conditions
Hepatocellular	↑ALT, ↑AST	Hepatitis (viral, toxic, autoimmune)
Cholestatic	↑ALP, ↑GGT, ↑Bilirubin	Obstruction, PBC, PSC
Synthetic failure	↓Albumin, ↑PT/INR	Cirrhosis, liver failure
Isolated ↑ALP	If GGT normal → think bone disease

⚠️ Special Notes for Exams

• ALT is more liver-specific than AST.

• AST is also found in muscle, RBCs, and heart → check CK if muscle injury is suspected.

• Bilirubin types:

  • Unconjugated ↑ in hemolysis and Gilbert’s syndrome

  • Conjugated ↑ in cholestasis and hepatocellular disease

• GGT is elevated in alcoholic liver disease, but not bone disease.

• Prothrombin time is often the first to derange in acute liver failure.

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📚 High-Yield MCQs for Licensing Exams (USMLE, NCLEX, FMGE)

1. A 45-year-old male with a history of alcohol use presents with fatigue and jaundice. LFTs show AST 145 U/L and ALT 60 U/L. What is the most likely diagnosis?

A. Viral hepatitis

B. Alcoholic hepatitis ✅

C. Hemochromatosis

D. Primary biliary cholangitis

Explanation: AST:ALT > 2 is characteristic of alcoholic hepatitis.

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2. A patient presents with jaundice, dark urine, and pale stools. Labs show ALP 320 U/L, GGT elevated, and direct bilirubin 4.5 mg/dL. What is the likely diagnosis?

A. Viral hepatitis

B. Hemolytic anemia

C. Obstructive jaundice ✅

D. Gilbert syndrome

Explanation: Cholestatic pattern with elevated ALP, GGT, and direct bilirubin → biliary obstruction.

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3. Which LFT abnormality is most indicative of impaired liver synthetic function?

A. Elevated ALT

B. Elevated AST

C. Low albumin and elevated PT/INR ✅

D. High GGT

Explanation: PT and albumin reflect liver synthesis of proteins and clotting factors.

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4. In acute viral hepatitis, which of the following is the most likely pattern?

A. ALT > AST ✅

B. AST > ALT

C. Isolated ALP elevation

D. Normal LFTs

Explanation: Viral hepatitis typically shows a hepatocellular pattern, with ALT > AST.

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5. A young woman presents with fatigue and pruritus. Labs show elevated ALP and positive anti-mitochondrial antibodies (AMA). What is the most likely diagnosis?

A. Primary sclerosing cholangitis

B. Autoimmune hepatitis

C. Primary biliary cholangitis ✅

D. Gallstones

Explanation: Female + pruritus + ↑ALP + AMA → PBC.

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🧷 Conclusion

Understanding the interpretation of liver function tests is essential for diagnosing and managing hepatic disorders. Focus on recognizing patterns, correlating with clinical history, and knowing high-yield associations—especially for exams like USMLE, NCLEX, and FMGE. Mastery of this topic is not only vital for exams but also for real-world clinical reasoning.