Liver function test 6 marks
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Liver Function Tests (LFTs)
Liver function tests (LFTs) are a panel of biochemical tests used to assess hepatic integrity, metabolism, and synthetic function. No single test reflects all liver functions; they are used as a battery to detect and classify liver disease.
1. Serum Bilirubin
Normal total bilirubin: 0.2-0.9 mg/dL (upper limit ~1.5 mg/dL); conjugated fraction normally <0.3 mg/dL.
Bilirubin exists in two fractions:
| Fraction | Also Called | Solubility | Significance |
|---|---|---|---|
| Unconjugated | Indirect | Water-insoluble (albumin-bound) | Elevated in hemolysis, Gilbert's, Crigler-Najjar |
| Conjugated | Direct | Water-soluble | Elevated in liver/biliary tract disease |
- The rate-limiting step of bilirubin metabolism is transport of conjugated bilirubin into the bile canaliculi, not conjugation itself
- Conjugated hyperbilirubinemia almost always implies hepatic or biliary disease
- Bilirubinuria (bilirubin in urine) = always conjugated bilirubin = confirms hepatic/obstructive disease
- In most liver diseases, both fractions rise together
- Bilirubin is an important component of the MELD score (Model for End-Stage Liver Disease) for liver transplant prioritization
(Harrison's Principles of Internal Medicine 22E, p. 2677)
2. Serum Aminotransferases (Transaminases)
Tests: AST (aspartate aminotransferase) and ALT (alanine aminotransferase). Normal: 10-40 IU/L.
- ALT is found primarily in the liver - more specific for liver injury
- AST is present in liver, cardiac muscle, skeletal muscle, kidneys, brain, pancreas (less specific)
- These enzymes are released into blood when liver cell membrane permeability increases (necrosis is NOT required)
- Poor correlation between enzyme level and degree of liver damage - absolute elevation has no prognostic significance in acute disorders
Interpretation by level:
- Up to 300 IU/L: nonspecific, seen in any liver disorder
-
1000 IU/L: seen almost exclusively in viral hepatitis, ischemic liver injury, or toxin/drug-induced liver injury
AST:ALT ratio:
- ALT ≥ AST: most acute hepatocellular disorders
- AST:ALT ratio >2:1 (especially >3:1): strongly suggests alcoholic liver disease
- AST:ALT >1 may also be seen in cirrhosis of any cause
(Harrison's Principles of Internal Medicine 22E, p. 2678)
3. Serum Alkaline Phosphatase (ALP)
Normal: 30-120 IU/L (varies by lab).
ALP exists in multiple isoforms - liver, bone, intestine, placenta. In liver disease, ALP is derived from the bile duct epithelium and is induced by bile acids.
- Elevated in cholestasis (intra- or extra-hepatic), bile duct injury, and space-occupying lesions of the liver
- Mild elevation (<3x normal): many liver and non-liver conditions
- Marked elevation (>4x normal): strongly suggests cholestasis or infiltrative disease
- To confirm hepatic origin: measure GGT or 5'-nucleotidase (both hepatic-specific; bone disease elevates ALP but NOT GGT)
Pattern: Predominant ALP + GGT elevation with lesser aminotransferase rise = cholestatic pattern
4. Gamma-Glutamyl Transpeptidase (GGT)
- A microsomal enzyme elevated in cholestatic disease
- Induced by alcohol and drugs (phenytoin, barbiturates) - useful marker of alcohol abuse
- Not elevated in bone disease (unlike ALP) - helps confirm hepatic origin of ALP elevation
- Very sensitive but not specific for liver disease
5. Serum Albumin
Normal: 3.5-5.0 g/dL; half-life ~20 days.
- Synthesized exclusively by the liver
- Reflects hepatic synthetic function
- Low albumin indicates chronic liver disease (cirrhosis, chronic hepatitis)
- Not useful in acute liver disease due to the long half-life (serum levels take weeks to fall)
- Non-hepatic causes of hypoalbuminemia: malnutrition, nephrotic syndrome, protein-losing enteropathy
6. Prothrombin Time (PT) / INR
- Coagulation factors I, II, V, VII, IX, X are synthesized by the liver; factor VII has the shortest half-life (~6 hours)
- A prolonged PT/elevated INR indicates impaired hepatic synthetic function
- Most sensitive marker of acute liver synthetic failure (responds within hours, unlike albumin)
- Used in Child-Pugh score and MELD score for prognosis
- PT that does not correct with Vitamin K administration indicates severe hepatocellular failure (vs. simply Vitamin K deficiency)
Pattern Recognition Summary
| Pattern | Predominant Abnormality | Likely Diagnosis |
|---|---|---|
| Hepatocellular | ALT/AST elevated (>>ALP) | Viral hepatitis, NASH, ischemia |
| Cholestatic | ALP/GGT elevated (>>ALT) | Obstruction, PBC, PSC |
| Alcoholic | AST:ALT >2:1, elevated GGT | Alcoholic liver disease |
| Synthetic failure | Low albumin, prolonged PT | Cirrhosis, fulminant hepatic failure |
| Fulminant failure | All markedly elevated + hyperammonemia | Acute liver failure |
(Harrison's Principles of Internal Medicine 22E, p. 2677-2678; Tietz Textbook of Laboratory Medicine 7th Ed; Henry's Clinical Diagnosis, p. 3821-3834)
Book of biochem satynarayan
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Satyanarayana biochemistry textbook medical students
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Unfortunately, the textbook library does not contain U. Satyanarayana's Biochemistry (the popular Indian medical biochemistry textbook). The library only has these biochemistry titles:
- Basic Medical Biochemistry - A Clinical Approach, 6th Ed (Marks)
- Biochemistry, 8th Ed - Lippincott Illustrated Reviews
- Harper's Illustrated Biochemistry, 32nd Ed
Satyanarayana's Biochemistry (by U. Satyanarayana & U. Chakrapani) is not part of the available collection here.
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If you have a specific topic you'd like covered - for example, from Satyanarayana's syllabus - I can answer it using the equivalent content from Lippincott's, Harper's, or Harrison's, which are all available in the library and cover the same biochemistry topics. These are internationally recognized references.
Common topics from Satyanarayana's Biochemistry include:
- Carbohydrate metabolism (glycolysis, TCA cycle, glycogen metabolism)
- Lipid metabolism (beta-oxidation, ketone bodies, cholesterol)
- Protein and amino acid metabolism
- Enzymes and enzyme kinetics
- Vitamins and minerals
- Nucleotide metabolism
- Molecular biology (DNA replication, transcription, translation)
- Clinical/applied biochemistry (liver function tests, kidney function tests, etc.)
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