Liver Function and Testing: Structure, Metabolism, and Clinical Assessment

Overview of Liver Function

Anatomy and Blood Supply

The liver is a vital organ with a central role in metabolism, detoxification, and synthesis of key biomolecules. It is divided into four lobes and weighs approximately 1.2–1.6 kg. Its unique dual blood supply consists of:

• Hepatic artery: Supplies ~25% of blood (oxygenated).

• Portal vein: Supplies ~75% of blood (from the gastrointestinal tract), delivering nutrients and potential toxins directly from the gut.

Key Point: The liver receives 'first pickings' of absorbed nutrients and toxins, making it the primary site for detoxification.

Major Functions of the Liver

• Waste Management: Detoxifies xenobiotics, conjugates compounds for excretion, and converts ammonia to urea.

• Recycling: Recycles iron, bile salts, and amino acids.

• Production and Storage: Synthesizes glycogen, lipoproteins, plasma proteins, and stores vitamins and minerals.

• Regeneration: The liver is the only human organ capable of full regeneration.

Example: Living donor liver transplantation is possible due to this regenerative capacity.

Carbohydrate, Lipid, and Protein Metabolism

• Carbohydrate Metabolism:

  • Maintains blood glucose homeostasis via glycogen synthesis and breakdown (glycogenolysis).

  • Performs gluconeogenesis (synthesis of glucose from non-carbohydrate sources).

• Lipid Metabolism:

  • Synthesizes lipoproteins (VLDL, HDL), cholesterol, phospholipids, and triglycerides.

  • Excretes cholesterol breakdown products via bile.

• Protein Synthesis:

  • Produces most plasma proteins (albumin, clotting factors, carrier proteins).

  • Exception: Immunoglobulins are synthesized by B-cells, not the liver.

Storage and Cofactor Metabolism

• Stores fat-soluble vitamins (A, D, E, K), vitamin B9 (folate), B12, copper, fats, and iron.

• Site of vitamin D metabolism.

Example: Iron is stored in a bioavailable form, essential for hemoglobin and cellular respiration.

Excretion and Detoxification

• Detoxifies drugs and environmental toxins (xenobiotics).

• Converts ammonia to urea for safe excretion.

• Conjugates compounds (e.g., glucuronidation, sulfation) to enhance solubility and excretion.

• Acts as a 'gatekeeper' by intercepting toxins from the gut before systemic circulation.

Bilirubin Metabolism

Plasma vs Serum

Plasma	Serum
Contains clotting factors	Clotting factors removed
Preferred for prothrombin time	Preferred for most laboratory tests

Key Point: Serum is used for most LFTs; plasma is required for prothrombin time.

Stepwise Bilirubin Metabolism

1. Red Blood Cell Breakdown: Senescent erythrocytes are degraded, releasing hemoglobin, which splits into globin (recycled) and haem.

2. Haem to Bilirubin: Haem oxygenase removes iron, forming biliverdin (green), which is reduced to bilirubin (yellow, water-insoluble).

3. Transport to Liver: Bilirubin binds to albumin for transport to the liver.

4. Conjugation in Hepatocytes: UDP-glucuronyl transferase conjugates bilirubin with glucuronic acid, making it water-soluble (conjugated bilirubin).

5. Excretion into Bile: Conjugated bilirubin is secreted into bile and enters the intestine.

6. Intestinal Breakdown: Gut bacteria convert bilirubin to urobilinogen and stercobilinogen; stercobilin colors feces brown. Some urobilinogen is reabsorbed and excreted in urine as urobilin (yellow).

Clinical Note: Pale stools and dark urine suggest bile duct obstruction.

Bilirubin's Protective Role

• At low levels, bilirubin acts as an antioxidant, inversely related to coronary artery disease risk.

• High levels are neurotoxic, especially in newborns (risk of kernicterus).

Liver Function Tests (LFTs)

Purpose and Types of LFTs

• No single test covers all liver functions; a panel is used for diagnosis, staging, monitoring, and screening.

Test	Indication
ALT, AST	Hepatocellular damage
ALP, GGT	Biliary obstruction/cholestasis
Albumin, Prothrombin time	Synthetic function impairment
Bilirubin (total, direct, indirect)	Impaired metabolism/excretion

Bilirubin Measurement

• Direct bilirubin: Conjugated, water-soluble; reacts directly with diazonium salt.

• Total bilirubin: Conjugated + unconjugated; unconjugated is solubilized before reaction.

• Indirect bilirubin: Calculated as total minus direct; represents unconjugated bilirubin.

Jansen Reaction: Formation of colored azidopyroles, measured spectrophotometrically at 600 nm.

Other LFTs and Their Principles

• ALT/AST: Enzyme leakage indicates hepatocellular injury. Measured by coupled reactions monitoring NADH consumption at 340 nm.

• ALP: Elevated in cholestasis; measured by conversion of p-nitrophenyl phosphate to p-nitrophenol (yellow) at 415 nm.

• GGT: Sensitive but non-specific marker; measured by release of colored product at 405 nm.

• Total Protein (Biuret Test): Cu2+ reacts with peptide bonds, forming a purple complex measured at 540 nm.

• Albumin (BCG Method): Bromocresol green binds albumin, forming a blue complex measured at 628 nm.

• Prothrombin Time (PT): Measures clotting factor synthesis; requires plasma. Expressed as INR.

• Alpha-Fetoprotein (AFP): Marker for hepatocellular carcinoma; detected by antibody-based assays.

Non-Laboratory Tests

• Ultrasound: Non-invasive, detects large masses or fatty liver, but not early cirrhosis.

• ERCP: Visualizes bile ducts and pancreas; invasive and expensive.

• Liver Biopsy: Gold standard for diagnosis; invasive, risk of complications.

Interpretation of LFT Results

Patterns of Abnormality

Pathology	ALT/AST	ALP/GGT	Albumin	Prothrombin Time	Bilirubin
Hepatocellular damage	↑↑	Normal	Normal	Normal	↑
Biliary obstruction	Normal/↑	↑↑↑	Normal	Normal	↑
Synthetic impairment	Normal/↑	Normal/↑	↓↓	↑	↑

Key Point: ALT/AST elevation = hepatocellular injury; ALP/GGT elevation = cholestasis; albumin/prothrombin time = synthetic function.

Caveats in LFT Interpretation

• ALT is more liver-specific than AST; AST is also found in heart, muscle, and other tissues.

• In end-stage liver disease, ALT/AST may be normal due to loss of hepatocytes.

• GGT is sensitive but not specific; can be induced by drugs and non-liver diseases.

Jaundice (Hyperbilirubinaemia)

Types and Causes

Type	Cause	Unconjugated Bilirubin	Conjugated Bilirubin	Urine Urobilinogen	Urine Bilirubin	Stool Colour	ALT/AST	ALP/GGT
Pre-hepatic	Haemolysis, neonatal jaundice	↑↑	Normal/slight ↑	↑↑	Normal/absent	Normal/dark	Normal	Normal
Hepatic	Hepatitis, cirrhosis, metabolic defects	↑	↑	↑	↑	Normal to pale	↑↑↑	Normal to ↑
Post-hepatic	Bile duct obstruction	↑ (secondary)	↑↑	↓ or absent	↑↑	Pale/chalky white	Normal to ↑	↑↑↑

Unconjugated Hyperbilirubinaemia

• Caused by haemolysis, Gilbert's syndrome, or neonatal enzyme immaturity.

• Liver function tests (ALT/AST/ALP/GGT) are normal; unconjugated bilirubin is elevated.

• Urinary urobilinogen is increased; bilirubin is absent in urine.

Conjugated Hyperbilirubinaemia

• Results from leakage of conjugated bilirubin due to hepatocellular or biliary tract disease.

• Conjugated bilirubin is water-soluble and appears in urine (dark urine).

• Obstructive jaundice: pale stools, absent urobilinogen in urine, elevated ALP.

Hepatic Jaundice Subtypes

• Conjugation Failure: Crigler-Najjar (complete deficiency), Gilbert's (partial deficiency), neonatal jaundice (immaturity).

• Post-Conjugation Failure: Dubin-Johnson syndrome (MRP2 transporter defect).

• Diffuse Hepatocellular Damage: Both unconjugated and conjugated bilirubin elevated; increased urinary urobilinogen and bilirubin.

Post-Hepatic (Obstructive) Jaundice

• Caused by obstruction (gallstones, tumors, strictures).

• Conjugated bilirubin cannot reach the intestine; leaks into plasma and urine.

• Stool is pale; urine is dark; ALP is markedly elevated.

• Secondary hepatic damage may eventually elevate ALT/AST.

Liver Disease: Types and Biochemical Patterns

Common Liver Diseases

• Hepatitis: Acute or chronic inflammation, often viral or toxic.

• Cirrhosis: Irreversible scarring and loss of function; complications include portal hypertension and liver failure.

• Tumors: Often metastatic; primary hepatocellular carcinoma is less common.

Clinical Note: Hepatic encephalopathy results from ammonia accumulation due to impaired detoxification.

Toxic and Viral Hepatitis

• Toxic Hepatitis: Acute (paracetamol overdose, CCl4) or chronic (alcohol).

• Viral Hepatitis:

  • Hepatitis A: Fecal-oral, no chronicity, vaccine available.

  • Hepatitis B: Blood/sexual, chronic possible, vaccine available.

  • Hepatitis C: Blood, chronic in 20%, no vaccine.

  • Hepatitis D: Requires Hep B co-infection.

  • Hepatitis E: Fecal-oral, generally acute, vaccine available.

Virus	Transmission	Chronic Disease?	Vaccine?
Hep A	Fecal-oral	No	Yes
Hep B	Blood/sexual	Yes	Yes
Hep C	Blood	Yes (20%)	No
Hep D	Blood (with Hep B)	Worsens Hep B	Yes (via Hep B vaccine)
Hep E	Fecal-oral	No	Yes

Biochemical Changes in Hepatitis and Cirrhosis

• Acute hepatitis: ALT/AST rise dramatically (>50× normal), bilirubin rises more slowly.

• Chronic hepatitis: Moderate, persistent elevation of aminotransferases.

• Cirrhosis: ALT/AST may normalize due to loss of hepatocytes; synthetic markers (albumin, prothrombin time) worsen.

Master Reference Table: Typical LFT Results

Condition	Bilirubin	ALT/AST	ALP	Albumin	Prothrombin Time
Acute Hepatitis	N to ↑↑	↑↑↑	N to ↑	N	N to ↑
Chronic Hepatitis	N to ↑	↑	N to ↓	↑ (sometimes ↓)	N to ↑
Cirrhosis	N to ↑	N to ↑	N to ↑↑	N to ↓↓	N to ↑
Cholestasis (Obstruction)	↑ to ↑↑↑	N to ↑	↑↑↑	N	N to ↑
Malignancy/Infiltration	N	N to ↑	↑↑	N to ↓	N

Key Exam and Clinical Points

• Liver as Detox Organ: Portal vein delivers gut contents directly to the liver for first-pass metabolism and detoxification.

• Abnormal Bilirubin: Causes include increased production (haemolysis), decreased conjugation (genetic or neonatal), biliary obstruction, and hepatocellular damage.

• Unconjugated Hyperbilirubinaemia: Urinary urobilinogen is elevated, but bilirubin is absent in urine.

• Post-Hepatic Jaundice: ALP is the most elevated test; conjugated bilirubin is high in plasma and urine, urobilinogen is absent in urine, and stools are pale.

Formulas and Equations

• ALT Reaction: $\mathrm{L\text{-}alanine + \alpha\text{-}ketoglutarate \xrightarrow{ALT} pyruvate + L\text{-}glutamate}$

• AST Reaction: $\mathrm{L\text{-}aspartate + \alpha\text{-}ketoglutarate \xrightarrow{AST} oxaloacetate + L\text{-}glutamate}$

• ALP Reaction: $\mathrm{p\text{-}nitrophenyl\ phosphate + H_2O \xrightarrow{ALP} p\text{-}nitrophenol + phosphate}$

• Prothrombin Time (INR): $\mathrm{INR = \frac{Patient\ PT}{Mean\ Normal\ PT}}$

• Indirect Bilirubin Calculation: $\mathrm{Indirect\ Bilirubin = Total\ Bilirubin - Direct\ Bilirubin}$

Additional info: This guide integrates and expands upon lecture notes, providing context for clinical interpretation and exam preparation. For case studies, refer to the bilirubin schematic and master LFT table as directed in the lecture.