Page 1 of 32

PA24.{1,6} | Bilirubin Metabolism, Jaundice & LFT Interpretation — SDL Guide

Learning Objectives

• Trace the complete pathway of bilirubin from haemoglobin breakdown to faecal excretion, naming the key biochemical steps and transporters at each stage.
• Classify jaundice into prehepatic, hepatic, and posthepatic types and explain the underlying mechanism for each.
• Distinguish conjugated from unconjugated hyperbilirubinaemia using clinical features, urine findings, and LFT patterns.
• Identify the four hereditary hyperbilirubinaemia syndromes (Gilbert, Crigler-Najjar, Dubin-Johnson, Rotor) and assign each to conjugated or unconjugated category.
• Interpret a liver function test panel by recognising hepatocellular, cholestatic, and prehepatic patterns.
• Interpret hepatitis B serology (HBsAg, anti-HBs, anti-HBc IgM/IgG, HBeAg) to determine acute infection, chronic infection, resolved infection, or vaccination status.
• Explain the pathophysiology of neonatal jaundice and the risk of kernicterus in preterm infants.

INSTRUCTIONS

Jaundice is one of the most common presentations in clinical medicine, and interpreting a liver function test is a core skill you will use daily in wards, OPD, and emergencies. This module builds a mechanistic map: from the molecular steps that process bilirubin, through the three sites where that processing can fail, to the pattern-recognition framework that turns a set of LFT numbers into a differential diagnosis. Hepatitis serology interpretation is woven in because viral hepatitis is the leading cause of hepatocellular jaundice in India. Everything here links to your Physiology base (bilirubin conjugation, enterohepatic circulation) and prepares you for the clinical reasoning you will need in Surgery and Medicine postings.

References

• Robbins & Cotran Pathologic Basis of Disease, 10th ed., Ch 18 (Liver) (textbook)
• Harsh Mohan Textbook of Pathology, 8th ed., Ch 20 (textbook)
• Chatterjea & Shinde Manual of Practical Medical Biochemistry, 12th ed. (Liver function tests) (textbook)

---

Version 2.0 | NMC CBUC 2024

Bilirubin Production: From Haem to Unconjugated Bilirubin

Bilirubin Production from Haem

Bilirubin is the end-product of haem catabolism. Approximately 80% derives from the breakdown of senescent red blood cells (RBC lifespan ≈ 120 days) in the reticuloendothelial system (RES) — principally the spleen, liver Kupffer cells, and bone marrow. The remaining 20% comes from premature destruction of defective erythroblasts (ineffective erythropoiesis) and from turnover of non-haemoglobin haem proteins (myoglobin, cytochrome P450).

The pathway has two enzymatic steps:
1. Haem oxygenase (in RES macrophages) opens the porphyrin ring, releasing iron (recycled via transferrin) and producing biliverdin + carbon monoxide.
2. Biliverdin reductase immediately reduces biliverdin to unconjugated bilirubin (UCB), also called indirect bilirubin.

UCB is non-polar and hydrophobic (fat-soluble). It is insoluble in water and cannot be excreted in bile or urine in this form. To travel in plasma, UCB binds tightly (but reversibly) to albumin — typically as a 1:1 complex. Free (unbound) UCB is neurotoxic: it crosses the blood-brain barrier and causes kernicterus.

Normal serum bilirubin: 0.3–1.2 mg/dL, predominantly unconjugated.

Bilirubin Production from Haemoglobin

Hepatic Uptake, Conjugation, and Excretion

Hepatic Bilirubin Uptake, Conjugation, and Excretion with Enterohepatic Circulation

Once the albumin-UCB complex reaches the hepatic sinusoid, the bilirubin is removed from albumin and taken up by hepatocytes via OATP1B1/1B3 transporters (organic anion transporting polypeptides) on the basolateral membrane. Inside the hepatocyte, UCB is bound to intracellular carrier proteins (ligandin/Y protein) and transported to the endoplasmic reticulum.

Conjugation occurs in the smooth ER, catalysed by UDP-glucuronosyltransferase (UGT1A1). Two molecules of glucuronic acid are added to bilirubin, producing bilirubin diglucuronide — also called conjugated bilirubin or direct bilirubin. Conjugated bilirubin (CB) is:
- Water-soluble → can be excreted in bile and filtered into urine
- No longer albumin-bound → faster clearance from plasma
- Detected by the van den Bergh direct reaction (reacts without methanol)

Biliary excretion: CB is actively transported across the canalicular membrane into bile by MRP2 (multidrug resistance protein 2, also called ABCC2). This is the rate-limiting step in bilirubin clearance and the most common site of inherited transport defects (Dubin-Johnson syndrome).

In the bile duct and intestine, CB passes to the colon where colonic bacteria deconjugate and reduce it sequentially to urobilinogen (colourless) → stercobilin (brown — normal stool colour).

Enterohepatic circulation: ~20% of urobilinogen is reabsorbed from the terminal ileum and colon into portal blood. The liver re-excretes most of it into bile; a small fraction (~1%) escapes into systemic circulation and is excreted by the kidneys as urinary urobilinogen (normal: trace amounts detectable).

Hepatic Bilirubin Metabolism: Molecular Transporters, Conjugation, and Enterohepatic Circulation

Jaundice: Definition, Detection, and Classification

Jaundice: Detection and Mechanistic Classification

Jaundice (icterus) is the yellow discolouration of skin, sclerae, and mucous membranes due to elevated serum bilirubin. It becomes clinically detectable when total bilirubin exceeds 2.5 mg/dL (scleral icterus at 1.5–2 mg/dL, as the sclera has high elastin with strong bilirubin affinity).

Latent jaundice: 1.2–2.5 mg/dL — biochemical, no visible staining.

Mechanistic classification (the framework used in clinical reasoning):

Type	Site of problem	Mechanism
Prehepatic	Before liver	Excess UCB production (haemolysis, ineffective erythropoiesis) overwhelms hepatic conjugation capacity
Hepatic	In liver cells	Hepatocellular damage impairs uptake, conjugation, AND excretion → mixed picture
Posthepatic	Bile ducts (extra- or intrahepatic)	Obstruction prevents CB from reaching gut → CB regurgitates into blood

A fourth mechanistic category — functional/transport defects — covers hereditary syndromes where the bile ducts and hepatocytes are structurally intact but specific transporters or enzymes are deficient.

Classification of Jaundice