Jaundice and Bilirubin Metabolism

Jaundice and Bilirubin Metabolism

Overview

Jaundice = yellow skin/sclera pigmentation from ↑blood bilirubin. High affinity for elastic tissue → sclera shows first. Normal: 0.3-1.3mg/dl (~80% unconjugated). Visible >2mg/dl; below that = LATENT jaundice.

Bilirubin metabolism — 4 phases

1. Source: 80-85% from senescent RBC Hb catabolism (RE system: marrow/spleen/liver, 120-day lifespan). Remainder: non-Hb haem pigments (myoglobin/catalase/cytochromes) + ineffective erythropoiesis. Haem→biliverdin (haem oxygenase, needs O2+NADPH)→bilirubin (biliverdin reductase).

2. Transport: unconjugated, albumin-bound. Sulfonamides/salicylates DISPLACE bilirubin from albumin → neonates → ↑kernicterus risk.

3. Hepatic phase (3 steps):

• Uptake: dissociates from albumin → binds GST/ligandin
• Conjugation: unconjugated (alcohol-soluble)→water-soluble mono/diglucuronide via bilirubin-UDP-glucuronosyl transferase (ER; inducible by phenobarbital). Long-standing serum presence (cholestasis/obstruction/chronic active hepatitis)→IRREVERSIBLE albumin binding = DELTA BILIRUBIN (biliprotein) — NOT renally excreted, persists after recovery
• Secretion: active transport into canaliculi

4. Intestinal phase: →stercobilinogen (stool colour) OR urobilinogen (bacterial). Conjugated NOT reabsorbed; urobilinogen IS reabsorbed (enterohepatic)→part resecreted in bile, part urine.

Unconjugated vs conjugated table

Liver cell necrosis types

1. DIFFUSE (submassive-massive) — all cells, groups of lobules. Viral hepatitis, drug toxicity
2. ZONAL (3 sub-types by blood-supply logic):
   • CENTRILOBULAR (zone3, around central vein) — COMMONEST. Worst-perfused→ischaemia (shock/CHF), also CCl4/chloroform
   • MIDZONAL (zone2) — uncommon. Yellow fever, viral hepatitis (→Councilman bodies, acidophilic)
   • PERIPORTAL (zone1) — best-perfused→FIRST hit by circulating toxins (phosphorus poisoning, eclampsia)
3. FOCAL — small irregular groups. Microbiologic (viral hepatitis, miliary TB, typhoid), drug-induced

Pathophysiologic classification

4 mechanisms: ↑production, ↓uptake, ↓conjugation (all →mainly UNCONJUGATED), ↓excretion into bile (→mainly CONJUGATED). Quick test: urine bilirubin present=conjugated (water-soluble, filtered); absent=unconjugated.

I. Predominantly UNCONJUGATED

1. ↑Production (haemolytic/acholuric/prehepatic) — haemolysis/ineffective erythropoiesis overwhelms conjugating capacity. Premature infants: deficient enzyme+high RBC destruction→icterus neonatorum, worst in HDN. KERNICTERUS risk if unconjugated >20mg/dl. Labs: normal transaminases/ALP/proteins, absent urine bile pigment (ACHOLURIC), dark stools.
2. ↓Hepatic uptake — drugs, starvation, sepsis
3. ↓Conjugation — hereditary (Gilbert’s, Crigler-Najjar) or acquired (drugs/hepatitis/cirrhosis). Physiologic neonatal jaundice = enzyme immaturity + ↑RBC destruction

II. Predominantly CONJUGATED (cholestasis)

Cholestasis = failure of bile reaching duodenum; bile accumulates in cells/passages. Intrahepatic(medical) vs extrahepatic(obstructive/surgical) — DISTINCTION MATTERS: extrahepatic often surgically fixable, intrahepatic can WORSEN with surgery. Prolonged either type→biliary cirrhosis.

1. Intrahepatic:

• Hereditary (“pure cholestasis”): Dubin-Johnson, Rotor, cystic fibrosis pancreas, benign familial recurrent cholestasis, intrahepatic atresia, pregnancy cholestasis
• Acquired (“hepatocellular cholestasis”): viral hepatitis, alcoholic hepatitis, drugs (chlorpromazine, OCPs)
• Features: conjugated hyperbilirubinaemia, bilirubinuria, ↑bile acids→pruritus, ↑ALP, hyperlipidaemia, hypoprothrombinaemia. Pure vs hepatocellular cholestasis: ↑transaminases ONLY in hepatocellular (liver injury)
• Biopsy: MILDER than extrahepatic. Elongated bile plugs in canaliculi, feathery hepatocyte degeneration→ductular proliferation→periportal fibrosis (resembles biliary cirrhosis)

2. Extrahepatic — mechanical obstruction: gallstones, strictures, pancreatic head CA, bile duct tumours, sclerosing cholangitis, congenital atresia. Complete/sudden→progressive; partial→intermittent.

• Shares conjugated hyperbilirubinaemia/bilirubinuria/pruritus/ALP/hyperlipidaemia, PLUS distinguishing features: fat-soluble vit A/D/E/K malabsorption+steatorrhoea→vit K deficiency→prolonged PT that IMPROVES with parenteral vit K (contrast: hepatocellular hypoprothrombinaemia does NOT improve). Clay-coloured stools (no stercobilin), urine urobilinogen vanishes. Fever common (ascending cholangitis)
• Biopsy: MORE MARKED than intrahepatic. Retrograde extension→dilated ducts, ruptured canaliculi→BILE LAKES (toxic, surrounded by focal necrosis). Bile stasis→ascending bacterial infection

Clinical relevance

Urine bilirubin test = fastest bedside split (conjugated=water-soluble=filtered). Vit-K response test = THE discriminator extrahepatic(improves, surgical) vs intrahepatic(doesn’t improve, medical) — directly drives whether surgery helps or harms. Zonal necrosis = blood-supply logic: worst-perfused(centrilobular)=ischaemia-sensitive; best-perfused(periportal)=toxin-first-exposed — generates all 3 mappings without rote memory. Kernicterus threshold (20mg/dl unconjugated) = shared endpoint of physiologic + HDN routes, high clinical-management yield.

---

Jaundice and Bilirubin Metabolism

Overview

Jaundice = yellow skin/sclera pigmentation from ↑blood bilirubin. High affinity for elastic tissue → sclera shows first. Normal: 0.3-1.3mg/dl (~80% unconjugated). Visible >2mg/dl; below that = LATENT jaundice.

Bilirubin metabolism — 4 phases

1. Source: 80-85% from senescent RBC Hb catabolism (RE system: marrow/spleen/liver, 120-day lifespan). Remainder: non-Hb haem pigments (myoglobin/catalase/cytochromes) + ineffective erythropoiesis. Haem→biliverdin (haem oxygenase, needs O2+NADPH)→bilirubin (biliverdin reductase).

2. Transport: unconjugated, albumin-bound. Sulfonamides/salicylates DISPLACE bilirubin from albumin → neonates → ↑kernicterus risk.

3. Hepatic phase (3 steps):

• Uptake: dissociates from albumin → binds GST/ligandin
• Conjugation: unconjugated (alcohol-soluble)→water-soluble mono/diglucuronide via bilirubin-UDP-glucuronosyl transferase (ER; inducible by phenobarbital). Long-standing serum presence (cholestasis/obstruction/chronic active hepatitis)→IRREVERSIBLE albumin binding = DELTA BILIRUBIN (biliprotein) — NOT renally excreted, persists after recovery
• Secretion: active transport into canaliculi

4. Intestinal phase: →stercobilinogen (stool colour) OR urobilinogen (bacterial). Conjugated NOT reabsorbed; urobilinogen IS reabsorbed (enterohepatic)→part resecreted in bile, part urine.

Unconjugated vs conjugated table

Liver cell necrosis types

1. DIFFUSE (submassive-massive) — all cells, groups of lobules. Viral hepatitis, drug toxicity
2. ZONAL (3 sub-types by blood-supply logic):
   • CENTRILOBULAR (zone3, around central vein) — COMMONEST. Worst-perfused→ischaemia (shock/CHF), also CCl4/chloroform
   • MIDZONAL (zone2) — uncommon. Yellow fever, viral hepatitis (→Councilman bodies, acidophilic)
   • PERIPORTAL (zone1) — best-perfused→FIRST hit by circulating toxins (phosphorus poisoning, eclampsia)
3. FOCAL — small irregular groups. Microbiologic (viral hepatitis, miliary TB, typhoid), drug-induced

Pathophysiologic classification

4 mechanisms: ↑production, ↓uptake, ↓conjugation (all →mainly UNCONJUGATED), ↓excretion into bile (→mainly CONJUGATED). Quick test: urine bilirubin present=conjugated (water-soluble, filtered); absent=unconjugated.

I. Predominantly UNCONJUGATED

1. ↑Production (haemolytic/acholuric/prehepatic) — haemolysis/ineffective erythropoiesis overwhelms conjugating capacity. Premature infants: deficient enzyme+high RBC destruction→icterus neonatorum, worst in HDN. KERNICTERUS risk if unconjugated >20mg/dl. Labs: normal transaminases/ALP/proteins, absent urine bile pigment (ACHOLURIC), dark stools.
2. ↓Hepatic uptake — drugs, starvation, sepsis
3. ↓Conjugation — hereditary (Gilbert’s, Crigler-Najjar) or acquired (drugs/hepatitis/cirrhosis). Physiologic neonatal jaundice = enzyme immaturity + ↑RBC destruction

II. Predominantly CONJUGATED (cholestasis)

Cholestasis = failure of bile reaching duodenum; bile accumulates in cells/passages. Intrahepatic(medical) vs extrahepatic(obstructive/surgical) — DISTINCTION MATTERS: extrahepatic often surgically fixable, intrahepatic can WORSEN with surgery. Prolonged either type→biliary cirrhosis.

1. Intrahepatic:

• Hereditary (“pure cholestasis”): Dubin-Johnson, Rotor, cystic fibrosis pancreas, benign familial recurrent cholestasis, intrahepatic atresia, pregnancy cholestasis
• Acquired (“hepatocellular cholestasis”): viral hepatitis, alcoholic hepatitis, drugs (chlorpromazine, OCPs)
• Features: conjugated hyperbilirubinaemia, bilirubinuria, ↑bile acids→pruritus, ↑ALP, hyperlipidaemia, hypoprothrombinaemia. Pure vs hepatocellular cholestasis: ↑transaminases ONLY in hepatocellular (liver injury)
• Biopsy: MILDER than extrahepatic. Elongated bile plugs in canaliculi, feathery hepatocyte degeneration→ductular proliferation→periportal fibrosis (resembles biliary cirrhosis)

2. Extrahepatic — mechanical obstruction: gallstones, strictures, pancreatic head CA, bile duct tumours, sclerosing cholangitis, congenital atresia. Complete/sudden→progressive; partial→intermittent.

• Shares conjugated hyperbilirubinaemia/bilirubinuria/pruritus/ALP/hyperlipidaemia, PLUS distinguishing features: fat-soluble vit A/D/E/K malabsorption+steatorrhoea→vit K deficiency→prolonged PT that IMPROVES with parenteral vit K (contrast: hepatocellular hypoprothrombinaemia does NOT improve). Clay-coloured stools (no stercobilin), urine urobilinogen vanishes. Fever common (ascending cholangitis)
• Biopsy: MORE MARKED than intrahepatic. Retrograde extension→dilated ducts, ruptured canaliculi→BILE LAKES (toxic, surrounded by focal necrosis). Bile stasis→ascending bacterial infection

Clinical relevance

Urine bilirubin test = fastest bedside split (conjugated=water-soluble=filtered). Vit-K response test = THE discriminator extrahepatic(improves, surgical) vs intrahepatic(doesn’t improve, medical) — directly drives whether surgery helps or harms. Zonal necrosis = blood-supply logic: worst-perfused(centrilobular)=ischaemia-sensitive; best-perfused(periportal)=toxin-first-exposed — generates all 3 mappings without rote memory. Kernicterus threshold (20mg/dl unconjugated) = shared endpoint of physiologic + HDN routes, high clinical-management yield.

Jaundice and Bilirubin Metabolism

Overview

Jaundice (icterus) is yellow pigmentation of skin/sclerae by bilirubin, resulting from elevated blood bilirubin (hyperbilirubinaemia). Bilirubin has high affinity for elastic tissue, hence jaundice shows up preferentially in elastin-rich tissue (sclera). Normal serum bilirubin: 0.3–1.3 mg/dl (~80% unconjugated). Jaundice becomes clinically visible above 2 mg/dl; a rise between normal and 2 mg/dl without visible yellowing is latent jaundice.

Normal Bilirubin Metabolism — Four Phases

1. Source: ~80–85% of bilirubin comes from catabolism of haemoglobin in senescent red cells, destroyed at the end of their 120-day lifespan in the reticuloendothelial system (bone marrow, spleen, liver). The remaining 15–20% comes partly from non-haemoglobin haem pigments (myoglobin, catalase, cytochromes) and partly from ineffective erythropoiesis. Haem is converted to biliverdin by microsomal haem oxygenase (needs O₂ and NADPH), then biliverdin to bilirubin by biliverdin reductase.

2. Transport: released bilirubin circulates as unconjugated bilirubin tightly albumin-bound. Drugs like sulfonamides and salicylates compete for albumin binding, displacing bilirubin and — in neonates — facilitating its entry into the brain, raising kernicterus risk.

3. Hepatic phase — three steps:

• Uptake: albumin-bound unconjugated bilirubin dissociates at the hepatocyte surface; bilirubin binds cytoplasmic glutathione-S-transferase (GST/ligandin).
• Conjugation: unconjugated bilirubin (alcohol-soluble, not water-soluble) is converted to water-soluble bilirubin mono-/diglucuronide in the endoplasmic reticulum by bilirubin-UDP-glucuronosyl transferase — a process inducible by drugs like phenobarbital. Conjugated bilirubin binds albumin reversibly (like unconjugated) or, if present long in serum (cholestasis, long-standing biliary obstruction, chronic active hepatitis), irreversibly as delta bilirubin (biliprotein) — which is not renally excreted and stays detectable after recovery.
• Secretion into bile: conjugated bilirubin is actively transported into bile canaliculi.

4. Intestinal phase: conjugated bilirubin in the gut lumen is either excreted directly as stercobilinogen (normal stool colour) or converted to urobilinogen by intestinal bacteria. Conjugated bilirubin itself is not reabsorbed, but urobilinogen is reabsorbed via enterohepatic circulation — part re-secreted into bile, part excreted in urine.

Unconjugated vs Conjugated Bilirubin

Types of Liver Cell Necrosis

All hepatic injury (microbiologic, toxic, circulatory, traumatic) causes liver cell necrosis; extent within the lobule varies:

1. Diffuse (submassive to massive) — extensive necrosis involving all cells in groups of lobules; commonest causes: viral hepatitis, drug toxicity.
2. Zonal necrosis — three sub-patterns by lobular zone:
   • Centrilobular (zone 3, around central vein) — commonest; characteristic of ischaemic injury (shock, CHF, since zone 3 is farthest from blood supply), also chloroform/carbon tetrachloride poisoning.
   • Midzonal (zone 2) — uncommon; yellow fever, viral hepatitis (necrosed mid-zone hepatocytes become acidophilic Councilman bodies).
   • Periportal (zone 1, closest to arterial/portal supply) — most vulnerable to circulating hepatotoxins since best perfused, e.g. phosphorus poisoning, eclampsia.
3. Focal necrosis — small, irregularly distributed hepatocyte groups; most often from microbiologic infection (viral hepatitis, miliary TB, typhoid, various bacterial/viral/fungal infections); also drug-induced hepatitis.

Pathophysiologic Classification of Jaundice

Four underlying mechanisms: increased bilirubin production, decreased hepatic uptake, decreased hepatic conjugation, decreased excretion into bile. The first three yield mainly unconjugated hyperbilirubinaemia; the last yields mainly conjugated hyperbilirubinaemia — this is the modern basis for classification (superseding the older pre-hepatic/hepatic/post-hepatic split). A simple discriminating test: bilirubin in urine → conjugated hyperbilirubinaemia (unconjugated bilirubin isn’t filtered by the glomerulus, so its absence from urine suggests unconjugated hyperbilirubinaemia).

I. Predominantly Unconjugated Hyperbilirubinaemia

1. Increased bilirubin production (haemolytic/acholuric/prehepatic jaundice) — from excessive red cell destruction (intra-/extravascular haemolysis, ineffective erythropoiesis); hyperbilirubinaemia develops once the liver’s conjugating capacity is exceeded. In premature infants, deficient conjugating enzyme plus high red cell destruction causes icterus neonatorum, particularly severe in haemolytic disease of the newborn — danger of permanent brain damage (kernicterus) once unconjugated bilirubin exceeds 20 mg/dl. Lab findings: predominant unconjugated hyperbilirubinaemia, normal transaminases/alkaline phosphatase/proteins, absent urine bile pigment (acholuric jaundice), dark brown stools (excess faecal bile pigment excretion).

2. Decreased hepatic uptake — impaired dissociation/binding to GST/ligandin, from drugs, prolonged starvation, sepsis.

3. Decreased bilirubin conjugation — from glucuronosyl transferase defect/deficiency: hereditary (Gilbert’s syndrome, Crigler-Najjar syndrome) or acquired (drugs, hepatitis, cirrhosis — though hepatocellular damage more often deranges excretory than conjugating capacity). Physiologic neonatal jaundice partly reflects relative UDP-glucuronosyl transferase deficiency plus increased neonatal red cell destruction.

II. Predominantly Conjugated Hyperbilirubinaemia (Cholestasis)

Cholestasis = failure of normal bile amounts to reach the duodenum; morphologically, accumulation of bile in liver cells and biliary passages. The excretory defect may be intrahepatic (medical jaundice) or extrahepatic (obstructive jaundice, from mechanical obstruction). Distinguishing the two matters clinically: extrahepatic cholestasis is often surgically treatable; intrahepatic cholestasis is not, and may worsen after surgery. Prolonged cholestasis of either type may progress to biliary cirrhosis.

1. Intrahepatic cholestasis — impaired hepatic excretion:

• Hereditary (“pure cholestasis”): Dubin-Johnson syndrome, Rotor syndrome, fibrocystic disease of pancreas, benign familial recurrent cholestasis, intrahepatic atresia, cholestatic jaundice of pregnancy.
• Acquired (“hepatocellular cholestasis”): viral hepatitis, alcoholic hepatitis, drug-induced (chlorpromazine, oral contraceptives).
• Features: predominant conjugated hyperbilirubinaemia (regurgitation into blood), bilirubinuria, raised serum bile acids with pruritus, raised alkaline phosphatase, hyperlipidaemia, hypoprothrombinaemia. “Pure cholestasis” is distinguished from “hepatocellular cholestasis” by elevated transaminases in the latter (reflecting liver cell injury).
• Biopsy: milder cholestasis than extrahepatic disease; dilated biliary canaliculi with elongated green-brown bile plugs; feathery degeneration of hepatocyte cytoplasm; canalicular bile stasis eventually causes ductular proliferation and periportal fibrosis, resembling biliary cirrhosis.

2. Extrahepatic cholestasis — mechanical obstruction of large bile ducts outside the liver or within the porta hepatis: gallstones, inflammatory strictures, carcinoma of the pancreatic head, bile-duct tumours, sclerosing cholangitis, congenital atresia of extrahepatic ducts. Obstruction may be complete/sudden (progressive obstructive jaundice) or partial/incomplete (intermittent jaundice).

• Shares conjugated hyperbilirubinaemia/bilirubinuria/pruritus/raised alkaline phosphatase/hyperlipidaemia with intrahepatic cholestasis, but has distinguishing features: fat-soluble vitamin (A, D, E, K) malabsorption and steatorrhoea → vitamin K deficiency → prolonged prothrombin time that improves with parenteral vitamin K (unlike hepatocellular hypoprothrombinaemia, which does not improve). Clay-coloured stools (no stercobilin), virtual disappearance of urine urobilinogen. Fever is common from ascending bacterial infection (ascending cholangitis).
• Biopsy: more marked cholestasis than intrahepatic disease; progressive retrograde extension of bile stasis into intrahepatic ducts → dilated ducts, ruptured canaliculi, bile extravasation forming bile lakes (toxic bile surrounded by focal hepatocyte necrosis); bile stasis predisposes to ascending bacterial infection with periductal polymorph accumulation (ascending cholangitis).

Clinical Relevance

• The urine-bilirubin test (present = conjugated, absent = unconjugated) is the single fastest bedside discriminator between the two broad categories, and follows directly from the physical-chemistry fact that only conjugated bilirubin is water-soluble and glomerulus-filterable.
• The vitamin-K response test is the classic discriminator between extrahepatic (obstructive, surgically correctable) and intrahepatic/hepatocellular cholestasis — prothrombin time correcting with parenteral vitamin K points to a mechanical, malabsorption-driven deficiency rather than a synthetic-failure one, and this distinction directly drives whether surgery helps or harms the patient.
• Zonal necrosis patterns are worth memorising as a “blood-supply logic” set: centrilobular = worst-perfused zone = ischaemia-sensitive; periportal = best-perfused zone = toxin-exposure-sensitive (since toxins arrive via that blood first) — this single logic generates all three zone-to-cause mappings without rote memorisation.
• Kernicterus is the shared danger across both the physiologic (neonatal enzyme immaturity) and haemolytic-disease-of-the-newborn routes to unconjugated hyperbilirubinaemia — the specific numeric threshold (20 mg/dl) is worth retaining given its direct clinical-management implication.

---

Jaundice and Bilirubin Metabolism

Overview

Jaundice (icterus) is yellow pigmentation of skin/sclerae by bilirubin, resulting from elevated blood bilirubin (hyperbilirubinaemia). Bilirubin has high affinity for elastic tissue, hence jaundice shows up preferentially in elastin-rich tissue (sclera). Normal serum bilirubin: 0.3–1.3 mg/dl (~80% unconjugated). Jaundice becomes clinically visible above 2 mg/dl; a rise between normal and 2 mg/dl without visible yellowing is latent jaundice.

Normal Bilirubin Metabolism — Four Phases

1. Source: ~80–85% of bilirubin comes from catabolism of haemoglobin in senescent red cells, destroyed at the end of their 120-day lifespan in the reticuloendothelial system (bone marrow, spleen, liver). The remaining 15–20% comes partly from non-haemoglobin haem pigments (myoglobin, catalase, cytochromes) and partly from ineffective erythropoiesis. Haem is converted to biliverdin by microsomal haem oxygenase (needs O₂ and NADPH), then biliverdin to bilirubin by biliverdin reductase.

2. Transport: released bilirubin circulates as unconjugated bilirubin tightly albumin-bound. Drugs like sulfonamides and salicylates compete for albumin binding, displacing bilirubin and — in neonates — facilitating its entry into the brain, raising kernicterus risk.

3. Hepatic phase — three steps:

• Uptake: albumin-bound unconjugated bilirubin dissociates at the hepatocyte surface; bilirubin binds cytoplasmic glutathione-S-transferase (GST/ligandin).
• Conjugation: unconjugated bilirubin (alcohol-soluble, not water-soluble) is converted to water-soluble bilirubin mono-/diglucuronide in the endoplasmic reticulum by bilirubin-UDP-glucuronosyl transferase — a process inducible by drugs like phenobarbital. Conjugated bilirubin binds albumin reversibly (like unconjugated) or, if present long in serum (cholestasis, long-standing biliary obstruction, chronic active hepatitis), irreversibly as delta bilirubin (biliprotein) — which is not renally excreted and stays detectable after recovery.
• Secretion into bile: conjugated bilirubin is actively transported into bile canaliculi.

4. Intestinal phase: conjugated bilirubin in the gut lumen is either excreted directly as stercobilinogen (normal stool colour) or converted to urobilinogen by intestinal bacteria. Conjugated bilirubin itself is not reabsorbed, but urobilinogen is reabsorbed via enterohepatic circulation — part re-secreted into bile, part excreted in urine.

Unconjugated vs Conjugated Bilirubin

Types of Liver Cell Necrosis

All hepatic injury (microbiologic, toxic, circulatory, traumatic) causes liver cell necrosis; extent within the lobule varies:

1. Diffuse (submassive to massive) — extensive necrosis involving all cells in groups of lobules; commonest causes: viral hepatitis, drug toxicity.
2. Zonal necrosis — three sub-patterns by lobular zone:
   • Centrilobular (zone 3, around central vein) — commonest; characteristic of ischaemic injury (shock, CHF, since zone 3 is farthest from blood supply), also chloroform/carbon tetrachloride poisoning.
   • Midzonal (zone 2) — uncommon; yellow fever, viral hepatitis (necrosed mid-zone hepatocytes become acidophilic Councilman bodies).
   • Periportal (zone 1, closest to arterial/portal supply) — most vulnerable to circulating hepatotoxins since best perfused, e.g. phosphorus poisoning, eclampsia.
3. Focal necrosis — small, irregularly distributed hepatocyte groups; most often from microbiologic infection (viral hepatitis, miliary TB, typhoid, various bacterial/viral/fungal infections); also drug-induced hepatitis.

Pathophysiologic Classification of Jaundice

Four underlying mechanisms: increased bilirubin production, decreased hepatic uptake, decreased hepatic conjugation, decreased excretion into bile. The first three yield mainly unconjugated hyperbilirubinaemia; the last yields mainly conjugated hyperbilirubinaemia — this is the modern basis for classification (superseding the older pre-hepatic/hepatic/post-hepatic split). A simple discriminating test: bilirubin in urine → conjugated hyperbilirubinaemia (unconjugated bilirubin isn’t filtered by the glomerulus, so its absence from urine suggests unconjugated hyperbilirubinaemia).

I. Predominantly Unconjugated Hyperbilirubinaemia

1. Increased bilirubin production (haemolytic/acholuric/prehepatic jaundice) — from excessive red cell destruction (intra-/extravascular haemolysis, ineffective erythropoiesis); hyperbilirubinaemia develops once the liver’s conjugating capacity is exceeded. In premature infants, deficient conjugating enzyme plus high red cell destruction causes icterus neonatorum, particularly severe in haemolytic disease of the newborn — danger of permanent brain damage (kernicterus) once unconjugated bilirubin exceeds 20 mg/dl. Lab findings: predominant unconjugated hyperbilirubinaemia, normal transaminases/alkaline phosphatase/proteins, absent urine bile pigment (acholuric jaundice), dark brown stools (excess faecal bile pigment excretion).

2. Decreased hepatic uptake — impaired dissociation/binding to GST/ligandin, from drugs, prolonged starvation, sepsis.

3. Decreased bilirubin conjugation — from glucuronosyl transferase defect/deficiency: hereditary (Gilbert’s syndrome, Crigler-Najjar syndrome) or acquired (drugs, hepatitis, cirrhosis — though hepatocellular damage more often deranges excretory than conjugating capacity). Physiologic neonatal jaundice partly reflects relative UDP-glucuronosyl transferase deficiency plus increased neonatal red cell destruction.

II. Predominantly Conjugated Hyperbilirubinaemia (Cholestasis)

Cholestasis = failure of normal bile amounts to reach the duodenum; morphologically, accumulation of bile in liver cells and biliary passages. The excretory defect may be intrahepatic (medical jaundice) or extrahepatic (obstructive jaundice, from mechanical obstruction). Distinguishing the two matters clinically: extrahepatic cholestasis is often surgically treatable; intrahepatic cholestasis is not, and may worsen after surgery. Prolonged cholestasis of either type may progress to biliary cirrhosis.

1. Intrahepatic cholestasis — impaired hepatic excretion:

• Hereditary (“pure cholestasis”): Dubin-Johnson syndrome, Rotor syndrome, fibrocystic disease of pancreas, benign familial recurrent cholestasis, intrahepatic atresia, cholestatic jaundice of pregnancy.
• Acquired (“hepatocellular cholestasis”): viral hepatitis, alcoholic hepatitis, drug-induced (chlorpromazine, oral contraceptives).
• Features: predominant conjugated hyperbilirubinaemia (regurgitation into blood), bilirubinuria, raised serum bile acids with pruritus, raised alkaline phosphatase, hyperlipidaemia, hypoprothrombinaemia. “Pure cholestasis” is distinguished from “hepatocellular cholestasis” by elevated transaminases in the latter (reflecting liver cell injury).
• Biopsy: milder cholestasis than extrahepatic disease; dilated biliary canaliculi with elongated green-brown bile plugs; feathery degeneration of hepatocyte cytoplasm; canalicular bile stasis eventually causes ductular proliferation and periportal fibrosis, resembling biliary cirrhosis.

2. Extrahepatic cholestasis — mechanical obstruction of large bile ducts outside the liver or within the porta hepatis: gallstones, inflammatory strictures, carcinoma of the pancreatic head, bile-duct tumours, sclerosing cholangitis, congenital atresia of extrahepatic ducts. Obstruction may be complete/sudden (progressive obstructive jaundice) or partial/incomplete (intermittent jaundice).

• Shares conjugated hyperbilirubinaemia/bilirubinuria/pruritus/raised alkaline phosphatase/hyperlipidaemia with intrahepatic cholestasis, but has distinguishing features: fat-soluble vitamin (A, D, E, K) malabsorption and steatorrhoea → vitamin K deficiency → prolonged prothrombin time that improves with parenteral vitamin K (unlike hepatocellular hypoprothrombinaemia, which does not improve). Clay-coloured stools (no stercobilin), virtual disappearance of urine urobilinogen. Fever is common from ascending bacterial infection (ascending cholangitis).
• Biopsy: more marked cholestasis than intrahepatic disease; progressive retrograde extension of bile stasis into intrahepatic ducts → dilated ducts, ruptured canaliculi, bile extravasation forming bile lakes (toxic bile surrounded by focal hepatocyte necrosis); bile stasis predisposes to ascending bacterial infection with periductal polymorph accumulation (ascending cholangitis).

Clinical Relevance

• The urine-bilirubin test (present = conjugated, absent = unconjugated) is the single fastest bedside discriminator between the two broad categories, and follows directly from the physical-chemistry fact that only conjugated bilirubin is water-soluble and glomerulus-filterable.
• The vitamin-K response test is the classic discriminator between extrahepatic (obstructive, surgically correctable) and intrahepatic/hepatocellular cholestasis — prothrombin time correcting with parenteral vitamin K points to a mechanical, malabsorption-driven deficiency rather than a synthetic-failure one, and this distinction directly drives whether surgery helps or harms the patient.
• Zonal necrosis patterns are worth memorising as a “blood-supply logic” set: centrilobular = worst-perfused zone = ischaemia-sensitive; periportal = best-perfused zone = toxin-exposure-sensitive (since toxins arrive via that blood first) — this single logic generates all three zone-to-cause mappings without rote memorisation.
• Kernicterus is the shared danger across both the physiologic (neonatal enzyme immaturity) and haemolytic-disease-of-the-newborn routes to unconjugated hyperbilirubinaemia — the specific numeric threshold (20 mg/dl) is worth retaining given its direct clinical-management implication.

Jaundice and Bilirubin Metabolism

• The four-phase bilirubin metabolism pathway (source → transport → hepatic phase’s three steps → intestinal phase) as a flow diagram
• Hepatic lobule cross-section with the three necrosis zones (centrilobular/midzonal/periportal) marked against blood supply direction
• Pathophysiologic classification tree: unconjugated (3 mechanisms) vs conjugated/cholestasis (intrahepatic vs extrahepatic) with their distinguishing lab features

---

Jaundice and Bilirubin Metabolism

• The four-phase bilirubin metabolism pathway (source → transport → hepatic phase’s three steps → intestinal phase) as a flow diagram
• Hepatic lobule cross-section with the three necrosis zones (centrilobular/midzonal/periportal) marked against blood supply direction
• Pathophysiologic classification tree: unconjugated (3 mechanisms) vs conjugated/cholestasis (intrahepatic vs extrahepatic) with their distinguishing lab features