Direct Bilirubin

Direct bilirubin is the conjugated form of bilirubin.
It is formed in the liver when unconjugated bilirubin combines with glucuronic acid (via UDP-glucuronyl transferase).

It is:

• Water soluble
• Excreted in bile
• Measured directly by the diazo reaction

1️⃣ Obstructive (Post-Hepatic) Jaundice – Most Important

Markedly Increased in:

• Gallstones
• Carcinoma head of pancreas
• Bile duct obstruction
• Cholestasis

🔎 Mechanism:

• Conjugated bilirubin cannot pass into intestine
• Regurgitates back into bloodstream

👉 Findings:

• ↑ Direct bilirubin
• Dark urine (bilirubin present)
• Pale/clay-colored stools
• ↑ ALP

2️⃣ Hepatocellular Jaundice

Seen in:

• Viral hepatitis
• Cirrhosis
• Drug-induced liver injury

🔎 Mechanism:

• Liver conjugates bilirubin
• But damaged hepatocytes fail to excrete it properly
• Mixed increase (both direct and indirect)

3️⃣ Neonatal Cholestasis

• Biliary atresia
• Neonatal hepatitis

⚠ Direct hyperbilirubinemia in newborn is always pathological and needs evaluation.

4️⃣ Differentiation of Types of Jaundice

Type of Jaundice:
Hemolytic (Pre-hepatic) 
Obstructive (Post-hepatic) 

5️⃣ Urine Bilirubin

Since direct bilirubin is water-soluble:

• It appears in urine
• Causes dark-colored urine

Unconjugated bilirubin does NOT appear in urine.

Normal Value

Direct bilirubin: 0 – 0.3 mg/dL