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Bilirubin, Direct

SI UNITS (recommended)

CONVENTIONAL UNITS

Synonym

Direct Bilirubin, DBIL; Neonatal Bilirubin, Conjugated Bilirubin, Haematoidin

Units of measurement

mmol/l, µmol/l, mg/dl, mg/100ml, mg%, mg/l, µg/ml

BILIRUBIN, DIRECT

(Conjugated Bilirubin – Water-soluble Bilirubin – Liver Excretory Function Marker)

Synonyms

  • Direct bilirubin
  • Conjugated bilirubin
  • Bilirubin diglucuronide
  • CBil
  • Direct-reacting bilirubin
  • Bilirubin (D)

Units of Measurement

  • mmol/L
  • µmol/L
  • mg/dL
  • mg/100 mL
  • mg%
  • mg/L
  • µg/mL

Description

Direct bilirubin is the water-soluble, conjugated form of bilirubin produced in the liver by UDP-glucuronyl transferase, forming:

  • Bilirubin mono-glucuronide
  • Bilirubin di-glucuronide (major component)

Direct bilirubin reflects hepatic conjugation and biliary excretion.

Laboratory measurement of "direct bilirubin" corresponds mainly to:

  • Conjugated bilirubin
  • Delta-bilirubin (bilirubin covalently bound to albumin)

It is elevated when bilirubin can be conjugated but cannot be excreted, typically from cholestasis or obstruction.

Physiological Role

  • Conjugation converts toxic, lipid-soluble bilirubin → water-soluble form
  • Enables transport into bile → intestine
  • Excreted as urobilinogen / stercobilin

Direct bilirubin is normally very low because liver excretion is rapid.

Clinical Significance

Elevated Direct Bilirubin (Conjugated Hyperbilirubinemia)

Seen when the liver conjugates bilirubin normally, but excretion is impaired.

1. Cholestasis (Most Important)

  • Extrahepatic obstruction
    • Gallstones
    • Pancreatic cancer
    • Biliary atresia
    • Strictures
  • Intrahepatic cholestasis
    • Primary biliary cholangitis
    • Primary sclerosing cholangitis
    • Drugs (OCPs, steroids, antibiotics)

2. Hepatocellular Injury

  • Viral hepatitis
  • Cirrhosis
  • Drug-induced liver injury
  • Alcoholic hepatitis

3. Neonatal Cholestasis

  • Biliary atresia
  • Neonatal hepatitis
  • Sepsis

4. Genetic Disorders

  • Dubin–Johnson syndrome
  • Rotor syndrome
     (Route of excretion impaired but conjugation preserved)

5. Sepsis

Impaired biliary transport → cholestasis.

Low Direct Bilirubin

  • Usually normal and not clinically significant
  • Indirect bilirubin predominates in hemolysis or ineffective erythropoiesis

Reference Intervals

(Tietz 8E + IFCC + Mayo/ARUP)

Adults

  • 0.0 – 0.3 mg/dL
  • 0 – 5 µmol/L

Mild elevation

  • 0.3 – 1.0 mg/dL (5–17 µmol/L)

Moderate–Severe

  • > 1.0 mg/dL (>17 µmol/L)

Cholestasis

  • Direct bilirubin usually > 50% of total bilirubin

Neonates

Conjugated bilirubin:

  • >1 mg/dL → always abnormal, needs evaluation
  • 20% of total bilirubin → cholestasis

Units Description & Conversion Factors

Molecular weight (MW) of bilirubin = 584.7 g/mol

Unit Meanings

UnitMeaning
mmol/Lmillimole per liter
µmol/Lmicromole per liter
mg/dLmilligram per deciliter
mg%mg per 100 mL
mg/100 mLsame as mg%
mg/Lmilligram per liter
µg/mLmicrogram per milliliter

Conversions

mg/dL ↔ µmol/L

µmol/L=mg/dL×17.1\text{µmol/L} = \text{mg/dL} \times 17.1µmol/L=mg/dL×17.1 mg/dL=µmol/L÷17.1\text{mg/dL} = \text{µmol/L} \div 17.1mg/dL=µmol/L÷17.1

mg/L ↔ mg/dL

mg/dL=mg/L÷10\text{mg/dL} = \text{mg/L} \div 10mg/dL=mg/L÷10

µg/mL ↔ mg/L

1 µg/mL=1 mg/L1\ \text{µg/mL} = 1\ \text{mg/L}1 µg/mL=1 mg/L

mmol/L ↔ µmol/L

1 mmol/L=1000 µmol/L1\ \text{mmol/L} = 1000\ \text{µmol/L}1 mmol/L=1000 µmol/L

mg% = mg/100 mL = mg/dL

Diagnostic Uses

1. Differentiation of Jaundice Types

  • Direct predominant → cholestatic/obstructive
  • Indirect predominant → hemolytic or unconjugated disorders

2. Assessing Liver Excretory Function

Elevated direct bilirubin indicates impaired bile flow.

3. Neonatal Evaluation

Critical for early diagnosis of:

  • Biliary atresia
  • Neonatal hepatitis
  • Metabolic disorders

4. Monitoring Liver Diseases

Useful in:

  • Cholestatic liver diseases
  • Cirrhosis
  • Drug-induced liver injury

5. Sepsis-associated Cholestasis

Direct bilirubin rises early.

Analytical Notes

  • Direct bilirubin measured by diazo reaction (modified Jendrassik–Grof).
  • Hemolysis interferes minimally; lipemia may increase values.
  • Light-sensitive → store away from sunlight.
  • Delta-bilirubin elevates in chronic cholestasis → prolonged rise even when obstruction resolves.

Clinical Pearls

  • Direct bilirubin > 1 mg/dL in infants is always abnormal and suggests cholestasis.
  • If direct bilirubin >50% of total, consider obstructive or hepatocellular processes.
  • In acute hepatitis, both direct and indirect bilirubin rise.
  • Biliary obstruction: high direct bilirubin + high ALP + high GGT.
  • Dubin–Johnson syndrome: direct bilirubin elevated with normal enzymes.

Interesting Fact

Direct bilirubin is also present as delta-bilirubin, a conjugated bilirubin covalently linked to albumin with a long half-life — explaining why jaundice resolves slowly after cholestasis relief.

SEO Unit Converter Text

Direct bilirubin unit converter — convert between mg/dL, µmol/L, mg/L, µg/mL, and mg%. Includes reference ranges and interpretation for liver disease, cholestasis, and neonatal jaundice.

References

  1. Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition — Bilirubin Metabolism.
  2. IFCC Bilirubin Measurement Standardization.
  3. AASLD Guidelines — Cholestatic Liver Disease.
  4. Mayo Clinic Laboratories — Direct Bilirubin.
  5. ARUP Consult — Jaundice Evaluation.
  6. MedlinePlus / NIH — Bilirubin Test.
  7. Pediatric Hepatology References — Neonatal Cholestasis.

Last updated: December 9, 2025

Reviewed by : Medical Review Board

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