Bilirubin is a yellow bile pigment produced during the physiological breakdown of haem-containing proteins, predominantly haemoglobin from senescent erythrocytes. Following its formation, bilirubin is transported to the liver, where it undergoes conjugation before being excreted into bile and eliminated through the gastrointestinal tract. Measurement of serum bilirubin is a routine biochemical investigation and provides valuable information regarding hepatic function, biliary drainage, and red blood cell turnover.
Serum bilirubin forms part of standard liver function tests (LFTs) and is widely used in primary care, emergency medicine, hepatology, gastroenterology, general medicine, and critical care. Abnormal bilirubin concentrations may reflect hepatocellular injury, cholestasis, haemolysis, inherited disorders of bilirubin metabolism, or systemic disease.
Bilirubin Metabolism
Bilirubin exists in two principal forms:
- Unconjugated (indirect) bilirubin is generated following the breakdown of haemoglobin within the reticuloendothelial system. It is lipid-soluble and transported to the liver bound to albumin.
- Conjugated (direct) bilirubin is produced following hepatic conjugation with glucuronic acid, rendering it water-soluble and allowing excretion into bile.
Routine laboratory analysis commonly reports total bilirubin, representing the combined concentration of unconjugated and conjugated bilirubin. Fractionation into direct and indirect bilirubin may assist in determining the underlying mechanism responsible for hyperbilirubinaemia.
Hyperbilirubinaemia
Hyperbilirubinaemia refers to an elevated serum bilirubin concentration and has a broad differential diagnosis. The underlying mechanism may be classified as pre-hepatic, hepatic, or post-hepatic.
Common causes include:
- Acute and chronic liver disease
- Viral hepatitis
- Alcohol-related liver disease
- Metabolic dysfunction-associated steatotic liver disease (MASLD)
- Biliary obstruction, including gallstones and malignancy
- Haemolytic anaemias
- Gilbert syndrome
- Drug-induced liver injury
- Sepsis and critical illness
- Pancreatic disease causing biliary obstruction
Marked elevation of serum bilirubin may result in jaundice, characterised by yellow discolouration of the skin, sclerae, and mucous membranes. Associated clinical features vary according to the underlying pathology and may include dark urine, pale stools, pruritus, abdominal pain, fever, weight loss, or constitutional symptoms.
Low Bilirubin
Reduced serum bilirubin concentrations are generally of limited clinical significance and rarely indicate underlying disease. In most circumstances, isolated low bilirubin values do not require further investigation.
Clinical Interpretation
Serum bilirubin should always be interpreted in conjunction with the wider clinical picture and other biochemical markers of liver function. Assessment commonly includes:
- Alanine aminotransferase (ALT)
- Aspartate aminotransferase (AST)
- Alkaline phosphatase (ALP)
- Gamma-glutamyl transferase (GGT)
- Serum albumin
- International Normalised Ratio (INR)
Additional investigations may include full blood count, haemolysis screening, viral serology, autoimmune markers, abdominal ultrasonography, computed tomography (CT), magnetic resonance cholangiopancreatography (MRCP), or liver biopsy where clinically indicated. Interpretation should incorporate the patient’s medical history, medication profile, physical examination, and serial laboratory measurements, as trends often provide greater diagnostic value than isolated results.
Bilirubin in Clinical Scoring Systems
Serum bilirubin is incorporated into several validated clinical calculators and prognostic models. It forms part of the Sequential Organ Failure Assessment (SOFA) Score, where it is used to quantify hepatic dysfunction in critically ill patients. Bilirubin is also included in established liver disease severity scores, including the Child–Pugh classification, and contributes to prognostic assessment in patients with chronic liver disease and cirrhosis.
The inclusion of bilirubin within these validated scoring systems reflects its importance as an objective marker of hepatic function and disease severity.
Clinical Significance
Bilirubin remains a key biochemical marker in the investigation of hepatobiliary disease, haemolytic disorders, and systemic illness. Accurate interpretation requires consideration of the underlying clinical context, associated laboratory findings, and the pattern of bilirubin elevation. When integrated with comprehensive clinical assessment, serum bilirubin contributes to diagnosis, prognostic evaluation, monitoring of disease progression, and evidence-based clinical decision-making across a wide range of medical specialties.