Bilirubin
Bilirubin is a yellow pigment produced during the normal breakdown of red blood cells. As haemoglobin is metabolised, bilirubin is formed and transported to the liver, where it is processed, conjugated, and excreted into bile before ultimately leaving the body through the digestive system. Measurement of bilirubin is a routine laboratory investigation and provides valuable information about liver function, bile flow, and red blood cell turnover.
Bilirubin testing forms part of standard liver function tests (LFTs) and is commonly requested in primary care, emergency medicine, gastroenterology, hepatology, and critical care. Abnormal bilirubin levels may indicate liver disease, biliary obstruction, haemolysis, or a range of inherited and acquired medical conditions.
Types of Bilirubin
Laboratories typically measure total bilirubin, which represents the combined concentration of unconjugated (indirect) and conjugated (direct) bilirubin.
- Unconjugated bilirubin is produced following the breakdown of red blood cells and is transported to the liver bound to albumin.
- Conjugated bilirubin has been processed by the liver and is water-soluble, allowing it to be excreted in bile.
Differentiating between these forms helps clinicians determine the likely cause of an elevated bilirubin level.
Elevated Bilirubin
Raised bilirubin levels, known as hyperbilirubinaemia, may occur for a variety of reasons.
Common causes include:
- Acute or chronic liver disease
- Viral hepatitis
- Alcohol-related liver disease
- Biliary obstruction (such as gallstones)
- Haemolytic anaemia
- Gilbert’s syndrome
- Drug-induced liver injury
- Sepsis
- Pancreatic disease
Patients with significantly elevated bilirubin may develop jaundice, characterised by yellowing of the skin and sclerae. Other symptoms depend on the underlying condition and may include dark urine, pale stools, abdominal pain, itching, fatigue, or fever.
Low Bilirubin
Low bilirubin concentrations are generally not considered clinically significant and rarely require further investigation.
Bilirubin in Clinical Assessment
Bilirubin should always be interpreted alongside other liver function tests, including:
- Alanine aminotransferase (ALT)
- Aspartate aminotransferase (AST)
- Alkaline phosphatase (ALP)
- Gamma-glutamyl transferase (GGT)
- Albumin
- International Normalised Ratio (INR)
Clinical history, physical examination, imaging studies, and additional laboratory investigations often help determine the underlying cause of abnormal bilirubin levels.
Bilirubin and Clinical Calculators
Bilirubin is an important component of several validated clinical scoring systems. It contributes to the SOFA (Sequential Organ Failure Assessment) Score, where it is used to assess hepatic function in critically ill patients. Bilirubin is also incorporated into liver disease severity scores and prognostic models used in hepatology and intensive care medicine.
Accurate bilirubin measurements therefore play an important role in risk assessment, disease monitoring, and clinical decision-making.
Clinical Interpretation
Reference ranges vary slightly between laboratories, and bilirubin results should always be interpreted in the context of the patient’s symptoms, medical history, medications, and other laboratory findings. Trends over time often provide more useful clinical information than a single isolated result.
At MediCalc, bilirubin is incorporated into validated clinical calculators based on recognised medical literature and established clinical practice. These tools are intended to support healthcare professionals by providing accurate clinical calculations that complement comprehensive patient assessment and professional judgement.
Understanding bilirubin and its clinical significance is essential in the diagnosis and management of liver disease, biliary disorders, haemolytic conditions, and critical illness. When interpreted appropriately, bilirubin remains one of the key laboratory markers used throughout modern clinical practice.