Bilirubin, Fractionated
The fractionated bilirubin test measures total bilirubin and its direct (conjugated) and indirect (unconjugated) components. Fractionation distinguishes prehepatic, hepatic, and posthepatic causes of hyperbilirubinaemia — unconjugated elevation points to haemolysis or impaired conjugation, while conjugated elevation signals hepatocellular dysfunction or biliary obstruction.
3
Biomarkers
Blood (serum)
Sample
1–2 business days
Turnaround
Preferred but not mandatory
Fasting
What is the Fractionated Bilirubin test?
The Fractionated Bilirubin Test measures total bilirubin and its direct (conjugated) and indirect (unconjugated) components to classify the pattern of hyperbilirubinaemia and direct the diagnostic pathway. Jaundice is the clinical trigger — visible scleral icterus typically appears when total bilirubin exceeds approximately 50 µmol/L (3 mg/dL) — but fractionation is also ordered to investigate incidental or asymptomatic bilirubin elevation discovered on a routine metabolic panel. Fractionation distinguishes prehepatic causes (haemolysis, ineffective erythropoiesis) that raise unconjugated bilirubin, hepatocellular injury that may raise both fractions, and cholestatic or obstructive processes that predominantly elevate the conjugated fraction. Practitioners interpret fractionated bilirubin alongside aminotransferases, alkaline phosphatase, GGT, and albumin to separate hepatocellular from cholestatic injury and to prioritise further investigation — ultrasound or cross-sectional imaging for suspected obstruction, haemolysis markers for unconjugated hyperbilirubinaemia, and autoimmune or viral serology for hepatocellular disease.
What does the Fractionated Bilirubin test measure?
Total bilirubin is the sum of unconjugated (indirect) and conjugated (direct) fractions. Bilirubin is the end product of haem catabolism, principally from senescent erythrocytes. In the reticuloendothelial system, haem oxygenase cleaves haem to biliverdin, which is reduced to unconjugated bilirubin and transported bound to albumin to the hepatocyte. Inside the liver cell, UDP-glucuronosyltransferase conjugates bilirubin with glucuronic acid, rendering it water-soluble for biliary excretion. Total bilirubin rises when production exceeds hepatic clearance or when excretion is obstructed.
Total bilirubin is the initial marker of jaundice and the gateway to distinguishing prehepatic, hepatic, and posthepatic causes. Fractionation into direct and indirect components is essential to localise the defect — unconjugated hyperbilirubinaemia points to increased production or impaired conjugation, while conjugated hyperbilirubinaemia signals hepatocellular dysfunction or biliary obstruction.
Direct bilirubin comprises conjugated bilirubin and the delta fraction (bilirubin covalently bound to albumin). Conjugation in the hepatocyte renders bilirubin water-soluble and permits its excretion into bile. When hepatocellular injury or biliary obstruction impairs excretion, conjugated bilirubin refluxes into plasma, where it is measurable as the direct fraction and capable of renal filtration, producing bilirubinuria.
Elevated direct bilirubin indicates hepatocellular dysfunction or cholestasis. It is the hallmark of obstructive jaundice, intrahepatic cholestasis, Dubin-Johnson and Rotor syndromes, and conjugated hyperbilirubinaemia of any cause. The ratio of direct to total bilirubin helps classify the pattern — a direct fraction exceeding 15–20 per cent of total bilirubin (or above 30 per cent in some algorithms) defines conjugated hyperbilirubinaemia and triggers imaging and further hepatobiliary work-up.
Indirect bilirubin is the unconjugated, lipid-soluble fraction that circulates bound to albumin. It arises from haem breakdown and must be conjugated in the liver before excretion. Because it is not water-soluble, unconjugated bilirubin cannot be filtered by the kidney and does not appear in urine, but at very high concentrations — particularly in neonates — it can cross the blood-brain barrier and cause kernicterus.
Isolated elevation of indirect bilirubin points to increased production (haemolysis, ineffective erythropoiesis, large haematoma resorption) or impaired conjugation (Gilbert syndrome, Crigler-Najjar syndromes, or drug inhibition of glucuronidation). Gilbert syndrome is the most common cause of mild chronic unconjugated hyperbilirubinaemia in adults, typically unmasked by fasting or intercurrent illness.
Why is the Fractionated Bilirubin test ordered?
- Investigation of visible jaundice, scleral icterus, or dark urine in any patient
- Work-up of incidental hyperbilirubinaemia found on a routine metabolic or liver panel
- Evaluation of suspected haemolytic anaemia when reticulocytosis and anaemia coexist with unconjugated hyperbilirubinaemia
- Suspected Gilbert syndrome in a patient with mild chronic unconjugated hyperbilirubinaemia and normal liver enzymes
- Assessment of cholestatic liver injury, where an elevated direct fraction directs imaging to exclude biliary obstruction
- Neonatal jaundice that persists beyond the physiological period or rises excessively, though paediatric protocols differ
- Monitoring of known chronic liver disease or biliary pathology, where trends in fractionated bilirubin reflect disease progression or response
- Preoperative evaluation in patients with a history of jaundice or known hyperbilirubinaemia syndromes
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
Preferred but not mandatory
Collection method
Venipuncture at patient service centre
Turnaround
1–2 business days
Collection notes
Phlebotomy at Quest patient service centre. Fasting is preferred because it may unmask Gilbert syndrome by raising unconjugated bilirubin, though not strictly required. Avoid vigorous exercise before the draw. Protect the specimen from light immediately after collection by wrapping the tube in foil or using an amber tube if available; analyse promptly or store protected from light.
Specimen requirements
Minimum 0.5 mL serum (gel separator acceptable). Separate from cells within two hours. Protect from light at all stages of handling, transport, and storage to prevent photodegradation. Reject grossly haemolysed, lipaemic, or icteric specimens if interference is suspected; recollect if necessary.
What can affect Fractionated Bilirubin results?
Haemolysis releases haemoglobin, which interferes with spectrophotometric bilirubin assays; grossly haemolysed samples should be rejected and redrawn to avoid spurious elevation.
Bilirubin is photosensitive and degrades under ambient or fluorescent light; specimens should be protected from light and analysed promptly or stored in the dark.
Fasting increases unconjugated bilirubin, particularly in Gilbert syndrome, by reducing hepatic uptake and conjugation; postprandial samples may normalise or show lower values.
Fever, infection, and physical stress can unmask latent Gilbert syndrome by transiently impairing glucuronidation, causing mild unconjugated hyperbilirubinaemia that resolves with recovery.
Rifampicin and phenobarbital induce UDP-glucuronosyltransferase and may lower unconjugated bilirubin; atazanavir and indinavir inhibit the enzyme and can raise it; a full medication history is essential.
Some assays are subject to interference from strong reducing agents, which can spuriously lower measured bilirubin; clinical correlation is required when values are unexpectedly low.
What do Fractionated Bilirubin results mean?
Elevated
Unconjugated (indirect) hyperbilirubinaemia: haemolysis, Gilbert syndrome, Crigler-Najjar syndromes, large haematoma resorption, ineffective erythropoiesis (megaloblastic or sideroblastic anaemia), or drugs inhibiting conjugation. Conjugated (direct) hyperbilirubinaemia: biliary obstruction (choledocholithiasis, stricture, malignancy), intrahepatic cholestasis (primary biliary cholangitis, primary sclerosing cholangitis, drug-induced cholestasis), hepatocellular injury (viral or autoimmune hepatitis, cirrhosis), Dubin-Johnson and Rotor syndromes, or sepsis-associated cholestasis. Mixed hyperbilirubinaemia occurs in many hepatocellular diseases.
Normal
Normal fractionated bilirubin in the setting of jaundice is unusual and should prompt reconsideration of the diagnosis — carotinaemia and other pigment disorders may mimic icterus. Normal bilirubin does not exclude early or compensated liver disease, and a single normal result does not rule out intermittent haemolysis or stress-unmasked Gilbert syndrome.
Reduced
Low total bilirubin is not clinically significant and may be seen in conditions of increased oxidative stress or with some laboratory interference; it requires no specific follow-up.
Note: The ratio of direct to total bilirubin is critical — a direct fraction above 15–20 per cent (or 30 per cent by some definitions) signifies conjugated hyperbilirubinaemia and mandates imaging to exclude obstruction. Fractionated bilirubin must be interpreted with aminotransferases, ALP, GGT, albumin, and the clinical picture. Isolated unconjugated hyperbilirubinaemia with normal liver enzymes and no anaemia is Gilbert syndrome until proved otherwise. Protect specimens from light and avoid haemolysed samples.
Frequently asked questions
Direct (conjugated) bilirubin is water-soluble and has been processed by the liver for excretion; indirect (unconjugated) bilirubin is the lipid-soluble form that circulates bound to albumin before hepatic conjugation. The ratio of the two fractions classifies the cause of jaundice.
How to access the Fractionated Bilirubin test
The Fractionated Bilirubin reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- Roche SP, Kobos R. Jaundice in the adult patient. Am Fam Physician. (2004)
- Fargo MV, et al. Evaluation of jaundice in adults. Am Fam Physician. (2017)
- European Association for the Study of the Liver (EASL) clinical practice guidelines: management of cholestatic liver diseases. (2009)
- Bosma PJ, et al. The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome. N Engl J Med. (1995)
- Kwo PY, et al. ACG clinical guideline: evaluation of abnormal liver chemistries. Am J Gastroenterol. (2017)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
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