Bilirubin, Indirect
The indirect bilirubin test measures the unconjugated fraction of serum bilirubin, calculated as total minus direct bilirubin, to differentiate prehepatic causes of jaundice — haemolysis and inherited conjugation defects such as Gilbert syndrome — from hepatocellular and cholestatic disease.
1
Biomarkers
Blood (serum)
Sample
2–3 business days
Turnaround
No
Fasting
What is the Indirect Bilirubin test?
The Indirect Bilirubin test is a calculated value that isolates the unconjugated fraction of serum bilirubin, representing the portion that has not yet been conjugated in the liver. It is ordered to characterise the nature of hyperbilirubinaemia and to separate causes of jaundice that arise before the liver — haemolytic disorders and disorders of bilirubin metabolism — from those caused by hepatocellular dysfunction or biliary obstruction, where direct bilirubin predominates. Practitioners use the indirect fraction alongside total and direct bilirubin to build a diagnostic framework: a high total with a high indirect fraction points to haemolysis or a conjugation defect, while a high direct fraction implicates cholestatic or hepatocellular disease. The test is essential in the neonatal period to monitor for pathological unconjugated hyperbilirubinaemia, in the haematology setting to screen for haemolysis, and in primary care to investigate incidental mild jaundice where Gilbert syndrome is suspected.
What does the Indirect Bilirubin test measure?
Indirect bilirubin is the unconjugated fraction that results from haem catabolism in the reticuloendothelial system. Senescent erythrocytes are phagocytosed by splenic and hepatic macrophages, where haem oxygenase cleaves the porphyrin ring to biliverdin, which is then reduced to unconjugated bilirubin. This lipophilic molecule circulates bound to albumin and crosses into hepatocytes, where it is conjugated with glucuronic acid by UDP-glucuronosyltransferase to form water-soluble direct bilirubin for biliary excretion. Indirect bilirubin is calculated by subtracting the measured direct (conjugated) fraction from total bilirubin.
Isolated elevation of indirect bilirubin signals either increased bilirubin production (haemolysis, ineffective erythropoiesis) or impaired hepatic conjugation (Gilbert syndrome, Crigler–Najjar syndrome). The pattern of bilirubin elevation — predominantly indirect versus direct — is the first discriminator in the work-up of jaundice, guiding the clinician toward a prehepatic, hepatic, or posthepatic cause.
Why is the Indirect Bilirubin test ordered?
- Evaluation of jaundice or scleral icterus to differentiate prehepatic (haemolytic, conjugation defects) from hepatic and posthepatic causes
- Investigation of suspected haemolytic anaemia, interpreted with full blood count, reticulocyte count, haptoglobin, and lactate dehydrogenase
- Work-up of neonatal jaundice to quantify the unconjugated fraction and guide phototherapy or further investigation of haemolysis and conjugation disorders
- Assessment of incidental mild hyperbilirubinaemia in an asymptomatic patient, where Gilbert syndrome is the leading differential
- Monitoring of patients with known inherited disorders of bilirubin conjugation, such as Gilbert or Crigler–Najjar syndrome
- Differentiation of conjugated from unconjugated hyperbilirubinaemia when total bilirubin is elevated and the pattern is not clear from direct bilirubin alone
- Evaluation of suspected ineffective erythropoiesis in thalassaemia or megaloblastic anaemia, where indirect bilirubin may be chronically mildly elevated
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
2–3 business days
Collection notes
Phlebotomy required. Protect specimen from prolonged light exposure, as bilirubin is photodegradable. Reject grossly haemolysed or severely lipaemic samples.
Specimen requirements
Minimum 0.5 mL serum (gel separator acceptable). Separate from cells promptly and shield from bright light during handling. Stable refrigerated for up to 3 days when protected from light.
What can affect Indirect Bilirubin results?
In-vitro haemolysis releases haemoglobin, which can interfere with bilirubin assays and spuriously elevate results; grossly haemolysed specimens should be rejected and redrawn.
Prolonged fasting (more than 24–48 hours) can unmask or worsen unconjugated hyperbilirubinaemia in Gilbert syndrome by reducing hepatic UDP-glucuronosyltransferase activity; drawing after an overnight fast is standard but extreme fasting should be avoided.
Fever, sepsis, and other acute stressors can transiently raise indirect bilirubin in individuals with Gilbert syndrome, as conjugation capacity is reduced during illness.
Some protease inhibitors (atazanavir, indinavir) and a few other medications can inhibit UDP-glucuronosyltransferase and raise unconjugated bilirubin; the pattern resembles Gilbert syndrome and is generally benign.
Severe lipaemia can interfere with spectrophotometric bilirubin assays, producing spuriously high or low results; ultracentrifugation or sample clearing may be required.
Bilirubin is photosensitive and degrades when exposed to bright light, so specimens should be protected from prolonged light exposure before analysis.
What do Indirect Bilirubin results mean?
Elevated
Increased bilirubin production: haemolytic anaemias (hereditary spherocytosis, G6PD deficiency, autoimmune haemolytic anaemia, sickle-cell disease), ineffective erythropoiesis (thalassaemia, megaloblastic anaemia), and large haematoma resorption. Impaired conjugation: Gilbert syndrome (most common, benign, seen in 3–10% of the population), Crigler–Najjar syndrome types I and II (rare, severe in type I), neonatal physiological jaundice, and drug-induced inhibition of glucuronidation. The pattern is a high total bilirubin with a predominant indirect fraction and usually a normal direct bilirubin.
Normal
A normal indirect bilirubin, in the context of normal total and direct bilirubin, effectively excludes significant haemolysis and inherited disorders of conjugation as causes of jaundice. Interpret alongside the full bilirubin panel and haemolysis markers.
Reduced
Low indirect bilirubin is not clinically significant and is simply a reflection of low total bilirubin; it is occasionally seen as a calculated artefact when direct bilirubin measurement is at the lower detection limit.
Note: Indirect bilirubin is a calculated value (total minus direct), so errors in either component propagate. The test must be read with total and direct bilirubin, never in isolation. Gilbert syndrome is common and benign, characterised by unconjugated hyperbilirubinaemia without haemolysis or liver dysfunction, and can be unmasked by fasting or illness. In neonates, the threshold for intervention is based on total and indirect bilirubin plotted on hour-specific nomograms.
Frequently asked questions
Indirect bilirubin is the unconjugated, lipophilic fraction bound to albumin before hepatic conjugation, while direct bilirubin is the water-soluble conjugated form ready for biliary excretion. Indirect rises with haemolysis or conjugation defects; direct rises with hepatobiliary disease.
How to access the Indirect Bilirubin test
The Indirect 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)
- American Academy of Pediatrics Subcommittee on Hyperbilirubinemia. Management of hyperbilirubinemia in the newborn infant 35 or more weeks of gestation. Pediatrics. (2004)
- 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)
- Gallagher PG. Diagnosis and management of hemolytic anemia in the newborn and young child. Hematol Oncol Clin North Am. (2018)
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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