What lab tests are used to diagnose Gilbert's syndrome?

Laboratory Diagnosis of Gilbert's Syndrome

Gilbert's syndrome should be diagnosed by calculating the proportion of conjugated bilirubin, which must be less than 20-30% of the total bilirubin (or stated inversely, unconjugated bilirubin should comprise >70-80% of total bilirubin). 1

Primary Diagnostic Laboratory Tests

Bilirubin Fractionation (Essential)

• Measure total and direct bilirubin to calculate the unconjugated fraction, which should be >70-80% of total bilirubin in Gilbert's syndrome 1, 2
• Total bilirubin levels are typically mildly elevated, rarely exceeding 4-5 mg/dL (68-85 μmol/L) 2, 3
• Research using alkaline methanolysis shows unconjugated bilirubin comprises 95 ± 2% in Gilbert's syndrome patients versus 84 ± 5% in healthy controls 4
• Critical distinction: Direct bilirubin includes both conjugated bilirubin AND delta bilirubin (albumin-bound with 21-day half-life), so if hyperbilirubinemia persists unexpectedly, request fractionation of direct bilirubin into conjugated and delta components 5, 2

Baseline Liver Function Tests (Required to Exclude Other Pathology)

• ALT and AST should be normal in Gilbert's syndrome 5, 2
• Alkaline phosphatase should be normal 1, 5
• Complete blood count to exclude hemolysis as a cause of unconjugated hyperbilirubinemia 2

Confirmatory Testing

Genetic Testing (When Diagnosis Unclear)

• DNA testing for UGT1A1 gene mutations provides definitive confirmation, especially when total bilirubin elevations occur in combination with elevated ALP and aminotransferases 1
• The variant TATAA element in the upstream promoter region of the UGT1A1 gene is firmly associated with Gilbert's syndrome in Western populations 6
• Genetic testing may be necessary when bilirubin levels exceed 6 mg/dL, which is rare in Gilbert's syndrome 3

Provocative Tests (Alternative Diagnostic Approach)

• Rifampicin test: Measure unconjugated bilirubin before and 4 hours after 600 mg rifampicin administration; significant increase supports Gilbert's syndrome 7, 8
• Fasting test: 400 calorie/day diet for 24 hours causes significant increase in unconjugated bilirubin in Gilbert's syndrome 7, 8
• Important limitation: Both tests can cause increased unconjugated bilirubin in approximately 50% of patients with chronic liver disease, reducing specificity 7

Critical Diagnostic Pitfalls

When Gilbert's Syndrome Diagnosis Should Be Questioned

• Bilirubin levels >6 mg/dL are unusual and warrant investigation for hemolytic disease or other bilirubin metabolism disorders 3
• Any elevation in ALT, AST, or alkaline phosphatase suggests alternative diagnosis 5, 2
• Conjugated bilirubin >30% of total excludes Gilbert's syndrome and indicates hepatobiliary pathology 1

Conditions That Mimic Gilbert's Syndrome

• Hemolytic anemias (sickle cell, thalassemia, G6PD deficiency) also cause unconjugated hyperbilirubinemia but with evidence of hemolysis on CBC 2
• Crigler-Najjar syndrome types I and II have much higher bilirubin levels (300-850 μmol/L or 17-50 mg/dL) 6
• Drug-induced cholestatic liver injury can present with isolated bilirubin elevation before transaminase elevation, typically 2-12 weeks after drug initiation 5

Clinical Context Requirements

Before diagnosing Gilbert's syndrome, verify:

• Absence of hemolysis markers (normal hemoglobin, haptoglobin, reticulocyte count) 2
• No medications causing hyperbilirubinemia 5, 2
• No alcohol consumption causing liver disease 2
• No viral hepatitis risk factors or positive serologies 5, 2