What is Total Bilirubin (TBili)?
Total bilirubin (TBili or TBIL) is a laboratory measurement that represents the sum of both unconjugated (indirect) and conjugated (direct) bilirubin in the blood, serving as a key marker for assessing liver function, biliary excretion, and hemolysis. 1
Biochemical Basis
Bilirubin is predominantly the breakdown product of the heme component of hemoglobin processed by the reticuloendothelial system 1
Bilirubin exists in two forms:
Total bilirubin comprises both fractions, and elevations of either will increase the measured concentration 1
Clinical Interpretation
Pattern Recognition
Unconjugated hyperbilirubinemia typically indicates hemolysis or impaired conjugation (such as Gilbert's syndrome) 1
Conjugated hyperbilirubinemia typically reflects parenchymal liver disease or biliary obstruction 1
In adults, except during the neonatal period, the majority of measurable bilirubin should be conjugated, even in patients with significant liver disease 1
Gilbert's Syndrome Considerations
Gilbert's syndrome, present in up to 10% of the population, is the most common cause of isolated elevated bilirubin 1
Gilbert's syndrome can be presumed when conjugated bilirubin is normally less than 30% of total bilirubin in the absence of hemolysis 1
When Gilbert's syndrome or hemolysis is present, clinical decision-making should focus on direct bilirubin rather than total bilirubin 1
Clinical Applications
Drug-Induced Liver Injury Monitoring
Eligibility screening using TBIL is designed to identify patients with underlying liver pathology who may have increased risk with certain medications 1
Elevated direct bilirubin often reflects liver dysfunction that may affect drug metabolism and elimination 1
Measurement of both total and direct bilirubin is recommended to distinguish patients with indirect hyperbilirubinemia (Gilbert's syndrome or hemolysis) from true liver injury 1
Neonatal Context
In neonates and infants with conjugated bilirubin >25 μmol/L, urgent referral to a pediatrician for assessment of possible liver disease is essential 1
Direct-reacting bilirubin and conjugated bilirubin, though commonly used interchangeably, are not identical, but for clinical purposes the terms may be used interchangeably 1
If total serum bilirubin is at or below 5 mg/dL (85 μmol/L), a direct or conjugated bilirubin of more than 1.0 mg/dL is considered abnormal 1
Important Clinical Caveats
Laboratory measurement of direct bilirubin is not precise, and values between laboratories can vary widely 1
A raised TBIL has been proposed to potentially alter biliary excretion of certain oncology compounds, resulting in clinical toxicity 1
When evaluating elevated total bilirubin, alternative causes must be investigated, including hemolysis, multiple transfusions, and other causes of unconjugated hyperbilirubinemia 1