What is TBL (Total Bilirubin Level) in LFT (Liver Function Test)?

TBL in LFT: Total Bilirubin Level in Liver Function Tests

Total Bilirubin Level (TBL) is a key component of Liver Function Tests (LFTs) that measures the amount of bilirubin in the blood, serving as an important indicator of liver and biliary function.

What is Total Bilirubin?

Total bilirubin consists of two fractions:

• Unconjugated (indirect) bilirubin: Insoluble form that results from the breakdown of hemoglobin by the reticuloendothelial system
• Conjugated (direct) bilirubin: Soluble form that has been processed by the liver

Clinical Significance of TBL

Normal Values

• Normal TBL range: typically 0.3-1.0 mg/dL (or 5-17 μmol/L)
• Values vary slightly between laboratories

Elevated TBL Interpretation

Elevated TBL can indicate various conditions:

1. Hepatocellular injury: When accompanied by elevated transaminases (ALT/AST)

   • Particularly concerning when ALT ≥3× ULN with TBL ≥2× ULN (suggests severe liver injury) 1

2. Biliary obstruction: When accompanied by elevated alkaline phosphatase (ALP)

3. Hemolysis: When predominantly unconjugated bilirubin is elevated

4. Gilbert's syndrome: Common benign condition causing mild unconjugated hyperbilirubinemia 1

   • Affects up to 10% of the population
   • Unconjugated bilirubin typically <30% of total bilirubin 1

TBL in Drug-Induced Liver Injury (DILI) Assessment

TBL is a critical parameter in monitoring for DILI:

• Hy's Law: ALT ≥3× ULN with TBL ≥2× ULN suggests severe liver injury with poor prognosis 1

• Monitoring algorithms: In clinical trials, TBL is routinely monitored alongside ALT/AST 1

• Action thresholds: When ALT ≥3× ULN with TBL ≥2× ULN, drug interruption is typically recommended 1

TBL in Specific Clinical Scenarios

Heart Failure

• Elevated TBL in heart failure patients is independently associated with increased morbidity and mortality 2

COVID-19

• Liver dysfunction including elevated TBL is common in severe COVID-19 cases and associated with worse outcomes 3

Cancer Surveillance

• The combination of TBL and lymphocyte counts may help identify patients at risk for hepatocellular carcinoma 4

NAFLD

• Higher bilirubin levels in NAFLD patients are associated with lower risk of all-cause mortality 5

Pitfalls in TBL Interpretation

1. Gilbert's syndrome: May cause intermittent mild elevations in unconjugated bilirubin without indicating liver disease 1

2. Hemolysis: Can cause elevated unconjugated bilirubin without liver pathology

3. Fasting: Can temporarily increase unconjugated bilirubin

4. Medications: Some drugs can affect bilirubin levels without causing true liver injury

When to Act on Elevated TBL

• TBL ≥2× ULN with ALT ≥3× ULN: Requires immediate attention and drug interruption in clinical trials 1

• Isolated TBL elevation: If predominantly unconjugated, consider Gilbert's syndrome; if predominantly conjugated, evaluate for biliary obstruction 1

• Rising TBL in acute cholecystitis: Immediate imaging rather than serial monitoring is recommended 6

Remember that TBL is just one component of LFTs and should be interpreted in conjunction with other parameters including ALT, AST, ALP, GGT, and albumin for comprehensive liver assessment 7.