ATBioInfo Antimicrobial & LOINC Reference
Hepatic & Hepatobiliary Diagnostics • LOINC 1975-2

Total, Direct & Indirect Bilirubin Biomarker Reference Guide

Clinical interpretation of fractionated serum bilirubin, unconjugated hyperbilirubinemia (hemolysis, Gilbert syndrome), and conjugated cholestatic jaundice.

CLSI Susceptibility / Reference Standard: Total: 0.2–1.2 mg/dL | Direct (Conjugated): < 0.3 mg/dL | Indirect: 0.2–0.9 mg/dL

Serum Bilirubin (LOINC 1975-2) is the primary tetrapyrrole biomarker used to evaluate heme catabolism, hepatocellular conjugation capacity (via UDP-glucuronosyltransferase 1A1 / UGT1A1), and biliary tree patency.

1. Pre-Hepatic (Unconjugated) Hyperbilirubinemia

Elevated indirect bilirubin (>85% unconjugated fraction) with normal serum transaminases (ALT/AST) and alkaline phosphatase (ALP) typically reflects either accelerated erythrocyte turnover (hemolytic anemias, G6PD deficiency, sickle cell crisis) or benign inherited glucuronidation variants such as Gilbert syndrome (UGT1A1*28 promoter polymorphism).

2. Hepatocellular vs. Cholestatic (Conjugated) Hyperbilirubinemia

When direct (conjugated) bilirubin exceeds 50% of total bilirubin alongside urobilinogen/bilirubinuria, clinicians differentiate hepatocellular injury (markedly elevated ALT/AST in viral, alcoholic, or drug-induced hepatitis) from extrahepatic biliary obstruction (markedly elevated ALP and GGT in choledocholithiasis or pancreaticobiliary malignancy).

Related Clinical Practice & Patient Interpretation Guidelines:
• Complete Clinical Bilirubin & Jaundice Diagnostic Guide (Nirogi Lanka)
• Adult Jaundice Pathophysiology & Workup