In the classification of jaundice, which of the following is a characteristic feature that distinguishes unconjugated hyperbilirubinaemia from conjugated hyperbilirubinaemia?

## Distinction Between Unconjugated and Conjugated Hyperbilirubinaemia ### Physical and Chemical Properties **Key Point:** Unconjugated bilirubin is lipophilic (fat-soluble) and water-insoluble, whereas conjugated bilirubin is water-soluble and hydrophilic. ### Urinary Appearance **High-Yield:** Because unconjugated bilirubin is not water-soluble, it **cannot cross the glomerular filtration barrier** and therefore does NOT appear in urine, even when serum levels are elevated. Conjugated bilirubin, being water-soluble, is freely filtered by the glomerulus and appears in urine (bilirubinuria) when serum levels exceed the renal threshold (~2–3 mg/dL). ### Van den Bergh Reaction | Feature | Unconjugated | Conjugated | | --- | --- | --- | | **Direct van den Bergh** | Negative (indirect reaction) | Positive (within 30 sec) | | **Indirect van den Bergh** | Positive (after alcohol addition) | Negative | | **Urinary excretion** | Absent | Present (bilirubinuria) | | **Albumin binding** | Bound to albumin (not filtered) | Free in plasma (filtered) | ### Clinical Pearl The presence of bilirubinuria (dark urine) in a jaundiced patient always indicates **conjugated hyperbilirubinaemia**, because only conjugated bilirubin is water-soluble enough to be excreted by the kidneys. This is a key clinical discriminator. **Mnemonic:** **UNCI** = **UN**conjugated = **C**annot appear in urine, **I**ndirect reaction.