Conjugated vs Unconjugated Bilirubin Explained

Bilirubin is a orange-yellow pigment that is a metabolite of red blood cells. When bilirubin is formed in blood, liver takes it from the blood, processes into a chemical that can be passed through stools as bile. There are two types of bilirubin: conjugated and unconjungated. Knowing the difference between conjugated and unconjugated bilirubin facilitates pathologists to diagnose different health conditions. Abnormal levels in either type are not a positive sign for your health.

What is Conjugated Bilirubin

Conjugated bilirubin is also called direct bilirubin. It is a type of bilirubin processed by the liver. Conjugated bilirubin is water-soluble and excreted through bile into stool and urine. High levels are considered abnormal. They may indicate liver damage, bile duct obstruction, or metabolic disorders. Conjugated bilirubin does not bind to albumin like the unconjugated type. Its buildup can lead to dark urine and pale stools. In some cases, it signals serious liver-related complications.

What is Unconjugated Bilirubin

Unconjugated bilirubin is also called indirect bilirubin. It is a type of bilirubin that has not been processed by the liver. This form is fat-soluble and moves through the bloodstream bound to albumin. It is considered normal in small amounts, especially in newborns. However, when levels are high, it may point to indirect hyperbilirubinemia causes, e.g., hemolysis, an immature liver, or conditions like Rh or ABO incompatibility. It can build up in tissues and cause jaundice. In severe cases, it may cross the blood-brain barrier and lead to kernicterus.

Difference Between Conjugated and Unconjugated Bilirubin

Aspect	Unconjugated Bilirubin	Conjugated Bilirubin
Other Names	Indirect bilirubin, “Normal” type	Direct bilirubin, “Abnormal” type
Formation	Not processed by the liver	Processed by the liver
Toxicity Nature	Toxic for the human body	Non-toxic for the human body
Excretion Pathway	Re-enters enterohepatic circulation	Excreted through stool and urine
Solubility	Not soluble in water	Soluble in water
Binding	Binds to albumin or remains free in plasma	Bound and ready for excretion
Onset	Appears within the first days of life	Appears with longer-term conditions
Colour Association	Leads to yellowing (jaundice) in the skin and eyes	Dark urine and pale stools in severe cases
Causes of Increase	Immature liver, hemolysis, infection, lack of oxygen/glucose	Chronic TPN/Intralipid use, biliary obstruction, inborn errors of metabolism (e.g., galactosemia)
Risks	May cross the blood-brain barrier, causing kernicterus	Indicates liver or biliary system dysfunction
Treatment	Phototherapy, hydration, feeding, exchange transfusion (critical cases)	Adjust TPN/lipids, medications, special formulas

Conjugated and Unconjugated Bilirubin Normal Range

Parameter	Adults/Elderly/Children	Newborns
Total Bilirubin	0.3-1.0 mg/dL (5.1-17 μmol/L)	1.0-12.0 mg/dL (17.1-205 μmol/L)
Conjugated Bilirubin	0.2-0.8 mg/dL (3.4-12.0 μmol/L)	Included in total bilirubin levels
Unconjugated Bilirubin	0.1-0.3 mg/dL (1.7-5.1 μmol/L)	Typically part of total bilirubin levels
Critical Values (Total Bilirubin)	>12 mg/dL	>15 mg/dL

Source: Pagana KD, Pagana TJ, Pagana TN. Mosby’s Diagnostic & Laboratory Test Reference. 14th ed. St. Louis, Mo: Elsevier; 2019.

Causes of High Bilirubin Levels

Elevated bilirubin levels, whether conjugated or unconjugated, signal an underlying health issue. Indirect hyperbilirubinemia causes include conditions like hemolysis, preterm birth, or insufficient oxygen or glucose. On the other hand, high conjugated bilirubin often stems from liver or biliary system problems. It could include biliary obstruction or chronic TPN use.
Monitoring conjugated and unconjugated bilirubin levels is necessary for diagnosing conditions like jaundice, kernicterus, and metabolic disorders. Early detection leads to appropriate treatments. They include phototherapy for unconjugated bilirubin or medications like ursodiol for conjugated bilirubin.

Conclusion

Assessing bilirubin levels lead to liver function and overall health, but their interpretation requires a nuanced approach. The difference between conjugated and unconjugated bilirubin is part of it. Pathologists can use bilirubin patterns to identify rare genetic disorders like Gilbert's syndrome or Crigler-Najjar syndrome. These conditions often go undetected without a detailed bilirubin profile. Advanced diagnostics like fractionation of bilirubin types can further refine awareness.

Suggested read: Bilirubin Report Format