Crigler-Najjar syndrome: Bilirubin

Bilirubin is the main waste product that results from hemoglobin breakdown during normal red blood cells turnover. There are two forms of circulating bilirubin: unconjugated (indirect) and conjugated (direct) bilirubin.

Indirect Bilirubin (Unconjugated)	Direct Bilirubin (Conjugated)
Also referred as: insoluble, prehepatic, indirect-reacting, albumin-bound bilirubin	Also referred as: posthepatic, direct-reacting, unbound bilirubin
Normal plasmatic range: 0.2 - 0.9 mg/dL (see note below)	Normal plasmatic range: 0.1 - 0.2 mg/dL (see note below)
Derives mainly from hemoglobin metabolism	It is the product of bilirubin metabolism within the liver
Hydrophobic	Water soluble
Must be conjugated in the liver before it can be excreted	Is readily excreted into bile, stool, and urine
Most bound to albumin in plasma; a small amount free in the plasma	Only a small amount normally found within the plasma
Never present in the urine	May be found in the urine when serum levels exceed 3-4 mg/dL (see note below)
Can be measured only indirectly, by subtracting the direct bilirubin level from the total bilirubin level	Can be measured directly in plasma

Total serum bilirubin (referred as total bilirubin) equals the sum of direct (conjugated) and indirect (unconjugated) bilirubin.

Note: Milligrams/deciliter (mg/dL) is the unit of measure commonly used for bilirubin. Micromoles/liter (umol/L) is the designated Systeme International (SI) unit of measure recommended for reporting clinical laboratory results .

1 mg/dL of bilirubin is 17.1 umol/L. (see bilirubin unit converter calculator)

Due to the absence of the activity of the UGT1A1 enzyme (which is the liver enzyme involved in the process of transformation of unconjugated bilirubin to conjugated bilirubin) Crigler-Najjar type I patients have high levels (above 20 mg/dL) of unconjugated bilirubin and very low levels or absence of conjugated bilirubin.