Physiological basis
Bilirubin is the orange-yellow pigment derived from
the breakdown of hemoglobin (heme). The majority of bilirubin comes from senescent red cells. It is
biotransformed in the liver and excreted in bile and urine.
The conjugated form is water soluble
and reacts directly with diazo dyes in the absence of reaction accelerator, and is therefore called
direct bilirubin. The unconjugated form is fat-soluble and reacts with diazo dyes only in the
presence of accelerator; so it is called indirect.
Some conjugated bilirubin is bound to serum
albumin, so-called D (delta) bilirubin.
Increased in: Acute or chronic hepatitis, cirrhosis, biliary tract obstruction,
toxic hepatitis, neonatal jaundice (neonatal hyperbilirubinemia), congenital liver enzyme
abnormalities (Dubin-Johnson, Rotor, Gilbert, Crigler-Najjar syndromes), fasting, hemolytic
disorders. Hepatotoxic drugs.
Comments
Assay of total bilirubin includes conjugated (direct) and
unconjugated (indirect) bilirubin. The unconjugated (indirect) form is the difference between total
bilirubin (with reaction accelerator) and the direct bilirubin fraction. Delta bilirubin is
determined together with conjugated bilirubin. Delta bilirubin (half-life is about 17 days) accounts
for relatively slow regression of jaundice.
Only conjugated bilirubin appears in the urine, and
it is indicative of liver disease and biliary tract obstruction.
Hemolysis is associated with
increased unconjugated bilirubin. Unbound (free) serum or plasma bilirubin level correlates better
than total bilirubin with CNS bilirubin concentrations and bilirubin encephalopathy (kernicterus) in
newborn jaundice.