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Bilirubin (BR) (from the Latin for "red bile") is a red-orange compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates.This catabolism is a necessary process in the body's clearance of waste products that arise from the destruction of aged or abnormal red blood cells. [3]
In biliary obstruction, below-normal amounts of conjugated bilirubin reach the intestine for conversion to urobilinogen. With limited urobilinogen available for reabsorption and excretion, the amount of urobilin found in the urine is low. High amounts of the soluble conjugated bilirubin enter the circulation where they are excreted via the kidneys.
In acute appendicitis, total bilirubin can rise from 20.52 μmol/L to 143 μmol/L. In pregnant women, the total bilirubin level is low in all three trimesters. [6] The measurement of bilirubin levels in the newborns is done through the use of bilimeter or transcutanoeus bilirubinometer instead of performing LFTs.
In both settings of hemolysis mentioned above, only low levels of conjugated bilirubin may accumulate in the serum, with the amount falling within the normal limits of 4 percent of total bilirubin as conjugated bilirubin can be efficiently excreted in bile through being secreted across canalicular membrane. [25]
Urine: conjugated bilirubin present, urobilinogen > 2 units but variable (except in children) Plasma proteins show characteristic changes. Plasma albumin level is low, but plasma globulins are raised due to an increased formation of antibodies. Unconjugated bilirubin is hydrophobic, so cannot be excreted in urine.
The level of total bilirubin is often further increased if the blood sample is taken after fasting for two days, [40] and a fast can, therefore, be useful diagnostically. A further conceptual step that is rarely necessary or appropriate is to give a low dose of phenobarbital: [41] the bilirubin will decrease substantially.
Urobilin is generated from the degradation of heme, which is first degraded through biliverdin to bilirubin. Bilirubin is then excreted as bile, which is further degraded by microbes present in the large intestine to urobilinogen. The enzyme responsible for the degradation is bilirubin reductase, which was identified in 2024.
Serum tests evaluate plasma bilirubin, haemolysis and liver function and are essential for definitive diagnosis. When total serum bilirubin exceeds 2.5 mg/dL and unconjugated bilirubin occupies >85% of total amount, it is classified as unconjugated hyperbilirubinemia. [6]