Urinalysis - Bilirubin
Introduction
Bilirubin, urobilinogen, urobilin, stercobilinogen and stercobilin are commonly named bile pigmentation. Enhanced urinary excretion of bilirubin or urobilinogen is called urobilinogenuria or bilirubinuria.
Bilirubin is formed in the reticuloendothelial system as the product of hemoglobin degradation. Erythrocyte breakdown occurs after approximately 120 days of life, whereby hemoglobin is being released. A sequence of reactions in heme pigments results in the formation of unconjugated bilirubin, which is water-insoluble and cannot be excreted by the kidney route. It binds to albumins in the blood and should be rapidly eliminated as it is harmful to the body. Therefore, it binds to glucuronic acid in the liver to form conjugated, water-soluble bilirubin. A minor portion of conjugated bilirubin is rediffused into the blood via hepatic lymphatic system, while a majority is secreted from the liver via biliary duct, passing to the small intestine. Bilirubin is reduced into urobilinogen and stercobilinogen by bacterial activities in the small intestine, and is mostly excreted by feces in this form. A minor part of intestinal urobilinogen returns to the liver, then to the blood, and is excreted in the urine via the kidneys.
Bilirubinuria is the usual sequel of an increased concentration of bilirubin in the blood, due to a reduced bile secretion from the liver (e.g. biliary calculi, tumors, etc.). In case of a lesion to the liver parenchyma due to increased permeability of the cell membranes, the conjugated bilirubin reaches the blood, which may also result in the occurrence of bilirubinemia.
A bilirubin concentration of only 3.6 μmol/L can be detected by a test strip. However, it is not likely to demonstrate such a low concentration of bilirubin in practice. The test sensitivity limit for bilirubin is 7 to 14 μmol/L.
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