Red blood cells at the end of their circulating lives are broken down in the reticuloendothelial system, mainly the spleen. The resulting heme is converted to bilirubin upon removal of iron. This process accounts for about 80% of the 500 μmol (292 mg) of bilirubin formed daily. Other sources of bilirubin include the breakdown of myoglobin and cytochromes and the catabolism of immature red blood cells in the bone marrow. Once formed, bilirubin is transported to the liver bound to albumin. This fraction of bilirubin is referred to as indirect or unconjugated bilirubin. In the liver, bilirubin is conjugated to glucuronic acid (mono‑ and diglucuronides) by the enzyme uridyl diphosphate glucuronyl transferase to form conjugated bilirubin. Conjugated bilirubin or direct bilirubin is excreted via the biliary system into the intestine, where it is metabolized by bacteria to a group of products known collectively as stercobilinogen. Elimination is almost complete and serum levels are normally negligible.
Total bilirubin is the sum of the unconjugated and conjugated fractions. Total bilirubin is elevated in hepatitis, cirrhosis, hemolytic disorders,
several inherited enzyme deficiencies, and conditions causing hepatic obstruction.
1 mL Serum Refrigerated
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