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Bilirubin
Definition:
Bilirubin is a breakdown product of hemoglobin. Total and direct bilirubin are usually measured to screen for or to monitor liver or gall bladder dysfunction. Alternative Names:
Total bilirubin; Unconjugated bilirubin; Indirect bilirubin; Conjugated bilirubin; Direct bilirubin Text Continues Below
How the test is performed:
Blood is drawn from a vein (venipuncture) or capillary. The laboratory centrifuges the blood to separate the serum from the cells and the bilirubin test is done on the serum. How to prepare for the test:
Fast for at least 4 hours before the test. Your health care provider may instruct you to discontinue drugs that affect the test. Drugs that can increase bilirubin measurements include allopurinol, anabolic steroids, some antibiotics, antimalarials, azathioprine, chlorpropamide, cholinergics, codeine, diuretics, epinephrine, meperidine, methotrexate, methyldopa, MAO inhibitors, morphine, nicotinic acid, oral contraceptives, phenothiazines, quinidine, rifampin, salicylates, steroids, sulfonamides, and theophylline. Drugs that can decrease bilirubin measurements include barbiturates, caffeine, penicillin, and high-dose salicylates. Why the test is performed:
This test is useful in determining if a patient has liver disease or a blocked bile duct. Bilirubin metabolism begins with the breakdown of red blood cells. Red blood cells contain hemoglobin, which is broken down to heme and globin. Heme is converted to bilirubin, which is then carried by albumin in the blood to the liver. In the liver, most of the bilirubin is chemically attached to a glucuronide before it is excreted in the bile. This "conjugated" bilirubin is called direct bilirubin; unconjugated bilirubin is called indirect bilirubin. Total serum bilirubin equals direct bilirubin plus indirect bilirubin. Conjugated bilirubin is excreted into the bile by the liver and stored in the gall bladder or transferred directly to the small intestines. Bilirubin is further broken down by bacteria in the intestines to urobilins, which contribute to the color of the feces. A small percentage of these compounds are reabsorbed and eventually appear in the urine, where they are referred to as urobilinogen.
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