What is Urobilinogen?

Urobilinogen comes from the transformation of bilirubin, a product of the breakdown of hemoglobin in the red blood cells (=heme catabolism). 

Urobilinogen is formed from bilirubin by intestinal bacteria in the duodenum (=the first part of the small intestine). Most urobilinogen is excreted in the feces (90%). A portion of it is absorbed back into the blood (around 10%). Around 5% is transported back to the liver and re-secreted into bile again. The remaining urobilinogen (5%) is transported by the blood into the kidneys where it is converted into yellow urobilin and is excreted giving urine its characteristic yellow color.

What is Bilirubin?

Bilirubin is a breakdown product of red blood cells. Red blood cells carry oxygen throughout our bodies. The life-cycle of red blood cells is around 120 days or 4 months. When the erythrocytes (=red blood cells or RBCs) get old -- or are damaged -- they will be broken down (or degraded) into its parts: 

Hemoglobin molecules will be broken down into Heme and Globin.

Globin is a protein. This protein will be broken down into amino acids and will be re-used to create more red blood cells (=erythropoiesis).

The heme is being broken down into two parts. Unconjugated bilirubin and iron. The iron will re-enter circulation and re-used for erythropoiesis as well. The unconjugated bilirubin needs to be removed from the body, because it is toxic. It has a yellow/orange color. The unconjugated bilirubin is lipid soluble. The protein that carries bilirubin in the blood is called albumin. Bilirubin is now carried to the liver. In the liver the unconjugated (fat soluble) will be converted to conjugated bilirubin (water soluble). The water soluble bilirubin can now be excreted by the liver in bile and excreted into the small intestine through the common bile duct. The large intestine bacteria can now convert the conjugated bilirubin into urobilinogen. Urobilinogen is lipid soluble. 90% of that is oxidized into Stercobilin. Sterocobilin has a brown color pigment and gives feces the brown color when it is excreted.

Total bilirubin levels are indicative of both the destruction of red blood cells and the proper functioning of the liver, gallbladder, and bile ducts. 

There are a variety of instances in which the bilirubin level may be elevated:

Liver failure: The liver is no longer doing its job and bilirubin is not converted from unconjugated to conjugated bilirubin (to then be excreted through stool). The unconjugated bilirubin builds up in the body giving liver failure patients their yellow glow. 

Hemolytic anemia: Some patients have a rapid destruction of red blood cells. Their bodies cannot keep up with this breakdown and the patient becomes jaundiced as a result. Jaundice is a condition in which the skin, whites of the eyes and mucous membranes turn yellow because of a high level of bilirubin.

Blockage of bile ducts: The bilirubin is not able to be excreted through bile into the small intestine and has increased numbers because of this physical obstruction.

If the unconjugated bilirubin levels are higher than the conjugated bilirubin, this could be caused by hemolytic or pernicious anemia, transfusion reactions, and cirrhosis. 

If the conjugated bilirubin levels are higher than the unconjugated bilirubin, this is typically due to a malfunction of the liver cells in conjugating the bilirubin. This is commonly seen with drug reactions, hepatitis, as well as any type of liver disease. This can also be seen due to some sort of blockage of the bile ducts. Causes include gallstones, tumors, or scarring of the bile ducts. 

What are normal Urobilinogen conditions?

Urobilinogen is normally eliminated in the stool after passing through the liver and gallbladder. However, small quantities of urobilinogen are found in normal urine, where they contribute to the typical yellow colour of the specimen. Urobilinogen is normally present in the urine in low concentrations (0.2-1.0 mg/dL).

Comparing the urinary bilirubin result with the urobilinogen result may assist in distinguishing between red cell hemolysis [=rupturing (lysis) of red blood cells], hepatic disease (=any disturbance of liver function that causes illness), and biliary obstruction (=a blockage of the bile ducts), as shown in the table below:

References:

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