Urobilinogen is a colorless substance produced from the breakdown of bilirubin. It is usually converted into two other substances known as stercobilin and urobilin. In the intestines, urobilinogen is converted to stercobilin by normal resident bacteria. Urobilinogen absorbed from the intestines goes to the kidney for excretion and some is converted to urobilin. Urobilin is responsible for giving urine its light yellow color, and stercobilin gives the feces its brownish appearance.

Bilirubin comes from the breakdown of hemoglobin. Hemoglobin is the oxygen carrying component of red blood cells (RBCs). RBCs often stay in the circulation for approximately 120 days. After this time span, they die and break down.

The death of red blood cells precipitates the production of the unconjugated or insoluble form of bilirubin. Unconjugated bilirubin then goes to the liver, where it is converted to its conjugated or soluble form. Conjugated bilirubin becomes a component of bile, a greenish-yellow fluid produced by the liver and then stored in the gallbladder. Eating foods rich in fats usually stimulates the gallbladder to release some of the bile to help in the digestion of fats, thus, bile is released to travel down to the small intestines.

As bile reaches the intestines, the bilirubin present in it will once again undergo conversion with the help of resident bacteria that are normally present there. This conversion will result in the formation of urobilinogen. Some is absorbed by the intestines and goes to the kidneys for excretion when voiding. A little amount is converted to urobilin and also excreted when voiding. The remaining amount that was not absorbed by the intestines, is usually converted to stercobilin and excreted in the feces.

An increased amount of urobilinogen concentration in the urine is mostly indicative of hyperbilirubinemia, which is the presence of excessive amounts of bilirubin in the blood. The danger of this condition is commonly seen in newborn babies where the presence of bilirubin in its insoluble form often accumulates in the child’s brain and causes seizures, visual problems, and mental retardation. Hemolytic anemia, which is a condition characterized by abnormal and premature death of red blood cells, also increases urobilinogen levels in the urine.

Other factors that may significantly increase urobilinogen levels are inflammation of the liver and liver cirrhosis. Its level in the urine becomes abnormally low when there is an obstruction in the passageway of bile. The use of broad-spectrum antibiotics that could result in the death of resident bacteria in the intestines may also lead to low urobilinogen levels.