These tests follow much the same pattern as conjugated and unconjugated bilirubin. After bile reaches the duodenum, intestinal bacteria convert most of the bilirubin to urobilinogen. Much urobilinogen is lost in the feces, but part is absorbed into the bloodstream. Once in the blood, most of the urobilinogen goes through the liver and is extracted by hepatic cells. Then it is excreted in the bile and once again reaches the duodenum. Not all the blood-borne urobilinogen reaches the liver; some is removed by the kidneys and excreted in urine. Normally, there is less than 1 Ehrlich unit, or no positive result at greater than a 1:20 urine dilution.

Conjugated bilirubin, like urobilinogen, is partially excreted by the kidney if the serum level is elevated. Unconjugated bilirubin cannot pass the glomerular filter, so it does not appear in urine. However, when the serum unconjugated bilirubin level is high, more conjugated bilirubin is produced and excreted into the bile ducts; consequently, more urobilinogen is produced in the intestine. Additional urobilinogen is reabsorbed into the bloodstream and a portion of this appears in the urine, so that increased urine urobilinogen is found when increased unconjugated bilirubin is present. When increased serum unconjugated bilirubin is due only to increased RBC destruction, the serum conjugated bilirubin level is close to normal, because the liver excretes most of the conjugated bilirubin it produces into the bile ducts. Since the serum conjugated bilirubin level is normal in jaundice due to hemolytic anemia, the urine does not contain increased conjugated bilirubin.

When complete biliary obstruction occurs, no bile can reach the duodenum and no urobilinogen can be formed. The stool normally gets its color from bilirubin breakdown pigments, so that in complete obstruction the stools lose their color and become gray-white (so-called clay color). The conjugated bilirubin in the obstructed bile duct backs up into the liver, and some of it escapes (regurgitates) into the bloodstream. Serum conjugated bilirubin levels increase, and when these levels are sufficiently high, tests for urine conjugated bilirubin give positive results. In cases of severe hepatocellular damage, urobilinogen is formed by the intestine and absorbed into the bloodstream as usual. The damaged liver cells cannot extract it adequately, however, and thus increased amounts are excreted in urine. In addition, there may be conjugated bilirubin in the urine secondary to leakage into the blood from damaged liver cells. Incidentally, urine bilirubin is often called bile, which is technically incorrect, since conjugated bilirubin is only one component of bile. However, custom and convenience make the term widely used.

In summary, there is an increased urine conjugated bilirubin level when the serum conjugated bilirubin level is elevated but usually not until the serum conjugated bilirubin exceeds the reference range upper limit for total serum bilirubin. It is rarely necessary to order urine bilirubin determinations, since the serum bilirubin level provides more information. Increased urine urobilinogen may occur due to increased breakdown of blood RBCs or due to severe liver cell damage. Urine urobilinogen determination rarely adds useful information to other tests in conditions that produce increased urobilinogen excretion. In addition, there is the problem of inaccuracy due to urine concentration or dilution.