The urine test or urinalysis is the oldest complementary study and one of the most requested in medical practice. The interpretation of the results must consider each patient’s context to avoid diagnostic errors. In this partial test, the presence of some compounds is determined, among which is urobilinogen, which is responsible for giving the urine its yellowish coloration.
Urine urobilinogen concentration is usually assessed using dipsticks. These plastic tapes determine various parameters by visualizing reactive cushions arranged for each compound.
The consequent comparison of the coloration observed in each pad with the scale provided by the manufacturer makes it possible to estimate a relative concentration of the substance in the urine.
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How is urobilinogen produced?
Urobilinogen is a pigment derived from the metabolism of bilirubin. It comes mainly from the degradation of erythrocytes.
In this process, the heme group of hemoglobin is converted to biliverdin, which is then converted to bilirubin by an enzyme. This molecule, known as “unconjugated” or “indirect” bilirubin (BNC), has a configuration that makes it difficult to excrete, so it must bind to albumin, which transports it to the liver.
Upon reaching the liver, the BNC enters the cell, conjugating with glucuronic acid to form direct or conjugated bilirubin (BC). The more soluble configuration of this form allows it to be excreted into the intestine through the bile.
In the intestine, the bacterial flora transforms part of the bilirubin into a series of pigments known as “urobilinogen,” whose derivatives impart a brown color to the feces.
Enterohepatic circulation
However, a part of the urobilinogen is reabsorbed in the intestine into the blood vessels, forming the enterohepatic circulation. Because urobilinogen is a moderately soluble pigment, it does not need to bind to any protein for transport, being excreted back into the bile.
However, a small percentage remains in general circulation. Upon reaching the kidneys, it is filtered into the urine, which gives it its characteristic amber coloration.
Urobilinogen Normal Values
When urobilinogen is determined in urine through test strips, the value obtained is only an estimate. Therefore, this type of analysis is considered qualitative or semi-quantitative.
In the same way, since urobilinogen is a compound that is easily oxidized, its actual concentration can be affected by exposure to light, especially when the sample takes time to process.
Commercial houses consider a concentration lower than 1 mg/dL within normal limits. However, some authors accept that concentrations of up to 3 mg/dL may not be pathological. However, it should always be evaluated in the context of the patient.
On the other hand, the absence of urobilinogen in the urinalysis is not usually of clinical importance due to the easy degradation of this pigment.
Causes of increased urobilinogen in urine
Urobilinogen by itself is not considered a diagnostic parameter of disease, but it has clinical utility in certain conditions. Because it is a pigment that is the product of bilirubin metabolism, the interpretation of an altered result usually includes an analysis of the behavior of both substances.
Liver disease
Hepatitis causes inflammation and variable damage to liver tissue. The most common cause is infection with one of the hepatitis viruses.
In the acute forms, non-specific symptoms appear, leading to the jaundiced phase, characterized by the evident yellowish coloration of the skin and mucous membranes. This mucocutaneous pigmentation is the product of the accumulation of bilirubin in the bloodstream.
Elevated urobilinogen in the urine may be an early finding of liver injury. For this reason, it is considered that the presence of trivial clinical manifestations, and an abnormal urine test in which urobilinogen is increased, can serve as an early suspicion of some liver diseases.
Hemolytic anemia
In hemolytic anemias, there is increased destruction of erythrocytes. The reasons why this phenomenon appears vary.
As more hemoglobin is degraded, bilirubin increases, predominating the indirect fraction (BNC). However, since the liver function is unchanged, some of this substance is converted to conjugated bilirubin.
This slight increase in BC allows a greater molecule secretion through the bile, reaching the intestinal tract. The bacteria metabolize this compound there, producing a slightly higher amount of urobilinogen than usually is synthesized. Therefore, this excess urobilinogen is reabsorbed through the enterohepatic circulation, some filtering into the urine. There an increased concentration will be recorded.
Conditions that decrease urobilinogen in the urine
Although the absence of urobilinogen is not considered suggestive of disease, the correlation with the patient’s clinical manifestations makes this result somewhat helpful. Especially if the conditions for collecting the urine sample and its processing were optimal.
Therefore, it is not prudent to dismiss the finding of increased urobilinogen. Assuming that it is the oxidation of the compound is not always correct.
Negative urobilinogen in urine may express an obstructive pathology of the bile ducts. The patient usually presents with abdominal pain and jaundice (although the latter may be absent).
Also, the stools turn whitish ( acholia ) due to a lack of pigmentation. Urine, for its part, has a dark coloration (choluria) due to the passage of conjugated bilirubin (BC) through renal filtration.
On the other hand, the use of drugs can also decrease urobilinogen in the urine. Antibiotic therapy with sulfonamides is the primary representative of this phenomenon. So it is necessary to inquire about the medications ingested by the patient.
In the same way, general recommendations should be given for the correct collection and transport of the sample to the laboratory. This is to avoid alterations in the result.