Generality
The urobilin is a substance derived from the reduction of bilirubin by the intestinal bacterial flora.
A small amount of urobilin is instead reabsorbed and conveyed to the liver, then excreted again through the bile in the intestine.
Particularly modest, in normal conditions, is the proportion of urobilin removed from the kidney (3 mg / 24 hours). Higher concentrations are signaled by a chromatic variation of the urine, which takes on a mahogany red color (hyperchromic urine) and does not form foam.
Due to the key role of the liver in the intestinal elimination of urobilin, an increase in its urinary concentrations often, but not necessarily, reflects problems in the liver .
NOTE: the presence of urobiline in urine is the result of the oxidation of urobilinogen.
What's this
Urobilin is a bile pigment derived by oxidation from urobilinogen. The precursor of the latter is bilirubin, which, after having been conjugated with glucuronic acid in the liver, is eliminated in the bile.
Once in the small intestine, bilirubin undergoes a process of reduction, becoming urobilinogen, of which:
- One part goes into the faeces ( stercobilinogen );
- One part is reabsorbed by the intestinal mucosa, returns to circulation and is reported in the liver. Hence, the urobilinogen can be poured into the bile or reach the kidney to be oxidized to urobilin and be excreted in the urine.
If the amount of urobilin increases above the values considered normal, it is possible the presence of a dysfunction on the liver (viral hepatopathies, acute and chronic, toxic, cirrhosis, neoplasms) or of the gallbladder (obstruction of the biliary tract), or a hematological disorder (haemolytic anemias).
Even when urobilin values are particularly low or absent, changes in liver function are likely, with cholestasis or obstructive jaundice.
Urobilina: biological meaning
- Bilirubin derives from the degradation of hemoglobin, a protein found in red blood cells with the task of transporting oxygen and giving it to tissues.
- Bilirubin is produced in the spleen, in an insoluble form called indirect bilirubin, then transported to the liver bound to albumin. At the hepatic level the molecule acquires solubility in water through conjugation with two molecules of glycuronic acid (from this moment on it is called direct or conjugated bilirubin).
- The conjugated bilirubin is water-soluble and as such is introduced into the bile, channeled into the biliary tract, accumulated in the gall bladder and discharged into the intestine (duodenum).
- In the terminal ileum and colon the direct bilirubin is transformed into urobilinogen by the bacterial beta-glucuronidase, which cleaves it to glycuronic acid and bilirubin; the latter is further processed and converted into urobilinogen, mesobilinogen and stercobilinogen, all colorless substances.
- The urobilinogen is maximally excreted in the faeces, in the form of colored pigments (bilirubin → urobilin → stercobilin). A 20% is instead reabsorbed by the blood and conveyed to the liver, where it is again excreted with bile.
- A small proportion of the reabsorbed urobilinogen escapes the hepatic filter and is excreted in the urine, where it is oxidized to urobilin, a substance responsible for their characteristic coloring. In fact, urobilinogen is colorless, but it is transformed by light and pH into orange-red urobilin; for this reason, the urine left to "age" after excretion has a darker color than fresh urine.
- Non-conjugated bilirubin is fat-soluble. Therefore, if it is present in the blood at high levels it accumulates in the skin and in the ocular sclere, giving it yellowish tones (jaundice); in the child he can also reach the brain, causing more or less serious damage (nuclear jaundice).
Why do you measure
The test measures the concentration of urobilin in the urine .
This analysis allows the assessment of liver function and helps to diagnose any anemia caused by the destruction of red blood cells (haemolytic anemia). Furthermore, the urobilin test is useful for monitoring neonatal jaundice and checking the health of the kidneys.
Urine with excess urobilin is yellow-brown in color and does not foam.
Normal values
Under normal conditions, the urobilin is not present in the urine or is found only in traces.
- Urobiline in urine - Normal values : absent or traces.
Urobilina Alta - Causes
The urobilin concentration in urine may increase for two different reasons.
The main causes of a high biliary pigment value include:
- Deterioration, damage or injury to liver cells (secondary to neoplasms; viral, acute or chronic hepatitis; toxic hepatitis or liver cirrhosis);
- Increased destruction of red blood cells, as in the presence of haemolytic anemias and severe contusions with hematomas in the process of resorption.
Low Urobiline - Causes
An absence of bile pigments is typically observed in complete obstructive jaundices.
Furthermore, a low urobilin value can be observed under the following conditions:
- Enzymatic deficiencies;
- Serious liver failure;
- Alteration of intestinal bacterial flora;
- Prolonged intake of antibiotics.
How to measure it
The concentration of urobilin is measured with a urinalysis.
To ascertain whether the high level of this compound is due to excessive haemolysis or hepatic impairment, other examinations can be performed, such as:
- Complete blood count with red blood cell count;
- Liver function test.
Preparation
For the evaluation of urobilin it is necessary to collect a small amount of urine in the morning, on an empty stomach, after having carried out a thorough personal hygiene and after letting go of the very first emission (which may contain the germs present outside the apparatus) . In the case of women, it is good to take the exam away from the menstrual period.
The urine must be collected in a sterile container, which must be carefully closed immediately afterwards and taken to the laboratory within a short period of time.
Before undergoing urine tests useful for the evaluation of urobilin, it is good to pay attention to the type of medicines you are taking, as they can influence the result.
Your doctor may consider it necessary to temporarily stop taking certain medicines such as, for example, sulfa drugs, phenothiazine-based drugs, acetazolamide, chlorpromazine and cascara-based anthraquinone laxatives.
Interpretation of Results
The diagnosis of the causes of an increase in urobilin is reserved to the doctor, who can also recommend other more specific tests (such as blood tests or ultrasound scans) to complete the clinical picture.
Urobilina Alta
The values of urobilin increase in hyperemolytic states, therefore in the presence of an excessive catabolism of red blood cells, typical of the so-called haemolytic anemias, or of serious contusions with hematomas in the course of reabsorption.
As anticipated, high levels of urobiline in the urine can be a sign of hepatic cell injury (viral, acute or chronic hepatitis, toxic hepatitis, cirrhosis, neoplasms), resulting in an inability to fix the billets absorbed by the enteropathic circulation.
Low Urobilina
Low urobilin values are observed in the case of:
- Complete obstructive jaundice;
- Congenital jaundices due to enzymatic insufficiency (Crigler-Najjar syndrome, an inherited genetic syndrome in which glucoronyl transferase is missing, physiological jaundice of the newborn and maternal milk jaundice);
- Serious liver failure;
- Prolonged intake of antibiotics (on the contrary, in the presence of a bacterial overgrowth, as in the blind loop syndrome, an increase in urinary urobilin is appreciated).
Finally, significant changes in urinary pH can affect the values of urobilin: alkalinity causes an increase, and increased acidity decreases.
