BILIRUBIN AND UROBILINOGEN TEST FOR URINE

BILIRUBIN AND UROBILINOGEN

Urinalysis provides useful information on the state of not only the kidney, but also of other urinary tract organs and liver. Tests for urine bilirubin and urobilinogen are good indicators of liver function.

Bilirubin is a normal breakdown product of red blood cells. It is a vivid yellow pigment. It is excreted by the liver through the intestine. The bilirubin reaches the liver as water insoluble bilirubin reaches the liver as water insoluble bilirubin-albumin complex (free bilirubin or unconjugated bilirubin). 

In the liver, it is converted to a water soluble product by the Kupffer cells by conjugation with glucuronic acid. This water soluble bilirubin (conjugated bilirubin) is excreted by the liver into the bile and from there to the gall bladder where it is concentrated and emptied into the small intestine.

In the intestine, most of the bilirubin is reduced to urobilinogen. Most of urobilinogen enters the blood circulation, returns to the liver and begins the cycle again. However, a small amount of urobilinogen does not return to the liver but instead is excreted via urine. 

Urobilinogen is excreted through the intestine unchanged or oxidised to form the brown coloured complex, urobilin. In effect about 99% of urobilinogen formed from bilirubin is excreted along with faeces (as stercobilin) while only 1% by the urine. This means that normal urine contains no bilirubin but only a small amount of urobilinogen.

While the free or unconjugated bilirubin is not excreted by the kidney (because it cannot pass through the glomerular capsule), conjugated bilirubin can pass through the renal glomerulus. Abnormal concentration of bilirubin in blood will be excreted by the kidney.

Excretion of urobilinogen is of value in assessing or monitoring jaundice. Urobilinogen is colourless but the oxidised form, urobilin is orange-yellow in appearance. Urine containing bilirubin is yellow-brown in colour and produces yellow foam when shaken.

Detection of bilirubin in urine

The presence of bilirubin in urine is known as bilirubinuria.There are several methods available for the detection of bilirubin in urine. One of the oldest methods is the . It is a method of simply shaking the urine and looking for the presence of a yellow foam.

There are the reagent strip tests, most of which are based on a diazo reaction. Bilirubin is coupled with a diazonium salt in an acid medium to form azobilirubin. A positive reaction is indicated by the formation of coloured complexes. The tests use different diazonium salts and so different coloured compounds are formed. The tests are specific for bilirubin but the presence of other highly coloured substances in the urine may result in false positive tests.

The reagent strips are firm plastic strips with the reagent impregnated into a test area at the tip of the strip. The tests are rapid and convenient to use. "The test strip may be a single test trip for bilirubin or may be part of a multi-test strip. The produced colour is compared with a colour chart.

Test for Detection of bilirubin in urine

Fouchet's test

Ictotesttablet test (Ames)
 

Procedure for reagent strip test

Urine Urobilinogen-The measurement of the amount of urobilinogen in urine is a good index for assessing jaundice and monitoring liver function impairment.

Small traces or urobilinogen are found in normal urine.

This is a part of urobilinogen which is reabsorbed from the intestine. The amount of urobilinogen in urine depends on the ability of the liver to excrete urobilinogen from the intestine. This in turn will depend on the amount of bilirubin being produced and being transported to the intestine.

Urobilinogen which is colourless in freshly passed urine, is oxidised on standing to urobilin, an orange-pink pigment. This pigment and urochrome impart the normal colour of urine. Urine itself becomes orange-yellow when there is excess of urobilin pigment.

Clinical significance
High concentration of urobilinogen may be due to	Low concentration may be caused by
1. Haemolytic disease when the amount of bilirubin produced is raised resulting in the production of greater amount of urobilinogen.  2. Paralytic enterocolitis in which large amount of urobilinogen is produced in the intestine. 3. Cirrhosis of the liver. 4. Hepatic congestion leading to less urobilinogen being excreted by the liver. It passes into the general circulation resulting in larger quantity being excreted by the kidneys.	1. Obstruction of bile ducts blocking the flow of bilirubin to the intestine. Thus, there is no bilirubin to be converted to urobilinogen.  2. Destruction of normal bacterial flora of the intestine when urobilinogen cannot be produced. 3. Inability of liver cells to conjugate bilirubin into a soluble form, in which case it cannot be excreted in urine.

 

Two tests are commonly employed to detect urobilinogen

Ehrlich'stest

Urobilinogenstrip tests  

Bilugen(Boehringer)test for urobilin or urobilinogen

Watson-Schwartz Test forUrobilinogen  

Schlesinger's Test for Urobilin (Urobilinogen + Urobilin)