Bilirubin in the urine
Last reviewed: 23.04.2024
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Normally, bilirubin in the urine is not determined, since unconjugated bilirubin is insoluble in administration and is not secreted by the kidneys.
Conjugated bilirubin through the biliary pole of the hepatocyte is excreted into the intestine, where it is reduced by the action of dehydrogenases of the microbial flora of the intestine into urobilinogen bodies. In the intestine consistently formed D-urobilinogen, I-urobilinogen and L-urobilinogen. Most of the D- and I-urobilinogen, formed in the upper parts of the large intestine, is absorbed by the intestinal wall and again enters the liver in the portal vein, where it disintegrates, and the products of this decomposition are again released into the intestine and, apparently, are involved in the hemoglobin synthesis . Fraction Λ-urobilinogen (sterkobilinogen), formed in the lower intestine, partially falls through the lower hemorrhoidal veins in the general circulation of the circulatory system and can be excreted in the urine as urobilin; The larger part in the form of sterocilin is excreted with feces, staining it.
In children of the first months of life, in connection with the absence of putrefactive processes in the colon, stercobilin is not formed, and conjugated bilirubin is released unchanged.
It is known that a small amount of conjugated bilirubin in the intestine under the influence of beta-glucuronidase is hydrolyzed into non-conjugated and reabsorbed along the enterohepatic pathway to the liver (enterohepatic circulation).
With a high level of free bilirubin of plasma and low renal excretion, the gradient for free bilirubin contained in the plasma is higher in the intestine, and therefore a significant amount of free bilirubin can be isolated in infants by diffusion through the intestine.
[Causes of bilirubin in the urine
Obstruction of extrahepatic biliary tract serves as a classic cause of bilirubinuria. The determination of bilirubin in the urine (along with the sorobilinogen) is used in the differential diagnosis of jaundice. Bilirubinuria is observed with obstructive and parenchymal jaundice, but absent with hemolytic jaundice. In newborns and children of the first year of life, early urine testing helps with atresia of the biliary tract and helps to avoid early death by timely surgical intervention, in some cases without transplantation. With viral hepatitis, bilirubin can be detected in the urine prior to the development of jaundice. Excretion of bilirubin increases with alkalosis,
At present, diagnostic strips are used more often for the qualitative determination of bilirubin in the urine. In addition, Harrison and Fouchet oxidation methods are used based on the ability of bilirubin to oxidize to biliverdin having an emerald green color. Normally, qualitative methods for determining bilirubin in urine give a negative result.
Dosorectation with subsequent spectrophotometry is used to quantify the level of bilirubinuria, as well as to determine the level of bilirubin in the blood serum.
With parenchymal jaundice in the serum, the amount of conjugated (direct) bilirubin increases. Since conjugated bilirubin is soluble in water, it is easily excreted by the kidneys. The urine is colored in a dark color. Qualitative reactions to bilirubin are sharply positive. The amount of strobobilinogen in the feces decreases, but complete disappearance of it from feces is observed only with mechanical jaundice. The intensity of bilirubinemia is higher the higher the level of bound bilirubin in the blood. With pronounced parenchymal jaundice, as well as with subhepatic jaundice, the amount of urobilin in the urine decreases. Qualitative reactions to urobilin can be negative in this case. With the restoration of the functional capacity of the liver and biliary excretion in the serum, the content of conjugated bilirubin decreases, the intensity of bilirubinuria decreases, the amount of sterocilin in the feces increases, and the qualitative responses to urobilin again become positive.
With jaundice caused by a rise in the level of free (unconjugated) bilirubin in the blood, qualitative reactions to bilirubin in the urine become negative, and on urobilin, on the contrary, positive, the amount of sterocilin in the feces increases.
Therefore, the determination of bilirubin in the blood serum, as well as qualitative reactions to bilirubin and stercobilin, have a limited significance for the differentiation of parenchymal and subhepatic jaundices. In some cases, it may be important that parenchymal jaundice, especially in severe parenchyma lesions, not only conjugated but also unconjugated bilirubin becomes larger in the blood serum, while in the subhepatic jaundice there is more exclusively conjugated bilirubin in the blood. However, it should be borne in mind that in parenchymatous jaundice, which occur with pronounced cholestasis, almost exclusively conjugated bilirubin accumulates in the blood.