Bilirubin determination is part of the so called “liver function tests” to assess the function and status of the liver.
Bilirubin is a product of heme catabolism, a component of proteins such as hemoglobin, myoglobin and cytochromes. Heme is converted to biliverdin by the action of heme oxygenase and biliverdin gives rise to bilirubin by biliverdin reductase. Bilirubin is poorly soluble in water, so it circulates bound to albumin in plasma. Bilirubin is a potentially toxic compound. In the liver, bilirubin is conjugated with glucuronic acid. This step creates the so-called conjugated bilirubin (also called “direct”), which is soluble, non-toxic and easily excreted through the bile.
Measurement of bilirubin in plasma
The most common method of determination of bilirubin (van den Bergh) is based on the use of diazo compounds. This method over-estimates the proportion of direct bilirubin. Using more accurate methods, it has been found that in normal subjects virtually 100% of the circulating bilirubin is unconjugated (“indirect”). Normal bilirubin levels are less than 1 mg/dL (18 micromol/L). Conjugated bilirubin is less than 20% of the total.
In prolonged cholestasis, a fraction of bilirubin is covalently bound to albumin, which is known as delta bilirubin. Bilirubin delta is not excreted in the urine and has a prolonged plasma half-life, equal to that of albumin. The existence of this bilirubin explains jaundice continuing for long periods after a period of cholestasis, even after liver function has normalized.
Clinical manifestations of hyperbilirubinemia
Elevated bilirubin manifests as jaundice. The threshold for clinical detection of jaundice is between 2 and 3 mg/dL. When is direct hyperbilirubinemia (conjugated), there is elimination of bilirubin in urine, which produces a characteristic dark color called dark urine. During the period of recovery from an episode of prolonged jaundice, dark urine may disappear but keep the jaundice, which is explained by the bilirubin delta. If there is a complete obstruction of the bile duct or liver failure markedly excretion of bilirubin, it does not reach the intestine and produces no brown pigmentation of normal bowel movements. This explains the acolia, describing the presence of white stools.
Interpretation of elevated bilirubin
After determining an elevation of bilirubin in the blood tests, the first step is to verify whether it is predominantly conjugated hyperbilirubinemia (“direct” bilirubin) or unconjugated (“indirect” hyperbilirubinemia). Usually we speak of hyperbilirubinemia of direct dominance when direct bilirubin is more than 30% of the total bilirubin.
The cause of indirect hyperbilirubinemia is an increased production of bilirubin, usually by increasing the catabolism of hemoglobin, for example, hemolytic anemias. In these diseases are other signs of hemolysis in blood tests, such as anemia, high MCV, high LDH and decreased haptoglobin. Rarely produces hemolysis bilirubin elevations greater than 6 mg / dL. Another very common cause of indirect hyperbilirubinemia is Gilbert syndrome, characterized by a decreased capacity of hepatic bilirubin conjugation. The other liver tests are normal in Gilbert’s syndrome. A rare cause of elevated unconjugated bilirubin is Crigler-Najjar syndrome, usually diagnosed at birth with marked jaundice (> 20 mg/dL in Crigler-Najjar type I).
Liver disease is associated with direct hyperbilirubinemia due to inadequate capacity of the liver to excrete bilirubin. The elevation of conjugated bilirubin in blood is one of the characteristic findings of cholestasic diseases and is accompanied by elevation of alkaline phosphatase and GGT. Several reasons may explain an increase in direct bilirubin:
- Bile duct obstruction: Either caused by stones or tumors of the biliary tract or pancreas.
- Cholestatic liver disease: Primary biliary cirrhosis, primary sclerosing cholangitis or drug toxicity, among other causes.
- Acute hepatitis: An acute inflammation of the liver can cause significant elevations in bilirubin excretion by failure at the level of the liver cell. In these cases the elevation of bilirubin is predominantly direct and is accompanied by elevations of aminotransferases (transaminase, SGPT/ALT and SGOT/AST). Viral hepatitis (hepatitis A virus, hepatitis B), hepatitis and drug toxicity (acetaminophen toxicity) or toxic (eg. mushroom toxicity) can cause liver damage and jaundice.
- Liver cirrhosis may be accompanied by progressive elevation of bilirubin. Importantly, elevation of bilirubin is a relatively late phenomenon in chronic liver disease and reflects significant damage in liver function.
- Isolated elevations of direct bilirubin: Some rare genetic diseases are characterized by isolated elevations of direct bilirubin, with the rest of the liver tests in the normal range. These diseases include Rotor syndrome and Dubin-Johnson.