Serum bilirubin and fractions – metabolism, methods, interferences, reference ranges, interpretation of results.
Bilirubin is a yellowish pigment found in bile, a fluid made by the liver.
Metabolism: Bilirubin is the terminal product of heme metabolism. Heme is present in haemoglobin and in other oxidative compounds such as hepatic mitochondrial and microsomal cytochromes. Bilirubin is toxic to tissues; therefore, it is transported in the blood bound to albumin. Only a minute amount of free form is present in the blood. Bilirubin is taken up by hepatocytes at their sinusoidal surface. The free molecule of bilirubin enters the hepatocyte. Unconjugated bilirubinin is lipophilic. Its conjugation with glucuronic acid renders it hydrophilic, thus, it can be eliminated in the bile. Conjugation occurs in the endoplasmic reticulum and consists of forming an ester between glucuronic acid and one or both propionic side-chains of bilirubin.
Unconjugated (“indirect”) bilirubin. This is the bilirubin created from red blood cell breakdown. It travels in the blood to the liver.
Conjugated (“direct”) bilirubin. This is the bilirubin once it reaches the liver and undergoes a chemical change. It moves to the intestines before being removed through your stool.
A high level of bilirubin in the blood can lead to jaundice. Jaundice is a yellow colour in the skin, mucus membranes, or eyes. Jaundice is the most common reason to check bilirubin level. A bilirubin test is also ordered when a person has liver or gallbladder problems.
Methods: The direct spectrophotometric assay is based on the absorbance of bilirubin at 454 nm. The direct spectrophotometric assay is based on the absorbance of bilirubin at 454 nm
Interferences: Haemolysis, other pigments, such as carotenoids, also absorb at 454 nm can alter the result also Diet, medications, and exercise can alter results.
Reference Ranges: is normal to have some bilirubin in the blood.
A normal level is:
- Direct (also called conjugated) bilirubin: less than 0.3 mg/dL
- Total bilirubin: 0.1 to 1.2 mg/dL
Interpretation of results: In newborns, bilirubin level is higher for the first few days of life.
Jaundice can also occur when more red blood cells than normal are broken down. This can be caused by: erythroblastosis fetalis, haemolytic anaemia, Transfusion reaction in which red blood cells that were given in a transfusion are destroyed by the person's immune system
The following liver problems may also cause jaundice or a high bilirubin level: cirrhosis, hepatitis, gilbert disease,