The amount of total serum bilirubin in normal adults ranges from 3.4 to 17.1 μmol/L, of which about 80% is indirect bilirubin and the rest is direct bilirubin. Senescent erythrocytes are destroyed and the released hemoglobin is decomposed into bead protein and hemoglobin, the latter being converted to bilirubin through a series of reactions, after which it is released into the blood, where it combines with serum proteins, and this type of bilirubin is called unconjugated bilirubin. Its intramolecular hydrogen bonds are still present, so it cannot react directly with diazo reagents, hence the name indirect bilirubin. Bilirubin in the blood is transported to the liver and is taken up by the hepatocytes, where it binds to glucuronic acid in the endoplasmic reticulum of the slippery surface. This bilirubin is called conjugated bilirubin, which no longer has hydrogen bonds within its molecule and can react directly with diazotization reagents, and is therefore called direct bilirubin. Normal adult serum total bilirubin level is 3.4 to 17.1 μmol/L. When the plasma bilirubin level exceeds 17.1 μmol/L, it is called hyperbilirubinemia. When the level is 17.1 to 34.2 μmol/L, the yellowing of the skin and sclera is not visible to the naked eye and is called occult jaundice. When the level exceeds 34.2 μmol/L, yellowing of the skin and sclera can be seen with the naked eye, which is called overt jaundice. There are many causes of elevated bilirubin. If you have elevated bilirubin or yellowing of the skin and sclera, you should go to the hospital in time and receive standardized treatment under the guidance of your doctor to avoid delaying your condition.