Bilirubin is produced by red blood cells, which have an inherent life span. When reaching the life span, red blood cells will rupture and release hemoglobin, which is decomposed into nor-iron hemoglobin and heme, and heme reacts chemically with other substances in the serum and undergoes a series of transformations to produce bilirubin. The bilirubin in the serum of the body is an important marker to discern whether the liver function is abnormal. The bilirubin in the serum is mainly total bilirubin, direct bilirubin and indirect bilirubin, while total bilirubin is the sum of indirect bilirubin and direct bilirubin. Direct bilirubin, also known as conjugated bilirubin, is produced by indirect bilirubin entering the liver and combining with glucuronide by the action of glucuronosyltransferase in the liver. Direct bilirubin is soluble in water, reacts directly with azo reagents, and can be excreted in the urine through the kidneys; its normal value is less than 6.8 umol/L. Indirect bilirubin, also known as non-conjugated bilirubin, mainly comes from the destruction of red blood cells, and is not glucuronidated in the liver is called indirect bilirubin; its normal reference range is 3.4-20.0 μmol/L. An increase in direct bilirubin usually indicates bile duct opacification and biliary obstruction; elevated indirect bilirubin usually indicates a decrease in the liver’s ability to convert indirect bilirubin or an excessive source of indirect bilirubin, such as hemolytic jaundice.