The level of bile acids in serum is an important evidence reflecting liver parenchymal injury, especially in acute hepatitis, chronic active hepatitis, ethanol induced liver injury, and cirrhosis, where there are sensitive changes. At present, it is believed that the determination of bile acids in serum has a sensitivity of 76% and a specificity of 93% for the diagnosis of liver and gallbladder diseases. In chronic liver diseases such as cirrhosis, elevated bile acids occur earlier than changes in albumin, cholesterol, cholesterol esters, and bilirubin, making it of great significance for the diagnosis of chronic liver disease. Measurement principle: Bile acid α- Hydroxysteroid Dehydrogenase (3 α- HSD) and thio oxidized nicotinamide adenine dinucleotide (Thio NAD+) are specifically oxidized to produce 3 α- Ketone sterols (3 α- KS) and thio reduced nicotinamide adenine dinucleotide (Thio NADH). In addition, the generated 3 α- Ketone sterols in 3 α- Hydroxysteroid Dehydrogenase (3 α- In the presence of HSD and reduced coenzyme I (NADH), bile acids and oxidized coenzyme I (NAD+) are generated. This cycle amplifies the trace amount of bile acid, determines the absorbance change of the generated thio reduced nicotinamide adenine dinucleotide (Thio NADH) near the wavelength of 405nm, and calculates the total bile acid content.