Creatinine testing clinically includes blood creatinine and urine creatinine. In muscle, creatine is formed slowly mainly through irreversible non-enzymatic dehydration reactions, and then released into the blood and excreted in the urine. Therefore, blood creatinine is closely related to total body muscle mass and is not easily affected by diet. The level of blood creatinine depends mainly on the amount of creatinine excreted by the kidneys. Therefore the level of creatinine can reflect the level of kidney function. The methods are: 1. Alkaline picric acid endpoint colorimetric method. Creatinine in serum and picric acid produce yellow-red picrate complex in alkaline solution, and the initial method is endpoint colorimetric method, which measures the absorbance at 510-520nm and calculates the concentration of serum creatinine. 2. Enzymatic method. The enzymatic method of creatinine determination mainly uses creatinine enzyme to catalyze the hydrolysis of creatinine to produce creatine and its inverse reaction.