The determination of free fatty acids (FFA) in serum is an important biochemical detection indicator, which is closely related to human health. Due to the complex composition of serum and the variety of FFA types, which are subject to various interference factors in detection, the chromogen substrate TOPS is usually used to determine FFA. Steps for determining FFA using color source TOPS: 1. First, add a buffer solution that has been weighed according to the formula proportion to the container, then sequentially add ATP, CoA, 4-aminoantipyrine, ion activator (magnesium chloride), stabilizer (BSA), and preservative. Add an appropriate amount of water and then add a surfactant. Adjust the pH to pH 6-9 with concentrated hydrochloric acid. Finally, add acyl CoA synthase, stir and filter. Add distilled water to the resulting filtrate to obtain reagent A. 2. Add buffer solution to the container, then sequentially add chromogen TOPS, water, and surfactant, adjust the pH to pH 6-9, finally add HRP and acetyl CoA oxidase, stir and filter, and then add distilled water to the filtrate to obtain reagent B quantitatively. 3.Fill and divide the above reagents into small bottles and store at 2-8 ° C. Mix reagent A with the sample in a certain proportion, incubate for a certain time, and read the absorbance value A1; Add reagent B and mix well, incubate for a certain time, and read the absorbance value A2. FFA concentration in the sample=standard solution concentration Χ (Λ) Α_/ Λ Α_#)， Λ Α_=Α 2-A1. Preparation of buffer solution: In a clean glass container, first add HEPES buffer solution (Good's buffer solution, i.e. zwitterionic buffer solution, including HEPES, Tris, MES, MOPS, etc.) that has been weighed according to the formula ratio. Add an appropriate amount of water and then add a surfactant of 5ml/L TritonX-100. Adjust the pH to pH 6-9 with concentrated hydrochloric acid, with a dosage of about 5ml/L. Then stir and filter, and add distilled water to the resulting filtrate to a certain amount. The method of using TOPS as a chromogen substrate to detect FFA in serum or plasma has high accuracy, small measurement error, and is the most suitable chromogen among many Trinder's chromogen standards for low acetyl CoA, high stability, and high molar absorption coefficient. In addition to TOPS, the chromogen substrates produced by Desheng also include TOOS, MAOS, ADPS, etc., which are suitable for detecting other biochemical indicators such as blood sugar.