This ELISA kit uses the Competitive-ELISA principle. The micro ELISA plate provided in this kit has been pre-coated with 5-HIAA. During the reaction, 5-HIAA in the sample or standard competes with a fixed amount of 5-HIAA on the solid phase supporter for sites on the Biotinylated Detection Ab specific to 5-HIAA. Excess conjugate and unbound sample or standard are washed away, and Avidin-Horseradish Peroxidase (HRP) conjugate are added to each micro plate well and incubated. Then a TMB substrate solution is added to each well. The enzyme-substrate reaction is terminated by the addition of stop solution and the color turns from blue to yellow. The optical density (OD) is measured spectrophotometrically at a wavelength of 450 nm. The concentration of 5-HIAA in tested samples can be calculated by comparing the OD of the samples to the standard curve.