The test principle applied in this kit is Sandwich enzyme immunoassay. The microtiter plate provided in this kit has been pre-coated with an antibody specific to Complement Component 4 (C4). Standards or samples are then added to the appropriate microtiter plate wells with a biotin-conjugated antibody specific to Complement Component 4 (C4). Next, Avidin conjugated to Horseradish Peroxidase (HRP) is added to each microplate well and incubated. After TMB substrate solution is added, only those wells that contain Complement Component 4 (C4), biotin-conjugated antibody and enzyme-conjugated Avidin will exhibit a change in color. The enzyme-substrate reaction is terminated by the addition of sulphuric acid solution and the color change is measured spectrophotometrically at a wavelength of 450nm ± 10nm. The concentration of Complement Component 4 (C4) in the samples is then determined by comparing the O.D. of the samples to the standard curve.