Veratox for Histamine is a competitive direct enzyme-linked immunosorbent assay (CD-ELISA) that allows the user to obtain exact concentrations of histamine in parts per million (ppm). Free histamine in the samples and controls is allowed to compete with enzyme-labeled histamine (conjugate) for the antibody binding sites. After a wash step, substrate is added, which reacts with the bound conjugate to produce blue color. More blue color means less histamine. The test is read in a microwell reader to yield optical densities. The optical densities of the controls form the standard curve, and the sample optical densities are plotted against the curve to calculate the exact concentration of histamine.