The method is based on a competitive colorimetric ELISA assay. The trenbolone antibody has been coated in the plate wells. During the analysis, sample is added along with the trenbolone-horseradish peroxidase (Trenbolone-HRP) conjugate. If the trenbolone residue is present in the sample, it will compete for the trenbolone antibody, thereby preventing the Trenbolone-HRP from binding to the antibody attached to the well. The resulting color intensity, after addition of the HRP substrate (TMB), has an inverse relationship with the trenbolone residue concentration in the sample.