Potential Host Cell Protein (HCP) impurities are detected and quantified utilizing methods based on a two-site immunoenzymatic assay using an HRP-labelled anti-host antibody to detect cell residues. The samples are applied on anti-host antibody coated microtiter stripes forming a complex. After removal of unbound samples, 3,3,5,5-Tetramethylbenzidine (TMB) is added to the stripe and converted by the HRP, resulting in a color signal that is directly proportional to the concentration of the HCPs present in the sample. In the final step, this reaction is stopped by acidification and the optical density (OD) of each well is measured using the ELISA microplate reader SpectraMax.