An ELISA (enzyme linked immunosorbent assay) is an antigenic test which gives a quantitative measurement of the analyte antigen. It will measure active as well as latent forms of the analyte. In contrast, a chromogenic assay measures functional activity. A patient could have normal levels of an analyte, such as PAI-1 antigen, but there could be some mutation on the molecule leading to a decrease in analyte activity. An ELISA would therefore detect nothing abnormal, whereas a chromogenic assay would.