Abstract: Disclosed herein is a medium for analyzing a substance in a mixture and method of making and using. In particular, the medium comprises a network of at least one polymeric fiber having a derivatized polymeric surface which allows immobilization of at least one selective binding agent thereon in a highly dispersed and randomly spaced orientation which forms a tortuous interstitial path for passage of the mixture therethrough. The substance may be a chemical substance or a biological substance. Also disclosed are assay matrices, assay systems and kits comprising the medium.