An ion selective electrode (“ISE”) assembly may be installed within a sample path of a biological testing system to measure ion concentration within sample fluid flowing through the sample path. A transducer membrane is placed within a housing and positioned to contact the sample path. A threaded portion is advanced into a socket of the housing and forces the membrane against a sealing portion of the sample path to prevent retention of sample fluid within the housing after testing. Sealing may be accomplished without adhesives or sealants and instead relies upon mechanical pressure of the threaded portion.