An improved enzymatic electrode and electrode module specifically adapted for use in a medical analyzer device is disclosed which permits rapid analysis of substances of interest contained within undiluted body fluids such as whole blood, serum and/or plasma. The enzymatic electrode is in fluid communication with an axially reciprocating probe which selectively transports either a quantity of a buffer aqueous solution or a calibrant aqueous solution presented at a wash cell or a body fluid specimen disposed within a sample cup to an active enzyme bearing membrane positioned adjacent the enzymatic electrode. The membrane comprises a composite membrane structure having a protective membrane layer adapted to prevent the passage of blood cells and other particulate or cellular substances therethrough as well as adjust the diffusion rate of the substance of interest desired to be measured into the membrane, an active enzyme layer adapted to convert the desired substance of interest to be measured into a detectable substance and a limiting membrane formed to prevent the passage of interfering low molecular weight substance therethrough. The membrane/reagent chemistry kinetics of the enzymatic electrode are specifically defined to enable a novel pseudo-rate/created peak measurement technique to be utilized which reduces measurement inaccuracies as well as signal data processing requirements. The membrane/electrode support structure and membrane/electrode solution flow path are specifically formed to permit rapid replacement of the composite membrane and aqueous solutions by non-professional personnel.