An improved diagnostic analyte monitoring device has partially retractable hollow guide needles for the intradermal placement of diagnostic elements fixedly connected to measuring means within this device obviating the need to remove the guide needle and to connect the diagnostic elements to measuring means after placement into the skin. A flexible surface adhering to the skin serves for the subcutaneous implantation of the diagnostic elements within the guide needles and partial retraction of the guide needles exposes the active surface to body fluid, actuated by means designed for easy handling and safe operation. Concentration-time profiles of endogenous and exogenous analytes measured with the device are used to improve drug treatment modalities on an individualized basis.