A non-invasive testing apparatus for determining a concentration of a target substance such as blood sugar blood alcohol cholesterol etc. in a patients blood involves applying an output RF signal (102) to the skin of a patient via an antenna (20) and measuring the amplitude and phase of a response signal which is a function of the output RF signal (102) modified by an interaction with the patients blood. Measurements are taken at different output RF frequencies (310) and the response (300) is plotted as a function of frequency (310). Various parameters (552 554 556 550 408 410) are derived from the shape of the resulting plots namely any or more of: a resonance frequency shift a Q factor of the resonance a group delay a phase shift an amplitude variation a shape factor of the plot and a gradient of the plot at different frequencies. Models of the derived parameters as a function of concentration of the target substance in blood are used to arrive at a determination of the latter. Further elements are a circuit (1000) for obtaining the amplitude and phase measurements a calibration device (5008) a system (3000) for attaching the apparatus to the patients body using a pulley belt and an inflatable bladder system (4000) for ensuring proper sensor contact.
