Abstract: A non-invasive testing apparatus for determining a concentration of a target substance, such as blood sugar, blood alcohol, cholesterol, etc., in a patient's blood is disclosed. The apparatus and method involve 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 patient's blood. The invention takes measurements at different output RF frequencies (310), and plots the response (300) as a function of frequency (310). The invention is essentially characterised by deriving various derived parameters (552, 554, 556, 550, 408, 410) 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.