The estimation of the variation of the glucose level in the blood of a person by the variation of the volume of the interstitial fluid compartments in muscular tissue due to a shift of fluid between the extracellular and intracellular compartments caused by variations of the osmotic pressure of the extracellular fluids which is in turn correlated with the glucose level. The variation of the volume of the interstitial fluid compartments is detected by a non-invasive conductometry measurement using electrodes placed in contact with the skin of the person overlying a portion of soft tissue including muscular fibers. To eliminate the adverse effect of the conductivity of the capillary vessels, the conductivity of the tissue is measured independently in two directions, namely parallel and transverse to the muscular fibers.