Abstract: Apparatus for indicating a predisposition to kidney stone disease in humans, and also dehydration, and for monitoring the efficacy of hydration therapy, comprises a conductivity probe including a tip which can be inserted into urine so that a drop thereof adheres to the tip, and display means for indicating the result as a derived function of conductivity (electrolyte concentration) in terms of urine specific gravity or osmolality. Preferably the probe electrodes are small and are spaced closely together so that the streamlines between the electrode tips are such that maximum current flows substantially within the adherent drop of urine.