A method includes measuring a first impedance-magnitudebetween a distal-electrode fitted at a distal end of amedical instrument in an organ of a patient and asurface-electrode attached externally to the patient, at a firstelectrical frequency. A second impedance-magnitude betweenthe distal-electrode and the surface-electrode, is measuredat a second electrical frequency. A difference between thefirst and second measured impedance-magnitudes iscalculated. Based on the calculated difference, anindication of, whether the distal-electrode is in physicalcontact with tissue or immersed in blood is decided, andoutputted.
