A method includes measuring a first impedance-magnitude between a distal-electrode fitted at a distal end of a medical instrument in an organ of a patient and a surface-electrode attached externally to the patient, at a first electrical frequency. A second impedance-magnitude between the distal-electrode and the surface-electrode, is measured at a second electrical frequency. A difference between the first and second measured impedance-magnitudes is calculated. Based on the calculated difference, an indication of, whether the distal-electrode is in physical contact with tissue or immersed in blood is decided, and outputted.
