Systems for detecting when catheter electrodes enter into and exit from an introducer are disclosed. In one form, a system detects a relative position of a catheter (comprising a marker band and an electrode) and an introducer (comprising a proximity sensor adapted to sense the marker band), while the catheter and introducer are in a human body. The system may comprise an electronic control unit to analyze signals from the catheter and/or the introducer, to determine whether the catheter electrode is within the introducer; and to disregard data collected from the electrode when that electrode is in the introducer. The sensor may be on the catheter and the sensed element may be on the introducer. The sensed element may comprise one or several marker bands. A marker band may be applied during the manufacture of a medical device or during its use and is any element capable of electromagnetic detection.