A probe cover for medical thermometer has a matching feature for preventing its use with an incompatible thermometer. A mechanical matching feature of the probe cover includes at least one of a fold, a step, or a series of holes or indentations that are respectively matched to a ridge, a valley or series of pins formed on the front end of a compatible thermometer. When applied to the compatible thermometer, the mechanical matching feature permits the probe cover to be fully seated on and retained by the probe, thereby placing the thermometer in condition for use. Alternatively, an opto-electronic matching feature of the probe cover includes a reflective layer for use with an opto-electronic detection circuit of a compatible thermometer. When applied to the compatible thermometer, the opto-electronic detection detects the reflective layer and places the compatible thermometer in an operational state for use.