A medical sensor device includes a sensor assembly including an underside surface for attachment against a patient's skin, a sensor portion to detect a characteristic of the patient, and sensor assembly contacts which in operation carry signals representing the detected characteristic. The device also includes a transmitter assembly removably engageable with the sensor assembly and including circuitry to take the signals from the sensor assembly contacts and to transmit readings of the detected characteristic to external equipment. The device also includes mechanical interface components on the sensor assembly and the transmitter assembly which allow the transmitter assembly to be brought into abutment with the sensor assembly at a first angular position via relative axial movement between them, and then allow a relative rotation of the assemblies with respect to one another towards a second angular position and presents axial separation of the assemblies in the second angular position.