Apparatus and method for determining a calibration parameter for a blood pressure measurement device

According to an aspect there is provided a method of determining a calibration parameter for a first blood pressure, BP, measurement device, the method comprising obtaining a first physiological characteristic measurement of a subject using the first BP measurement device, wherein the first BP measurement device is for obtaining physiological characteristic measurements of a physiological characteristic of the subject and for determining a BP measurement of the subject from the physiological characteristic measurements using the calibration parameter, wherein the first physiological characteristic measurement is obtained when a torso of the subject is in a first posture; obtaining a second physiological characteristic measurement of the subject using the first BP measurement device, wherein the second physiological characteristic measurement is obtained when the torso of the subject is in a second, different, posture; determining the change in the posture of the torso from the first posture to the second posture; estimating a change in BP of the subject or a change in the physiological characteristic of the subject from the determined change in the posture of the torso; and determining the calibration parameter for determining BP measurements from physiological characteristic measurements obtained by the first BP measurement device from an analysis of the first physiological characteristic measurement, the second physiological characteristic measurement and the estimated change.