According to an aspect, there is provided an apparatus for measuring the blood pressure, BP, of a user, the apparatus comprising a volume-clamp BP monitoring device that comprises a first pressure device for applying pressure to a first part of the body of the user, a first photoplethysmogram, PPG, sensor for obtaining a first PPG signal from the first part of the body of the user, and a control unit that is configured to analyse the first PPG signal and to control the pressure of the first pressure device; wherein the control unit is configured to adjust the pressure of the first pressure device to maintain the first PPG signal at a constant level and to determine the BP of the user from the pressure of the first pressure device; and a second sensor, separate from the first PPG sensor, for measuring a physiological characteristic of the user in a second part of the body of the user, wherein the second part of the body is separate from the first part of the body; wherein the apparatus is configured to analyse the measured physiological characteristic to determine a measure of the blood perfusion in the second part of the body of the user, and to determine whether to perform a recalibration of the volume-clamp BP monitoring device on the basis of changes in the blood perfusion.