There is provided a method of assessing the reliability of a measurement of blood pressure of a subject, the method in a control unit of an apparatus comprising obtaining a first measurement of at least one physiological characteristic of the subject that is indicative of the level of pain and/or stress of the subject, wherein the first measurement is made prior to a measurement of blood pressurebeing made; obtaining the measurement of blood pressure of the subject; obtaining a second measurement of the at least one physiological characteristic of the subject, wherein the second measurement is made after the measurement of blood pressure is made and prior to a subsequent measurement of blood pressure being made; and determining if the measurement of blood pressure is reliable based on thefirst measurement and the second measurement, wherein the measurement of blood pressure is determined to be reliable if the first measurement and the second measurement indicate a low level of pain and/or stress. A corresponding apparatus that implements the method is also provided.