A randomised controlled trial of the effect of anticipation of a blood test on blood pressure
Blood pressure is affected by situational anxiety, such as the white coat effect. We hypothesised that blood pressure would also be affected by anticipation of a blood test. Volunteer subjects were recruited on the campus of Birmingham University. Subjects were randomly assigned to intervention and control groups. After a period of rest, three seated blood pressure measurements were taken at 1-min intervals using an electronic sphygmomanometer. Between the second and third measurements subjects in the intervention group were told that a blood test would be carried out after the last measurement. No blood test was carried out. Three blood pressure measurements were made in all 213 randomised subjects. Analysis was by intention-to-treat. In the control group mean systolic and diastolic blood pressure fell in successive measurements. Between the second and third measurements mean systolic blood pressure fell by 1.4 mm Hg in the control group and rose by 2.6 mm Hg in the intervention group (difference 4.0 mm Hg, P < 0.0001). A rise in diastolic blood pressure between the second and third measurements did not reach statistical significance. It was concluded that anticipation of a blood test affects measured systolic blood pressure in volunteers. The practice of taking blood tests at the same time as measuring blood pressure may potentially bias estimations of blood pressure.