Superiority of ambulatory to physician blood pressure is not an artifact of differential measurement reliability.

Background: Ambulatory blood pressure is a better predictor of target organ damage and the risk of adverse cardiovascular events than office measurements. Whether this is due to the greater reliability owing to the larger number of measurements that are usually taken using ambulatory monitoring, or the greater validity of these measurements independent of the number, remains controversial.
Methods: We addressed this issue by comparing physician readings and ambulatory measurements as predictors of left ventricular mass index. The number of readings was controlled by using the average of three physician readings and randomly selecting three awake readings from a 24-h ambulatory recording.
Results: In a multiple regression analysis that included both the ambulatory and physician blood pressure measurements, only the ambulatory systolic measurements significantly predicted left ventricular mass index (B=0.37, t=3.11, P=0.002); the coefficient for physician's systolic measurements was essentially zero (B=-0.01, t=-0.26, NS).
Conclusions: These findings suggest that the superiority of ambulatory blood pressure as a predictor of target organ damage, compared with physician measurements, cannot be adequately/fully explained by the impact of the larger number of measurements obtained with ambulatory monitoring.