Value of ambulatory blood pressure monitoring in clinical pharmacology.

Ambulatory monitoring of blood pressure can improve clinical trials in two ways, first, by the identification and exclusion of those patients in whom the blood pressure is raised only in the clinic environment and second, by improving the repeatability of blood pressure estimations. In 75 subjects the standard deviation of the difference in diastolic pressure between two clinic readings taken a month apart was 12.3 mmHg. On ambulatory monitoring this fell to 6.3 mmHg. Since the number of subjects needed in a trial is related to the square of the standard deviation of the difference, the improved repeatability leads to a substantial reduction in the number of subjects needed. However, to achieve an adequate repeatability, at least 20 blood pressure readings are required per day.