Clinical blood pressure monitoring does not provide a true picture.

Katherine Keenan and colleagues attempted to determine the value of monitoring blood pressure by quantifying the probability that observed changes in blood pressure reflect true changes. They used an analysis of blood pressure measurements of patients in the perindopril protection against recurrent stroke study (PROGRESS).

This is a randomised placebo-controlled trial carried out in 172 centres in Asia, Australasia, and Europe.

They looked at 1 709 patients with a history of stroke or transient ischaemic attack randomised to fixed doses of perindopril and indapamide.

Results were based on a mean of two blood pressure measurements in patients receiving treatment recorded to the nearest 2 mmHg with a standard mercury sphygmomanometer at baseline and at 3 months, 6 months, 9 months, and 15 months, and then every 6-33 months.

They found that there was no change in the mean blood pressure of the cohort during the 33-month follow-up. Six months after blood pressure was stabilised on treatment, if systolic blood pressure was measured as having increased by >10 mmHg, 6 of those measurements would be false positives for every true increase of 10 mmHg. The corresponding value for an increase of 20 mmHg was over 200. Values for 5 mmHg and 10 mmHg increases in diastolic blood pressure were 3.5 and 39, respectively. The likelihood that observed increases in blood pressure reflected true increases rose with the time between measurements such that the ratio of true positives to false positives reached parity at 21 months.

They conclude that the usual clinical approaches to monitoring patients taking drugs to lower blood pressure have a low probability of yielding reliable information about true changes in blood pressure. Evidence-based guidelines for monitoring treatment response are urgently required to guide clinical practice.

Keenan K, et al. BMJ 2009; 338: b1492.