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Objective: To assess the diagnostic accuracy of recommendations for self-monitoring blood pressure (BP) for diagnosing hypertension in primary care.

Methods: Two hundred and forty-seven consecutive participants with raised (>=130 mmHg systolic) BP measured by their general practitioner from four primary care practices in the United Kingdom underwent 28 days of self-monitoring followed by 24-h ambulatory BP monitoring (ABPM). Diagnostic accuracy of the first 7 days of self-monitored BP (minimum 4 days, discarding readings on day 1) in detecting hypertension with ambulatory blood pressure was taken as reference.

Results: Two hundred and three participants were included, 109 (53.7%) of whom were diagnosed with hypertension using daytime ambulatory BP. The average of days 2-7 self-monitored BP correctly classified 150 of 203 participants [sensitivity 93.6%, 95% confidence interval (CI) 87.2-97.4%; specificity 51.1%, 95% CI 40.5-61.5%). However, the average of days 2-5 self-monitoring correctly classified 152 of 203 participants due to better specificity (53.2%, 95% CI 42.6-63.6%). In sensitivity analysis, diagnostic accuracy was not improved by inclusion of readings beyond day 5, and inclusion of readings taken on day 1 had no impact on diagnostic accuracy. Self-monitoring in the clinic was more accurate than readings taken by the general practitioner, but not self-monitoring outside of the clinic.

Conclusion: Hypertension can be ruled out in the majority of patients with elevated clinic BP using the average of the first 5 consecutive days of self-monitored BP, supporting lower limits for self-monitoring readings in current guidelines. Performing readings beyond day 5 and including readings taken on the first day had no clinical impact on diagnostic accuracy.

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