An anonymous reader quotes a report from Ars Technica: In a study out this week, about 70 percent of home blood-pressure devices tested were off by 5 mmHg or more. That's enough to throw off clinical decisions, such as stopping or starting medication. Nearly 30 percent were off by 10 mmHg or more, including many devices that had been validated by regulatory agencies. The findings, published in The American Journal of Hypertension, suggest that consumers should be cautious about picking out and using such devices -- and device manufacturers need to step up their game. Lead author Raj Padwal and his colleagues set out to test the accuracy of the devices themselves. Funded by the University of Alberta Hospital Foundation, they compared the home blood-pressure monitors of 85 patients with a gold-standard blood-pressure measurement technique. The patients' monitors varied by type, age, and validation-status. But they all used an automated oscillometric method, which measures oscillations in the brachial artery and uses an algorithm to calculate blood pressure. The gold-standard method was the old-school auscultatory method, which involves the arm-squeezing sphygmomanometer and a clinician listening for thumps with a stethoscope. Of the 85 home devices, 59 were inaccurate by 5 mmHg or more in either their systolic (the top number that's the maximum pressure of a heart beat) or diastolic (the bottom number that's the minimum between-beat pressure). That's 69 percent inaccurate. Of those, 25 (or 29 percent) were off by 10 mmHg or more. And six devices (seven percent) were off by 15 mmHg or more.