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By Mandy Oaklander
October 28, 2014

When one nephrologist at an Ottawa hospital asked a patient to bring in his home blood pressure monitoring machine, he only planned to teach him how to use it. But he noticed something strange. The measurement was 21 units off from the device the doctor uses in his office, a gulf that can mean the difference between normal and high blood pressure.

“That took me aback, because in some patients we are looking at home readings to make our decisions for treatment,” says Dr. Swapnil Hiremath of The Ottawa Hospital. So he decided to check out the machines of all his patients, expecting no more than 1% of the machines to give the wrong reading. Instead, according to a new study he’ll present at American Society of Nephrology’s Kidney Week next month, he and his team found that home blood pressure readings might be off in up to 15% of patients.

The researchers asked 210 patients to bring in their home blood pressure monitors and tested them against the gold-standard in-office machines—known as mercury sphygmomanometers. A full 30% were more than 5 units off in the systolic measurement, or when the heart is contracting, and 8% were off by more than 10. Diastolic blood pressure measurements, when the heart is resting between beats, were even more askew: 32% were off by 5 units, and 9% by more than 10 units.

Depending on the patient, those discrepancies could be enough to influence treatment decisions. Hiremath says he doesn’t know why so many of the readings are off, especially since all the machines are validated by FDA and Health Canada before they appear on the market. “Despite that,” he says, “these errors are happening and we’re getting these variations in readings.”

Hiremath takes three readings to ensure accuracy, and instead of merely encouraging his patients to test their home monitors against the kind used in doctors’ offices, now he all but mandates it. “It’s a resource intensive thing to do,” he says. “There are no easy fixes here.”

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