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For most accurate blood pressure readings, 24-hour home monitoring may be needed, study finds

blood pressure readings
REUTERS/Lucy Nicholson
People are at a greater risk of an early death from cardiovascular disease when their blood pressure measures high on a home monitoring device but not at the clinic.

Measuring blood pressure with a portable 24-hour monitoring device is a more accurate way of assessing people’s risk of premature death from cardiovascular disease than measuring it during a single visit to a doctor’s office, according to a large study published recently in the New England Journal of Medicine (NEJM).

The study also found that people are at a greater risk of an early death from cardiovascular disease when their blood pressure measures high on a home monitoring device but not at the clinic.

“This research is a clear game-changer, as for the first time, it definitely shows that blood pressure measured regularly during a 24-hour period predicts the risk of heart disease, stroke and death much better than blood pressure measured in a doctor’s surgery or clinic,” said Dr. Bryan Williams, the study’s senior author and chair of medicine at University College London, in a released statement. 

“With a much more accurate assessment of a patient’s blood pressure, doctors will be able to provide the most effective treatments at the earliest opportunity, which will save many more lives,” he added.

Study details

The study used data collected on 63,910 patients at 223 medical clinics across Spain. All the patients had symptoms that met medical-guideline recommendations for 24-hour blood pressure monitoring. 

The patients had two blood pressure readings taken in the clinic. They were also given a blood pressure monitoring device to wear at home for 24 hours. The device measured and recorded blood pressure every 20 to 30 minutes.

Based on these readings, the patients were divided into four groups:

  • Those with “white coat” hypertension — high blood pressure readings while in the clinic but not while wearing the 24-hour monitor
  • Those with “masked” hypertension — high blood pressure readings while wearing the 24-hour monitor but not while in the clinic
  • Those with sustained high blood pressure — high blood pressure readings both in the clinic and at home
  • Those with no high blood pressure, either in the clinic or at home

The patients were followed for an average of almost five years. During that period, 3,808 of them died of all causes, and 1,295 died of heart attacks, stroke, heart failure and other cardiovascular causes. 

The study found that the blood pressure readings from the 24-hour home-monitoring devices were 50 percent more accurate at predicting a patient’s risk of early death than the readings made in the clinics.

That didn’t mean, however, that the clinic readings were not helpful. High blood pressure measurements in the clinic were also predictive of an early death. They just weren’t as predictive as the home devices.  

The study also found that masked hypertension — high blood pressure detected on the home devices but not in the clinic — was associated with the highest risk of death. That increased risk may be because the masking led to delays in diagnosing the condition, William and his colleagues suggest. The longer high blood pressure remains undiagnosed and untreated, the greater the damage it can do to the blood vessels and the heart.

Limitations and implications

The study comes with several important caveats. As an observational study, it can’t prove that 24-hour blood pressure monitoring is better than clinic readings at accurately diagnosing high blood pressure and predicting cardiovascular risk. Other factors, not addressed in the study, might explain the results. 

In addition, the blood pressure readings in the study were taken at single moments in time (one clinic visit and one 24-hour period). This factor limits the prognostic power of the readings, the study’s authors point out.

Another caveat can be found in the study’s conflict of interest statement, where Williams is listed as an adviser to a company that makes portable blood pressure devices.

Still, this research supports other — albeit smaller — studies that have also shown that measuring blood pressure over the course of a day often provides a truer assessment of a person’s blood pressure status than a single reading in a doctor’s office. 

Indeed, the American Heart Association (AHA), as well as other medical organizations, now recommends that people with high blood pressure  — and certain other groups of people who may be at risk of having the condition — monitor their blood pressure at home.  

FMI:You’ll find an abstract of the study on the NEJM website, but the full paper is behind a paywall. For more information about home blood pressure monitoring, go to the AHA website

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Comments (2)

Hmmm…

My experience has been closer to the "white coat" than I'd have guessed. For several years, I've taken a reading once a day for 24 days, twice a year, and the at-home readings have always been significantly lower than the readings taken at the clinic during my twice-a-year visits to "my" doctor. The highest reading recorded (for me) was a one-time occurrence at the office of my dentist. A dental visit produces considerable anxiety… My pressure-reading device is battery-operated, so I always put in fresh batteries before the start of a new round of at-home readings. If my at-home readings are inaccurate, at least they're consistently inaccurate in the same way, with the same device.

Interesting Study for Several Reasons

This is an interesting study, not least because of its large sample size. The result highlighted in most media reviews—that I found unsurprising—focuses on their model when both the clinic and the 24-hour ambulatory BP monitoring variables are included, where it “provides unequivocal evidence that ABPM is superior to clinic pressure at predicting total and cardiovascular mortality”, quoting the first author in an interview.

What I did find surprising, and yes concerning, was their model results using a different specification. Instead of the BP variables (ambulatory and clinic) in the first model, a set of indicator variables were included to designate enrollee membership in one of the four categories: normotensive (at home and clinic), white coat hypertensives (high in clinic, normal at home), masked hypertensives (high at home, low at clinic), and last those with sustained high blood pressure (at both). Here I give two relevant quotes (from a review of the article and from the NEJM article); note that the normotensive BP category is the reference one for these two others:

Looking at specific phenotypes, untreated white coat hypertension carried a risk of all-cause mortality (HR 1.79; 95% CI 1.38-2.32) that was similar to the risk seen with untreated sustained hypertension (HR 1.80; 95% CI 1.41-2.31); this finding contrasts with prior studies showing that risk was not elevated or only partially heightened in patients with white coat hypertension compared with normotensives.

“In our study, white coat hypertension was not benign, which may be due in part to the higher mean blood pressure over 24 hours in these patients (119.9/71.9 mm Hg vs. 116.6/70.6 mm Hg in normotensive patients; P less than .001) or to their metabolic phenotype,” the investigators wrote.

Those seem to be pretty small differences in the mean BP measurements to have accounted for that large an elevated risk. Which makes me wonder about any adverse selection into this WCH group of enrollees, at the level of metabolic phenotype or for other important factors.