Experts suggest that measuring BP can be done more accurately in a patient’s home rather than in the doctor’s rooms.
Measuring BP accurately can be tricky, and there's a good chance that, when it is taken in a doctor’s rooms, those numbers won’t reflect the patient's real blood pressure on an ordinary day.
Raj Padwal, a professor of medicine at the University of Alberta in Canada who runs a specialty hypertension clinic, told The Washington Post he generally doesn’t ask for in-clinic blood pressure readings for his patients “because 50% of the time they’re inaccurate”.
There are a few reasons that readings taken in a doctor’s office can be misleading, but there is an easy way for patients to get an accurate picture of their BP from the comfort of their homes.
Problem with doctor’s rooms BP checks
Blood pressure is exquisitely sensitive, changing from moment to moment. “It’s affected by emotional status, mental health, physical activity, ambient temperature, medications, whether you ate, whether you slept well, and so on,” Padwal said.
BP is measured with two numbers: systolic pressure, the top number, representing the pressure on your arteries as your heart contracts; and diastolic pressure, the bottom number, representing the pressure on the arteries when the heart is at rest.
Getting an accurate BP reading requires following a highly specific procedure that includes keeping the feet flat on the floor, the back against a sturdy chair, and the arm on a flat surface.
Also, the cuff should be level with the heart and placed directly on the skin (not over a shirt), and patients should empty their bladder and sit quietly for several minutes before the reading. But many of these requirements aren’t consistently met in a doctor’s office, where staff may be too rushed to put patients in the right position.
Plus, doctors’ offices can have equipment problems.
Patients might assume the BP monitors there are more accurate than at-home monitors, but that’s often not the case. Analogue devices that use a cuff and pressure gauge need calibration every few months, but this rarely happens, said Jordana Cohen, an associate professor of medicine and epidemiology at the University of Pennsylvania Perelman School of Medicine.
Digital BP monitors, also used in medical settings, are more reliable. But only a fraction of them have been independently verified for accuracy, a process called validation.
Also, the doctor’s office may not have the right cuff size for the patient’s arm, and a too-big or too-small cuff can result in an inaccurate reading.
Some people also experience what’s called white-coat hypertension, where their blood pressure rises in a doctor’s office, possibly due to the stress of a medical visit.
Others have masked hypertension, in which their blood pressure falls to a lower level in a doctor’s office – possibly because those with a lot of stress in their daily lives feel calmer at a doctor’s surgery.
The true prevalence of these conditions isn’t clear, but it’s thought that 15% to 30% of people have white-coat hypertension, and about 32% of those with normal in-office BP readings have masked hypertension.
All of this may explain why blood pressure readings at a doctor’s rooms can vary so much.
A 2023 study that evaluated data on more than half a million people found that between two consecutive doctor’s visits, a person’s systolic blood pressure reading changed by an average of 12mmHg in either direction.
That’s not just a small rounding error. When patients first start on a new blood pressure medication, doctors generally expect it to lower the BP pressure by less than 12mmHg, said one of the study’s authors, Harlan Krumholz, a cardiologist and professor of medicine at the Yale School of Medicine.
That means if the doctor relies only on in-office measurements, they can’t be sure that a drop in the patient’s blood pressure is due to the medication working or to something else entirely.
Krumholz said the only way to overcome this is to collect a lot of measurements – 20 or so – and average them. Fortunately, experts say, there’s a simple way to help mitigate all of these problems: home blood pressure monitoring.
Why home BP monitoring works well
Patients wanting to measure at home can buy a high-quality monitor with a cuff that fits their arm. They need to be sure to prepare properly and use the correct procedures when sitting. And they can take multiple measurements on different days – and provide those data to their doctor.
At-home monitoring is especially valuable for anyone whose in-office readings indicate high BP and for those at an elevated risk for heart disease because of pre-existing health conditions like diabetes, Padwal said.
It can also be useful for tracking the effect of a new medication or an adjusted dosage, or for women who are pregnant or postpartum and who should be on the lookout for the high blood pressure problem called pre-eclampsia.
Not only can home monitoring be more accurate than in-office checks, but some studies have found it can actually help with the control of BP, particularly in the first few months of using a monitor.
Study details
Quantifying Blood Pressure Visit-to-Visit Variability in the Real-World Setting: A Retrospective Cohort Study
Yuan Lu, George Linderman, Harlan Krumholz, et al.
Published in Circulation: Cardiovascular Quality and Outcomes on 8 March 2023
Abstract
Background
Visit-to-visit variability (VVV) in blood pressure values has been reported in clinical studies. However, little is known about VVV in clinical practice and whether it is associated with patient characteristics in real-world setting.
Methods
We conducted a retrospective cohort study to quantify VVV in systolic blood pressure (SBP) values in a real-world setting. We included adults (age ≥18 years) with at least 2 outpatient visits between January 1, 2014 and October 31, 2018 from Yale New Haven Health System. Patient-level measures of VVV included SD and coefficient of variation of a given patient’s SBP across visits. We calculated patient-level VVV overall and by patient subgroups. We further developed a multilevel regression model to assess the extent to which VVV in SBP was explained by patient characteristics.
Results
The study population included 537 218 adults, with a total of 7 721 864 SBP measurements. The mean age was 53.4 (SD 19.0) years, 60.4% were women, 69.4% were non-Hispanic White, and 18.1% were on antihypertensive medications. Patients had a mean body mass index of 28.4 (5.9) kg/m2 and 22.6%, 8.0%, 9.7%, and 5.6% had a history of hypertension, diabetes, hyperlipidemia, and coronary artery disease, respectively. The mean number of visits per patient was 13.3, over an average period of 2.4 years. The mean (SD) intraindividual SD and coefficient of variation of SBP across visits were 10.6 (5.1) mm Hg and 0.08 (0.04). These measures of blood pressure variation were consistent across patient subgroups defined by demographic characteristics and medical history. In the multivariable linear regression model, only 4% of the variance in absolute standardised difference was attributable to patient characteristics.
Conclusions
The VVV in real-world practice poses challenges for management of patients with hypertension based on blood pressure readings in outpatient settings and suggest the need to go beyond episodic clinic evaluation.
See more from MedicalBrief archives:
Home BP monitoring signifcantly improves hypertension control, cuts costs
Inter-arm difference in BP robustly linked to greater early death risk
Blood pressure: standing readings outperform seated
BP readings’ accuracy differences between men and women – Canadian study
Vital for women to know their BP, say cardiologists