Home blood pressure machines are often wrong. The FDA must speak up

Hypertension is considered to be the No. 1 risk factor for death globally. So it’s no wonder convenient, relatively low-cost blood pressure machines with single-size, standard cuffs can be found in both clinics and homes across the U.S. The “Get It, Slip It, Cuff It, Check It” campaign makes it sound incredibly simple.

However, these popular machines and simple-sounding advice may lead to wildly inaccurate results for the sizable portion of the U.S. population with wider or longer arms. Investigators at John Hopkins recently conducted a randomized controlled trial published in JAMA Internal Medicine and discovered an automated BP monitor overestimated systolic blood pressure (the top number) by between 5 to 20 mm Hg in patients for whom standard cuff size was too small and underestimated it by 4 mm Hg when too large. Almost 40% in the trial were misclassified as hypertensive using the standard cuff based on a common threshold for hypertension (130/80 mm Hg).

One risk of this is that misclassification could lead to readings being ignored. Another risk is millions could be overmedicated based on false readings or make unnecessary lifestyle changes to manage blood pressure.

This happened to me after a specialist clinician used a cheap BP machine and scared the living daylights out of me. I ended up buying five home BP machines ($30-$100 each) using standard sized cuffs only to get inconsistent readings. So I upgraded to professional BP machines (the $703 Omron HEM-907XL and the $399 Welch-Allyn ProBP 2400) after my primary physician demonstrated one to me.

By chance, I experimented with different cuff sizes that come with these professional machines and discovered my high systolic BP readings were due to using the standard, medium 5.5” cuff, which, it turns out, is too small for my arm. My systolic BP varied by 30 mm Hg on the same device just by changing the cuff.

After that little experiment, I Googled and learned about the Hopkins trial. My primary doctor confirmed using manual BP readings and wrote me a doctor’s note recommending large cuffs whenever my BP is taken.

With more accurate BP readings at home, for the first time then I could clearly see the effect of other factors identified by the American Heart Association (AHA) that throw off BP readings. The AHA’s recommendations include waiting at least five minutes before taking the readings, positioning arm at heart-level, and not consuming caffeine or alcohol at least 30 minutes prior. However, the AHA also says to choose a validated BP monitor that “comes with the correct cuff size.” It is unclear what this even means if validated, consumer blood pressure machines come with one-size-fits-all, standard cuffs that magically cover a wide range of arm circumferences (22-42 centimeters), but professional BP machines (by the same manufacturers) need to split the same range into two cuff sizes (medium and large). A majority of adults with hypertension require large or extra-large cuffs.

As a patient, I did not know that literature on BP mismeasurement from incorrect cuff size dates back to at least the 1990s with manual BP measurement using stethoscopes. A 2010 paper says cuff width needs to be proportional to circumference of the upper arm. However, results from another clinical trial in 2021 reported automated blood pressure machines further exaggerate mismeasurement from incorrect cuff size by an additional 7 points compared with manual machines. This calls into question all my blood pressure readings over the past 30 years because I was never offered a large cuff before.

Why would there be a difference between automated versus manual methods for larger arms? It could be due to differences in how heartbeat is sensed using cuff pressure (automated) versus stethoscopes (manual). With a stethoscope below the cuff, heartbeat is detected only after blood begins flowing all the way through the arm. Automated machines don’t have built-in stethoscopes. Instead, they use oscillations of air pressure inside the entire cuff spanning the upper to lower arm and transmit these oscillations to a pressure sensor inside the BP machine (via air in a hose). A 2018 paper explains how oscillometric sensors are prone to confusing knocking of the pulse against the bladder wall (cuff) with actual blood flow, before blood flows through the entire upper arm length. Until manufacturers address false positives in automated devices, using a stethoscope appears to be a simple, foolproof way to bypass this uncertainty. But, of course, that doesn’t work for most people who don’t have the training to take manual measurements at home.

Home blood pressure devices marketed as “clinically proven accurate” are inaccurate if resulting in false positives for a sizable fraction of the population. Clinicians, patients, manufacturers, and the FDA each need to take a lesson from this:

1. Clinicians: BP mismeasurement due to cuff size by automated machines is readily addressable by investing in a high-quality, professional BP machine supporting multiple cuff sizes for patients.
2. Patients: When clinicians do not offer it, patients need to be empowered with knowledge to ask for the right cuff size at their doctor’s office because too small of a cuff size misstates BP readings and can mislabel them as hypertensive (or vice versa). The Mayo Clinic offers tips to help you get the right cuff size, but it is best to consult your primary doctor about it. You can ask your primary doctor for a note that recommends the right cuff size along with best practices to follow during BP measurement like I did and then take it to other doctors’ offices.
3. Manufacturers: Low-cost, automated BP machines for consumers need to address false positives due to single-size cuffs. Oscillometric machines need to be upgraded to support multiple cuff sizes. In the longer term, perhaps they should be redesigned to use less indirect methods to sense pulse only after it flows completely through the arm.
4. The FDA: When I reached out to the FDA for comment, it said that it is evaluating the Hopkins trial as part of the body of evidence to further its understanding but declined to comment on the specific study. But the FDA’s duty is not simply to further its own understanding for decades. It is also supposed to proactively warn, advise, and protect public health by assuring safety and efficacy of biomedical devices such as automated BP monitors in a timely manner, e.g. with easily understandable package insert (for cuff size limitations), as is routine for in vitro (rapid) diagnostics.

Patients like me want to be able to take control of our health. But if the equipment available to us isn’t reliable and “Get It, Slip It, Cuff It, Check It” oversimplifies things, then what’s the point?

Devabhaktuni Srikrishna is founder of Patient Knowhow, which curates patient educational content for air safety.