Blood pressure checks are so common that the familiar arm cuff can seem almost foolproof.
But researchers at the University of Cambridge say the traditional method may sometimes underestimate the upper blood pressure number, potentially making high blood pressure harder to detect.
High blood pressure, or hypertension, puts extra strain on blood vessels and organs over time.
It is a major cause of heart disease, stroke and early death, yet it often causes no obvious symptoms, making accurate measurements especially important.
A blood pressure reading contains two numbers. Systolic pressure, the higher number, measures pressure when the heart pumps, while diastolic pressure, the lower number, measures pressure while the heart relaxes between beats.
In the traditional method, a cuff around the upper arm is inflated until it squeezes the artery closed. As the cuff slowly deflates, blood begins moving through the artery again and produces sounds that can be heard with a stethoscope.
The pressure when these sounds first appear is used to estimate systolic pressure. The pressure when the sounds disappear is used to estimate diastolic pressure.
Scientists have known for years that cuff measurements are not perfectly accurate. They can underestimate systolic pressure while overestimating diastolic pressure, but the physical reason for the systolic error has been difficult to explain.
The Cambridge team investigated the problem by building a mathematical and physical model of what happens to an artery while a cuff squeezes the arm. Earlier laboratory models often represented arteries using simple rubber tubes, but real arteries can flatten and close in more complicated ways.
The new work showed that inflating the cuff does more than temporarily stop blood from passing through the artery. It also leaves very low blood pressure in the part of the arm below the cuff while blood flow is blocked.
That low pressure on the far side of the cuff can keep the artery closed longer than expected as the cuff begins to deflate. Blood therefore may not start flowing again until cuff pressure has fallen below the person’s true systolic pressure.
Because the measurement depends on detecting the return of blood flow, this delay can make the recorded systolic number lower than the actual pressure. Even a modest difference could matter when a person’s reading is close to the level used to diagnose hypertension or decide whether treatment is needed.
The researchers estimate that measurement problems could contribute to hypertension being missed in a substantial number of people. However, blood pressure can also vary naturally from minute to minute, and factors such as stress, recent exercise, caffeine, posture and incorrect cuff size can affect a reading.
The team believes the newly identified problem might be reduced without completely replacing existing equipment. One possible approach involves briefly raising the arm before taking the measurement, which may change pressure below the cuff and help the artery reopen more accurately.
This idea still needs to be tested in people before it becomes part of routine medical practice. The researchers are seeking clinical studies to determine how large the effect is across different ages, body types and health conditions.
Future blood pressure devices could also use more information about the individual being measured to correct for some of these physical effects. For now, repeated measurements taken correctly remain more useful than relying on a single reading when diagnosing high blood pressure.
The University of Cambridge research was published in 2025 in the peer-reviewed journal PNAS Nexus. The study, led by Kate Bassil and colleagues, provides a new explanation for a long-standing source of error in one of medicine’s most familiar tests.
If the findings are confirmed in clinical trials, a small change in how blood pressure is measured could potentially improve the detection of hypertension. That could help more people receive treatment before years of unnoticed high blood pressure damage the heart, brain, kidneys and blood vessels.
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