The most popular test in medicine is blood pressure monitoring. Cuff-based monitors long were the gold standard of noninvasive measurement of blood pressure in clinics, pharmacies, and even at-home devices. But what happens when we fully trust these instruments, and they give us false results?
In a 2025 research report, Kate Bassil and Anurag Agarwal of the University of Cambridge reveal why cuff-based techniques underestimate systolic blood pressure (SBP) in a systematic manner: a finding that has important implications for the diagnosis of hypertension.
The Importance of Accurate Blood Pressure
The main cause of early mortality in the world is high blood pressure or hypertension. The fact that it typically does not have any symptoms makes regular and accurate testing very important to early diagnosis.
Although invasive arterial line monitoring is accurate, it cannot practically be used outside of intensive care. This is why auscultatory measurements by cuff are the main approach. Nonetheless, recently published meta-analyses suggest cuffs have a mean bias of -5.7 mmHg and +5.5 mmHg, respectively, overstating diastolic blood pressure (DBP) (Staats; Lurie)
This difference is not marginal because, as per the studies, almost 30 percent of the cases of systolic hypertension have been reported to be underestimated by the cuff. Such mistakes may cause delays in diagnosing and treating millions of people.
Auscultatory Method: Just a Recap
Under the auscultatory method:
Around the upper arm above the systolic pressure, a cuff is inflated.
Slow release of pressure causes blood to flow randomly and causes Korotkoff sounds audible through a stethoscope.
The pressure at the time of the first sounds is measured as SBP; at the time that the sounds die out, the pressure is measured as DBP.
Though this seems quite clear, in practice, it is more complicated. The behaviour of arteries is not that of stiff tubes, and local tissue has an effect on pressure transfer.
Why Does the Cuff Overestimate Systolic?
It took decades before researchers noticed why cuffs tend to measure higher diastolic pressure; related factors such as arterial stiffness and tissue thickness demand an increasingly higher cuff pressure to occlude arteries. However, the underestimation of systolic pressure on a consistent basis could not be explained.
The unknown variable in the study by Bassil and Agarwal is low downstream pressure.
When the cuff completely obstructs the artery, blood flow is halted to the vessels below. Such vessels lose their pressure rapidly, plateauing at 30 70 mmHg. Such low downstream pressure causes the cuff-covered artery to take a longer time to reopen after cuff deflation. The result? The systolic pressure has recorded lower than actual value.
Experimental Insights
The study authors prepared a novel laboratory rig to simulate blood flow:
The arteries were modeled using lay-flat tubes which could completely close (unlike previous use of rubber tubes).
Flow duplication was carried out by using water as a working fluid.
The downstream tanks that gave various pressures were variable.
The negative relationship between the underestimation of systolic values and downstream pressure was very high (R = -0.92) during the experiments. The more rapidly downstream pressure is reduced, the larger the error is.
Interestingly, when a uniform pressure chamber (rather than a cuff) was utilised, underestimation no longer relied on downstream pressure. This proved that unevenness of pressure in the cuff was a primary factor behind the error.
Clinical Implications
Such results are important since the value that justifies the current oscillometric and cuffless monitors is based on cuff-based readings. In case of a flawed working of the so-called gold standard, devices adjusted to this calibration are also faulty.
The important implications are:
Lost diagnoses of hypertension: As many as a third of patients with systolic hypertension may be lost to diagnosis.
Innovation in devices: Errors might be lower if the cuffs are designed such that they produce more homogeneous pressure.
Simple procedural modifications: Clinical regularities can help rectify estimation shortfall. The easiest to fix would be simple procedural adjustments: arm elevation to make downstream venous pressure uniform, etc.
Individual calibration: Individual factors such as age, arm circumference, and arterial stiffness might be employed to calibrate the readings.
Looking Forward: Is better monitoring ahead?
The stakes are high since more than 1.3 billion individuals living worldwide have hypertension. This study does not merely point out the reason behind the systolic underestimation but also provides a roadmap on how to improve that.
Potential innovations in the future are:
Improved next-generation pressure-uniformity cuffs.
Oscillometric and wearable monitors with software corrections that take downstream pressure effects into consideration.
Ultrasound validation research to sharpen up correction models.
Through better measurement and assessment of the level in detecting and treatment of hypertension, we would go a long way in saving lives.
Conclusion
The reason why the cuff-based monitors underestimate the systolic blood pressure is clearly explained in the 2025 Cambridge study, and it is given for the first time in the form of a physical explanation. The culprit is low downstream vessel pressure, which slows the opening of the renal arteries and distorts readings.
The development opens up possibilities of engineering and procedural remedies that have the potential to revolutionize cardiovascular care. Resolving this old issue, clinicians and device manufacturers gain the opportunity to provide more stable blood pressure control, one of the key concepts of worldwide health.
References & Resources
Bassil K, Agarwal A. Underestimation of systolic pressure in cuff-based blood pressure measurement. PNAS Nexus, 2025. DOI: 10.1093/pnasnexus/pgaf222
World Heart Federation. Hypertension. 2023. Link