The sphygmomanometer, invented in the 1890s, does not always give an accurate picture of a person’s blood pressure. (BIGSTOCK)

About 67 million Americans — a third of all adults — have hypertension, a symptom-less disease that is a major contributor to heart disease, stroke and kidney disease. But we’re tracking this silent killer with random readings from a device that was invented in the 1890s. A better solution is out there: Will we use it?

The original blood pressure cuff — the mercury sphygmomanometer — was invented in 1896, and it still works fine. But newer, user-friendly and portable devices make it clear that sporadic checks in a doctor’s office are not nearly as good at detecting hypertension as frequent readings taken throughout the day.

Ambulatory blood pressure monitors use a cuff outfitted with a microchip to measure and record blood pressure levels at regular intervals. Because these devices run on automatic pilot, they tend to provide the least biased, most reliable information. If you’re smoking when it inflates, your blood pressure will be likely be inflated; if you’re deeply into your yoga routine, it might reflect that calmness.

Home blood pressure monitors are more susceptible to human error and bias because they’re less mobile and the patient decides when to initiate a check. Also, people using a device that requires them to write down their blood pressure numbers sometimes either leave out their high readings or round them down a little.

Both of these monitors have been around for decades, but recent advances have made them more convenient and accurate. And their portability has allowed us to see what office-based blood pressure readings have been missing: a lot.

White coats and masks

About 20 percent of patients who are consistently hypertensive in their doctor’s office or clinic don’t have high blood pressure the rest of the day. This phenomenon, called “white-coat hypertension,” reflects the anxiety often caused by a trip to the traditional white-cloaked doctor.

Does this mean that the Centers for Disease Control and Prevention is wrong in claiming that 67 million Americans have hypertension? Have 13 million Americans been given a misdiagnosis?

In a way, yes, but data from ambulatory blood pressure devices show that although office readings routinely over-diagnose hypertension, they sometimes under-diagnose it. The flip side of white-coat hypertension is called “masked hypertension”: Office readings are normal, but those taken outside the office are high. About 10 percent of people whose pressure appears normal in an office in fact have hypertension, and their disease goes undetected.

And then there are folks with “resistant hypertension,” people who can’t get their office blood pressure under control even with a fistful of medications. Studies have shown that 30 percent or more of these patients are actually well controlled or even over-controlled. They don’t need more medication, and they may even need less.

While both ambulatory blood pressure monitors and home blood pressure cuffs provide better data than an office device does, the ambulatory models have the added benefit of working while the patient sleeps, a part of the day that scientists don’t understand very well with respect to blood pressure.

We do know this: An hour or so into sleep, the blood pressure of most healthy individuals falls by 10 to 20 percent and then stays low until shortly before they wake up. These people are called “dippers.”

But some people are “non-dippers.” They lack the normal drop in blood pressure during sleep. A growing body of evidence suggests that nocturnal blood pressure is a better predictor of risk for heart attack or stroke than daytime blood pressure. That has led some doctors to have “non-dippers” take their blood pressure medication in the evening instead of in the morning.

Despite all the advantages of ambulatory blood pressure monitoring (ABPM), its use remains limited primarily to hypertension specialists (there are about 1,5oo of these in the United States), and some cardiology and nephrology practices. But most hypertension is diagnosed and treated in primary-care clinics: Why aren’t more of them using ABPM?

Simply put, getting the machines and software, and training physicians and clinic staff how to use them requires more time and money than most bustling and often cash-strapped primary-care clinics have.

Numbers show the benefit

About a year go, a 15-physician clinic near my hospital in Minneapolis slogged through the requisite start-up complexities to develop its own ABPM program. The practice has now used it on more than 1,000 patients, and the results have been striking.

About a third of those who underwent ABPM had been suspected of being hypertensive based on office blood pressure readings, but they hadn’t been started on medicine. Monitoring revealed that about 45 percent of the people in this group had normal ambulatory blood pressure, despite clinic readings that were mildly elevated at 140 or as high as 160. (Less than 120 is optimal and 120 to 140 is considered pre-hypertension.)

“It’s been breathtaking to see how much difference there can be between office blood pressure readings and what we’ve found with ambulatory monitoring,” said David Ingham, one of the doctors in the practice. “Someone comes into the office with a blood pressure of 160 or 170, and you think, ‘Well, that’s too high to just be a white-coat effect — they’re going to have some high blood pressure underneath all that — but then the ABPM comes back as normal.”

Overall, 13 percent of the ABPM patients had their medication dose lowered or stopped altogether — which means that they had been over-treated. Another 18 percent had a dose increased or a new medication added as a result of the monitoring: They had been under-treated.

“This isn’t a randomized, placebo-controlled trial; we realize that,” Michael Cummings, another doctor in the practice, said. “But these are real-life patients in a regular clinic, and in that way the numbers are more useful and more realistic than what you’d get from a clinical trial.”

Medicare pays for ABPM, but only if white coat hypertension is suspected. Other insurers cover ABPM more broadly, though some patients may bear at least some of the cost. Cummings and Ingham wanted their clinic’s doctors to order ABPM whenever they thought it was necessary, without putting the patient at any financial risk, so they don’t bill anyone for the testing. By buying their own equipment and increasing the number of people using it, they pushed down the cost of each 24-hour test to less than $50. (Charges for tests conducted by outside companies can reach $900.)

In 2011, British doctors began using ABPM as a confirmatory test on nearly every patient suspected of having hypertension. The change occurred after Britain’s National Institute for Health and Clinical Excellence concluded that ABPM was the most accurate and cost-effective option for clinching the diagnosis.

An analysis published in the medical journal, the Lancet, projected that new approach will save Britain’s National Health Service $15 million over the first five years since it was adopted, mainly by avoiding treatment for those with white coat hypertension.

Any savings would be welcome in the United States, where the annual costs associated with hypertension are $156 billion, according to the American Heart Association.

Since the incidence of hypertension increases with age, those costs will likely increase as baby boomers grow older.

For now, our country’s leading guideline on hypertension describes ABPM as the best predictor of who will get hypertensive organ damage, but it stops short of enthusiastically recommending it for general usage, instead listing five clinical situations (including white-coat hypertension and resistant hypertension) where it “may be helpful.”

For diagnosing and treating hypertension, random office blood pressure measurements too often leave us in the dark. A flashlight — ABPM — could be exactly what the patient needs.

Bowron is a hospital-based internist in Minneapolis.