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By Erika Edwards and Maggie Fox

Blood pressure checks definitely save lives and are worth doing, experts said Monday, but patients need to get their blood pressure screened outside the doctor’s office, too.

That’s because some people’s blood pressure spikes when they see a doctor, while other people may have dangerous surges when they wake up in the morning, or through the day, the US Preventive Services Task Force says in new recommendations.

So-called ambulatory blood pressure monitoring will help doctors decide who might be able to lower their dose of blood pressure medications. And it might flag patients whose blood pressure looks perfectly normal at an office visit but who need help keeping it under control in day-to-day life.

“We're encouraging physicians to confirm a new diagnosis of high blood pressure in their patients using either ambulatory blood pressure monitoring, which means wearing a cuff for 24 hours while the blood pressure gets checked every 20 or 30 minutes, and confirming that the blood pressure isn't just elevated in the office, but that it's also elevated at home or at work,” said Dr. Mark Ebell of the University of Georgia and a member of the task force.

"There are a lot of people out there who have high blood pressure who don't know it or who have high blood pressure and it's not being adequately treated.”

“There probably are some people who could use a lower dose than they're currently taking because they have lower blood pressures at home than in the office, and their doctor doesn't know about that,” Ebell told NBC News.

“We think an even bigger concern is under-diagnosis and under-treatment. There are a lot of people out there who have high blood pressure who don't know it or who have high blood pressure and it's not being adequately treated.”

A third of U.S. adults have high blood pressure.

“High blood pressure is a major contributing risk factor for heart failure, heart attack, stroke, and chronic kidney disease. In 2010, it was the primary or contributing cause of death for more than 362,000 Americans,” the task force, which gathers independent panels of experts to make health recommendations for Americans, said in its report.

Right now, people are told to get their blood pressure to 140 or lower. That's the top number in a blood pressure reading, known as systolic blood pressure. But a study released earlier this year found that getting blood pressure to 120 or lower reduced rates of cardiovascular events, such as heart attack, stroke and heart failure by a third. It reduced the deaths from any cause by 25 percent.

The task force has long recommended that people get screened for high blood pressure, and it’s standard procedure in most office and hospital visits. But the group has always been aware that a one-time measurement during a medical visit may not accurately show what a person’s blood pressure really usually is.

“Blood pressure is affected by various short-term factors, such as emotions, stress, pain, physical activity, and drugs (including caffeine and nicotine),” the group’s report, published in the Annals of Internal medicine, reads.

Some people may be stressed by visiting the doctor, and when that makes blood pressure go up it’s called white coat syndrome. As many as 30 percent of people diagnosed with high blood pressure may have this effect, and while they may still need medication to bring their blood pressure down, they may need less than prescribed based on the office visit.

Other people may experience a morning spike in blood pressure that’s been linked to heart attacks and strokes. That wouldn’t show up at a doctor visit. And still others may be relaxed at a medical visit but stressed at home or at work and need help controlling their blood pressure there.

So the group recommends ambulatory blood pressure monitoring, too, to make sure a patient isn’t being over- or under-treated. It involves wearing an inflatable cuff all day and carrying a device, as well as keeping a diary to record activities.

If that’s not possible, home blood pressure monitoring may also serve, the group said.

“We don't have quite as much data on the monitoring in the pharmacies and the grocery stores, but certainly if someone can't do the monitoring at home, that's a good alternative,” Ebell said.

"I think you're more likely to get an accurate reading with a blood pressure measured in the upper arm."

“In general, most of the research has been done using blood pressure cuffs that measure on the upper arm and I think you're more likely to get an accurate reading -- not that the wrist ones are always wrong, but I think you're more likely to get an accurate reading with a blood pressure measured in the upper arm.”

Ebell said people need more than just one or two blood pressure measurements.

“Initially when we're trying to confirm the diagnosis, I think it's helpful to get the blood pressure several times a day and over several days,” he said.

“Once somebody has the diagnosis confirmed, they're on treatment, I don't think they need to be checking it every day, but periodically checking it is probably a good idea just to make sure that they're still well controlled or if they have symptoms, they feel light-headed. Or they don't feel well, maybe checking the blood pressure is a good idea as well.”

The American Heart Association recommends a low-salt diet high in fruits, vegetables and whole grains to lower blood pressure. But people often need to take drugs as well, including diuretics, beta-blockers and drugs called ACE inhibitors.