My blood pressure is one of those health benchmarks I’ve never worried much about. It has always been well below 120 over 80, a normal level according to the American College of Cardiology, the Centers for Disease Control and Prevention and other health organizations.

I learned to check my pressure at home to be sure it stayed normal, carefully following guidelines set by the American Heart Association and American Medical Association, including that the cuff should be placed over bare skin, the patient should not be talking, the arm should be supported with elbow at heart level, with back supported, legs uncrossed and both feet flat on the floor.

So I was surprised last winter when, as I waited to see my doctor, a nurse came in to the exam room and, in a rushed encounter, told me to hold my arm in the air as she placed the cuff over my sweater sleeve and took the measurement. My blood pressure was 130 over 66, which is still normal but on the high side of it. Some might classify it as borderline high pressure.

High blood pressure, considered a leading risk factor for heart attack and stroke, can be a scary thing. About 45 percent of people in the United States have high blood pressure and only 24 percent of them have their blood pressure under control, according to the CDC.

Having an accurate idea of your blood pressure is important — in 2018, there were nearly a half-million deaths in the United States that were caused by hypertension or in which it was a contributing cause.

Several studies have shown that how blood pressure is measured can alter the result and I wondered if that is what could have raised mine. A study of people who took part in the Systolic Blood Pressure Intervention Trial (SPRINT) found that blood pressure readings taken during routine clinic appointments were consistently higher than those taken during the study’s trial, which carefully adhered to AHA guidelines.

The U.S. Preventive Services Task Force, an independent body of medical experts in prevention and evidence-based medicine, found in 2015 that 15 to 30 percent of people with high blood pressure readings at routine clinic appointments may have lower readings outside of that setting.

There can be several explanations, including that some patients feel anxious at the doctor’s office — they experience the “white-coat syndrome” — which can cause a high reading. And others may have rushed to get to their appointment, navigating subways, traffic and parking without time to calm down before blood pressure is taken. But it’s also clear, experts say, that some clinics just aren’t doing it right.

Kevin Hwang, an associate professor of general internal medicine at McGovern Medical School at UTHealth who has studied the reasons for office blood pressure mistakes, says he has heard from patients about them.

“I’ve had a lot of patients come to me and they will say, ‘Hey, one reason I wanted to talk with you is because I got my blood pressure measured and they said it was 160 over 100, but I had no previous symptoms,’ ” Hwang said. “A lot of times, they will say, it was in a dentist’s office, they were lying back in a dentist’s chair or it was done in a pharmacy where they didn’t rest and their back was unsupported. By the time they are seen in our office, it’s 20-30 points lower.”

Hwang said one study showed up to 50 percent of patients had lower blood pressure at home, probably because of both white-coat hypertension and mistakes when taken in an office. He said even taking a measurement through a thin sleeve may boost a reading.

“It’s a mixed bag when you look at research, depending on thickness of the sleeve, age of patient and other factors,” he said. “For example, it may not make a difference in young patients but does so in older patients.”

Paul Whelton, Show Chwan professor of global public health at Tulane University’s School of Public and Tropical Medicine who has led many National Institutes of Health-funded blood pressure intervention trials, said the consequences of straying from guidelines can be serious, given the link between high blood pressure, heart disease and stroke.

“It leads to a lot of error and most of the time the error is going to be overestimation of the true average blood pressure, but sometimes it’s underestimation and there’s no way to deal with that,” he said.

Whelton pointed to one small study showing many medical students performed poorly on doing accurate blood pressure tests. He suggested technicians, or other staff with more time than nurses, medical assistants or doctors, should be trained to take blood pressures.

“Usually people who are pretty busy don’t take the time,” he said.

Ray Thomas, 67, of Chicago, is one of those patients who feels nervous when he goes to a doctor’s office, his “heart pounding.”

He wasn’t surprised when his blood pressure was high on a nurse’s reading, but then became normal after he had rested and his doctor took it. But he still wondered whether their methods played a part or whether he just needed some time to destress.

The next time, Thomas said he planned to be more assertive about his care by asking to rest first.

As for me, the next time I go in, I will certainly do whatever it takes to be sure my blood pressure is taken correctly. That will mean asking to sit quietly for a few minutes first and resting my arm on the armrest of a chair. I may get an annoyed look from the individual taking my pressure, but it will be well worth knowing if I really have hypertension.