HEALTH TOPICS 
To determine whether you have hypertension, a medical professional will take a blood pressure reading. How you prepare for the test, the position of your arm, and other factors can change a blood pressure reading by 10% or more. That could be enough to hide high blood pressure, start you on a drug you don't really need, or lead your doctor to incorrectly adjust your medications.
National and international guidelines offer specific instructions for measuring blood pressure. If a doctor, nurse, or medical assistant isn't doing it right, don't hesitate to ask him or her to get with the guidelines.
Here's what you can do to ensure a correct reading:
• Don't drink a caffeinated beverage or smoke during the 30 minutes before the test.
• Sit quietly for five minutes before the test begins.
• During the measurement, sit in a chair with your feet on the floor and your arm supported so your elbow is at about heart level.
• The inflatable part of the cuff should completely cover at least 80% of your upper arm, and the cuff should be placed on bare skin, not over a shirt.
• Don't talk during the measurement.
• Have your blood pressure measured twice, with a brief break in between. If the readings are different by 5 points or more, have it done a third time.
There are times to break these rules. If you sometimes feel lightheaded when getting out of bed in the morning or when you stand after sitting, you should have your blood pressure checked while seated and then while standing to see if it falls from one position to the next.
Because blood pressure varies throughout the day, your doctor will rarely diagnose hypertension on the basis of a single reading. Instead, he or she will want to confirm the measurements on at least two occasions, usually within a few weeks of one another. The exception to this rule is if you have a blood pressure reading of 180/110 mm Hg or higher. A result this high usually calls for prompt treatment.
It's also a good idea to have your blood pressure measured in both arms at least once, since the reading in one arm (usually the right) may be higher than that in the left. A 2014 study in The American Journal of Medicine of nearly 3,400 people found average arm- to-arm differences in systolic blood pressure of about 5 points. The higher number should be used to make treatment decisions.
In general, blood pressures between 160/100 mm Hg and 179/109 mm Hg should be rechecked within two weeks, while measurements between 140/90 and 159/99 should be repeated within four weeks. People in the prehypertension category (between 120/80 and 139/89 mm Hg) should be rechecked within four to six months, and those with a normal reading (less than 120/80 mm Hg) should be rechecked annually. However, your doctor may schedule a follow-up visit sooner if your previous blood pressure measurements were considerably lower; if signs of damage to the heart, brain, kidneys, and eyes are present; or if you have other cardiovascular risk factors. Also, most doctors routinely check your blood pressure whenever you go in for an office visit.
For more on getting your blood pressure under control, buy Controlling Your Blood Pressure, a Special Health Report from Harvard Medical School.