Your blood pressure is a measurement of the pressure your circulating blood is putting on the walls of your arteries, and is expressed as two numbers: Your systolic pressure (the top number, so the “120″ in 120/70) is the force exerted on your arteries when your heart beats, and the diastolic pressure (the bottom number) is the force exerted when your heart rests between those beats. This pressure is expressed as “mm Hg,” which stands for “millimeters of mercury.”
Normal blood pressure is usually considered to be less than 120 systolic and 80 diastolic (usually written out “120/80 mm Hg”), and anything higher than that is considered high blood pressure (hypertension). But it also shouldn’t get too low; anything under 90/60 is considered low blood pressure (hypotension). Both can indicate and cause health problems ranging from dizziness and confusion to stroke and heart attack.
That’s why your doctor consistently checks your blood pressure when you have any sort of examination, using the cuff wrapped around your upper arm (these days you can also check your own blood pressure at home pretty easily). But if you’re like me and always racing to your doctor’s appointments and arriving in an apologetic sweat, you might wonder if your blood pressure readings are accurate if they’re taken under unusual physical conditions. The answer is yes, your blood pressure results can be distorted by several factors.
White coat syndrome
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One main reason your blood pressure reading might be inaccurate is what’s known as white coat syndrome—the anxiety that some people feel when in a doctor’s office. According to the American Medical Association, this can result in a reading error of up to 26 mm Hg—which can easily push your reading into the “high blood pressure” range temporarily.
If your physician thinks you might be dealing with something like white coat syndrome, they may suggest checking your blood pressure regularly at home or recommend that you arrive early for your appointments to have some time to relax prior to being examined.
The mechanics
The physical arrangement when you have your blood pressure taken can have a big impact on your numbers, especially if you’re checking your blood pressure at home:
- Back support. Sitting in a chair that supports your back will reduce errors; having an unsupported back can add as much as 10 mm Hg to your reading.
- Arm support. If you hold your arm lower than your heart, your reading can be off by as much as 23 mm Hg. It’s best to support the arm so you can keep it at the right elevation easily.
- Legs and feet. Sitting with your legs crossed can add up to 10 mm Hg to your reading, and not supporting your feet can add another 10 mm Hg. Sit with your legs uncrossed and your feet flat on the floor.
- Bare arm. Taking your blood pressure with the cuff around one or more layers of clothing can add up to 40 mm Hg.
- Cuff size. Finally, the cuff of the blood pressure machine must be the right size. Studies have shown that heavier people often get inaccurately higher readings because the standard-size cuff is too small, and constricts their arms too much. It’s important that the cuff being used is appropriately sized.
Behavior
Even if your body is physically arranged properly for the reading, and you don’t suffer from anxiety when visiting the doctor, other behaviors can affect your reading:
- Talking. Yup, having a conversation while having your blood pressure taken can up your numbers by as much as 15 mm Hg.
- Full bladder. If you’re getting your blood pressure read, pee first: A full bladder can add up to 33 mm Hg to your systolic pressure.
- Food and drugs. It’s not surprising that eating a big meal shortly before a reading, or things like alcohol, nicotine, and caffeine ingested right before your test can distort the readings. You should avoid taking in food or substances for a while before your test.
It’s a good idea to build in a 5-minute rest period before having your blood pressure taken—if you blow into the doctor’s office or come home from errands and immediately sit down for a reading, you might get higher numbers than you should.
It’s important to note that healthcare professionals are aware of these factors and can make adjustments when reading the results to accommodate for them. But if you want the most accurate results possible so you don’t get any false alarms, keep these things in mind next time you strap that cuff on.