What’s your blood pressure? It’s a simple enough question, and you might think it’s like asking how tall you are or how much you weigh. But those are static numbers that are easy to pin down. Measuring blood pressure is a lot more like aiming at a moving target — it varies depending on many variables so needs a little more effort.

Blood pressure is measured using a device called a sphygmomanometer — that inflatable cuff that’s placed on your upper arm. After it is inflated, the air is slowly released as the doctor or nurse listens with a stethoscope for the sound of the artery in your arm. The first sound they hear identifies the systolic pressure; diastolic pressure is the point at which the sound disappears.

Normal blood pressure is 120 over 80 (120/80). If the systolic (first number) pressure is 140 or higher and diastolic is 90 or higher on two or more occasions, that is hypertension (high blood pressure). But if blood pressure falls between the two (120 to 140 and 80 to 90) this indicates pre-hypertension, putting you at elevated risk for developing full-blown hypertension.

The exact causes of high blood pressure are not known, but there are factors and conditions that may play a role in its development: smoking, being overweight, a lack of physical activity, too much salt in the diet, overconsumption of alcohol, stress, aging and genetics are all known to play a role.

Blood pressure is variable throughout the day, even from minute to minute. It is affected by complex bodily systems like the heart, kidneys, arteries, hormones and nerves. In addition, it varies depending on whether you are being active or completely at rest and your body position as you are tested. Even the fact your blood pressure is being measured can increase it in some people (called the “white coat effect”). Smoking, coffee or exercise in the half hour prior to testing can all affect the measure.

If your blood pressure is obviously high or clearly normal, measurement issues are not so critical, but if you are on the edge of having a problem (pre-hypertension), accuracy is much more important.

Best practice guidelines identify how to most accurately measure blood pressure. Ideally, you should wait in a quiet room for five minutes prior to testing. During the test, neither you nor the practitioner should talk. The best position is seated on a chair with both feet on the floor and your back supported (not on the exam table as is often the case). The doctor or nurse should be holding the full weight of your arm at heart level. Ideally, two tests should be done at least a minute apart, then averaged. And using the proper size of cuff is important, as too small a cuff will show an erroneously high reading.

When required, the best measure is 24-hour ambulatory testing, where a device measures blood pressure over the period of a full day, recording it every half hour. This level of information provides the most solid evidence for introducing medication.

Concern over accuracy can be moot in cases where the person tested is clearly in the normal range or is obviously hypertensive. Minor variations will affect neither diagnosis. It is the grey area in between those two extremes. Have a conversation with your doctor about your own situation.

Dr. Paul Martiquet is the Medical Health Officer for Rural Vancouver Coastal Health including Powell River, the Sunshine Coast, Sea-to-Sky, Bella Bella and Bella Coola.