Measuring blood pressure correctly

What blood pressure is and why it matters

Blood pressure is the force of blood pushing against the walls of your arteries as your heart pumps. It is written as two numbers, such as 120 over 80. The top number, systolic, is the pressure when the heart beats; the bottom number, diastolic, is the pressure when the heart rests between beats. It is measured in millimetres of mercury, written as mmHg. Blood pressure that is persistently high, called hypertension, makes the heart and arteries work harder and, over years, raises the risk of heart attack, stroke, kidney disease and other problems. Because high blood pressure usually causes no symptoms, many people do not know they have it. This is why accurate measurement, and regular checks, matter so much — the number is your early warning.

How to measure it correctly

Good technique makes a real difference. Before measuring, avoid caffeine, smoking and exercise for about half an hour, and empty your bladder. Sit and rest quietly for about five minutes first. Sit with your back supported and legs uncrossed, feet flat on the floor. Rest your arm on a table so the cuff is level with your heart, and use the correct cuff size for your arm — a cuff that is too small gives a falsely high reading. Put the cuff on bare skin, not over clothing. Stay still and do not talk during the measurement, as talking raises the reading. Take two or three readings a minute apart and record them; the first is often the highest, so later readings are usually more reliable. Always use a validated monitor.

Home and ambulatory monitoring

A single reading in a clinic can be misleading, because some people's pressure rises simply from being in a medical setting — this is called white coat hypertension. Others have normal clinic readings but raised pressure at home. To get a truer picture, clinicians often use readings taken away from the clinic. Home monitoring means measuring your own blood pressure with a validated machine, usually twice a day for several days, and recording the results. Ambulatory monitoring uses a device worn for 24 hours that takes readings automatically as you go about your day. Both give an average from many readings, which reflects your everyday blood pressure far better than one measurement. If you monitor at home, use a machine with an upper-arm cuff validated for accuracy, and share your recorded readings with your healthcare team.

Common mistakes that skew readings

Many things can push a reading up or down and lead to the wrong conclusion. Common errors include not resting beforehand, having a full bladder, crossing the legs, letting the arm dangle unsupported, and putting the cuff over clothing. Talking, checking a phone, or feeling anxious all raise the number, as does using a cuff that is too small. Measuring straight after coffee, a cigarette or exercise gives a falsely high result. On the other hand, relying on a single reading, or on wrist or finger monitors that are often less accurate, can mislead. Because a diagnosis of high blood pressure can mean lifelong treatment, it is worth taking the trouble to measure carefully and to repeat readings on different occasions rather than acting on one number.

Understanding the numbers

In the UK, blood pressure is generally considered normal when a home or clinic reading is around 120 over 80 or below, and consistently below about 140 over 90 in the clinic, or 135 over 85 at home. Readings above these levels, confirmed on more than one occasion, may indicate high blood pressure, while very high readings need prompt attention. A single high reading does not mean you have hypertension, and a single normal one does not rule it out — the pattern over time is what counts. If your readings are raised, your GP or nurse will look at the trend, check for other risks, and advise on lifestyle changes and, if needed, medicine. If you ever have a very high reading with symptoms such as severe headache, chest pain, breathlessness or visual problems, seek urgent medical help.