How is blood pressure measured?
Blood pressure is measured in millimetres of mercury (mm Hg).
Clinic/GP surgery blood pressure readings
These are readings taken by a doctor or nurse in a clinic or a GP surgery, using a standard blood pressure machine.
Home blood pressure readings
These are readings taken by a person whilst seated and at rest at home, using a standard blood pressure machine. You need to take readings twice a day for a week. This will give 14 top and 14 bottom readings. Add the top readings together and divide by 14. Then do the same for the bottom readings. This gives you an average reading. It's normal for your blood pressure to fluctuate, so a single raised reading isn't a cause for concern unless it's extremely high.
Ambulatory blood pressure readings
These are readings taken at regular intervals whilst you go about your normal activities. A small machine that is attached to your arm takes and records the readings, usually over a 24-hour period.
As a rule, an average of the ambulatory blood pressure readings gives the truest account of your usual blood pressure. Home blood pressure readings are a good substitute if an ambulatory machine is not available. Ambulatory and home readings are often a bit lower than clinic or GP surgery readings. Sometimes they are a lot lower. This is because people are often much more relaxed and less stressed at home than in a formal clinic or surgery situation.
What is high blood pressure?
High blood pressure (hypertension) is a blood pressure that is 140/90 mm Hg or above each time it is taken at the GP surgery (or home or ambulatory readings where the average is more than 135/85 mm Hg). That is, it is sustained at this level. High blood pressure can also be:
- Just a high systolic pressure - for example, 170/70 mm Hg.
- Just a high diastolic pressure - for example, 120/104 mm Hg.
- Or both - for example, 170/110 mm Hg.
However, it is not quite as simple as this. Depending on various factors, the level at which blood pressure is considered high enough to be treated with medication can vary from person to person.
Blood pressure of 140/90 mm Hg or above (or average home/ambulatory readings 135/85 mm Hg or above)
If your blood pressure is always in this range you will normally be offered treatment to bring the pressure down, particularly if you have:
- A high risk of developing cardiovascular diseases (see below); or
- An existing cardiovascular disease (see below); or
- Diabetes; or
- Damage to the heart or kidney (end-organ damage) due to high blood pressure.
Blood pressure of 160/100 mm Hg or above (or home/ambulatory readings 150/95 mm Hg or above)
If your blood pressure is always in this range, you will almost certainly be advised to have treatment to bring it down.
Blood pressure between 130/80 mm Hg and 140/90 mm Hg
For most people this level is fine. However, current UK guidelines suggest that this level is too high for certain groups of people. Treatment to lower your blood pressure if it is 130/80 mm Hg or higher may be considered if you:
- Have developed a complication of diabetes, especially kidney problems.
- Have had a serious cardiovascular event such as a heart attack, transient ischaemic attack (TIA) or stroke.
- Have certain ongoing (chronic) kidney diseases.
Clinical Editor's note
November 2017 - Dr Hayley Willacy has recently read guidelines from the American College of Cardiology and the American Heart Association Task Force - see Further Reading below. These have redefined the boundaries of elevated blood pressure. The earlier the risks of developing cardiovascular disease are picked up, the greater your chances to make good lifestyle changes which can make a difference:
- Normal: <120/80 mm Hg.
- Elevated: 120-129/<80 mm Hg.
- Stage 1: 130-139/80-89 mm Hg.
- Stage 2: >140/90 mm Hg.
Some people will not need medication. They will manage through diet and exercise, to lower their risks. Older adults with high blood pressure, other medical problems and limited life expectancy will be assessed individually to make the best decision for them.
Further reading and references
Guideline for the Prevention, Detection, Evaluation, and Management of High Blood Pressure in Adults; Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines, 2017
Hypertension: management of hypertension in adults in primary care; NICE Clinical Guideline (August 2011)
Guidelines for the management of arterial hypertension; ESH/ESC Clinical Practice Guidelines, European Society of Cardiology (2013)
He FJ, Li J, Macgregor GA; Effect of longer term modest salt reduction on blood pressure: Cochrane systematic review and meta-analysis of randomised trials. BMJ. 2013 Apr 3346:f1325. doi: 10.1136/bmj.f1325.
Description of the DASH (Dietary Approaches to Stop Hypertension) Eating Plan; National Institutes of Health
Ettehad D, Emdin CA, Kiran A, et al; Blood pressure lowering for prevention of cardiovascular disease and death: a systematic review and meta-analysis. Lancet. 2016 Mar 5387(10022):957-67. doi: 10.1016/S0140-6736(15)01225-8. Epub 2015 Dec 24.
Alcohol Guidelines Review – Report from the Guidelines development group to the UK Chief Medical Officers; Department of Health, January 2016
2016 European Guidelines on cardiovascular disease prevention in clinical practice; European Society of Cardiology (2016)
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