High Blood Pressure: Tests and Diagnosis
How is blood pressure measured?
Blood pressure is measured by using an instrument called a sphygmomanometer (also known as a ‘sphyg’) or an automatic electronic monitor. A cuff (which needs to be the right size) is wrapped around the upper arm, inflated, and then slowly deflated.
If a sphyg is used, the person taking your blood pressure will listen for a pulse in your arm using a stethoscope. Readings are taken from a mercury (or air) scale depending on the type of sphyg.
In automatic or electronic monitors, the cuff automatically inflates and releases and an electronic reading is made.
What do the readings mean? Blood pressure is measured in millimetres of mercury (mmHg). Two measurements are taken, giving the systolic pressure (the higher reading) and the diastolic pressure (the lower reading).
The systolic reading indicates what pressure blood is under within the blood vessels as the heart contracts. The diastolic reading indicates the pressure as the heart relaxes. The two readings are written one above the other as a short hand way of reading the results (e.g. 130/80). This means the systolic pressure is 130mmHg and the diastolic pressure is 80mmHg or 130mmHg when the heart contracts and 80mmHg when the heart relaxes.
Does my blood pressure stay the same? Your blood pressure fluctuates throughout the day, depending on what you are doing. During physical work for example, the muscles need a greater supply of food and oxygen and the blood pressure rises. When you are asleep your blood pressure is at its lowest.
Getting an accurate reading: A one off high reading is not enough to make a diagnosis of high blood pressure. There are many other things that can affect your blood pressure reading (some people suffer from a ‘white coat effect’ where their blood pressure rises at the thought of having their blood pressure taken) so expect to have to have your blood pressure rechecked on at least two further occasions, usually about a month apart.
- Be sure to tell the person taking your blood pressure about any medication or drugs you are taking.
- Sit quietly for at least 3 minutes and try to relax.
- Your arm should be supported at the level of your heart.
- Make sure your bladder is empty.
- Try and avoid having a heavy meal immediately before.
Sometimes your doctor may require you to monitor your blood pressure at home over a period of time. This can be either by home monitoring or by 24 hour ambulatory monitoring. Both these methods tend to produce lower levels than in a clinic setting.
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Home monitoring: Home monitoring can reduce white coat effect and improve interpretation of readings by having multiple recordings possibly over many days. It also allows you to become involved in your own care and management of your blood pressure. Your practice may lend you a monitor if this is felt to be useful. The Blood Pressure Association can provide information about recommended equipment if you would like to purchase your own (www.bpassoc.org.uk).
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Ambulatory monitoring: Ambulatory monitoring involves using a monitor that automatically takes your blood pressure at intervals over a 24 hour period. Blood pressure is taken at 15 – 30 minute intervals during the day and 30 – 60 minute intervals at night. This can be more helpful than either home or clinic measurements as it provides a full 24 hour profile that can be used to work out average day and night time readings and so establish more accurately if blood pressure is elevated for long periods.
What is normal blood pressure? 
Up to 140/90mmHg is considered to be within the normal range. Levels above this may need treatment and any level equal to or above 160/100 would almost certainly require to be treated
What other tests should I have?
If you are found to have high blood pressure the doctor will examine you to look for any evidence of damage that may already have been done as well as possible causes of high blood pressure.
- Your urine will be checked for protein and blood. This will show whether your kidneys need to be examined more closely.
- Your blood will be taken: to check kidney function; check hormone levels; blood sugar and cholesterol levels.
- Your heart will be checked by listening for evidence of heart muscle damage and by having an ECG (electrocardiograph) which gives a tracing of the electrical activity of the heart.
- Your blood vessels at the back of the eye will be checked using an ophthalmoscope.
- Your circulation may be assessed by checking pulses in your wrists, legs and feet.
The information in this section has been adapted from the CHSS leaflet Living with High Blood Pressure.
Last updated: 22/11/2007