Having no symptoms, hypertension can only be diagnosed by measuring it. Two readings are taken: the pressure of the blood when pumping (systolic), and the resting pressure between each heartbeat (diastolic). The numbers refer to the height of a column of mercury that the pressure can sustain, measured in millimetres of mercury (mmHg). Although it’s a routine medical procedure, blood pressure (BP) measurement is not nearly as reliable as doctors often make out.

First, BP readings can vary widely and yet still be normal. BP will rise with physical exertion or stress of any kind, however minor. The time of day, room temperature, a full bladder, eating, drinking or smoking within the previous hour, standing, sitting or lying down can all influence BP measurements. Even having a conversation increases BP and, if the person being spoken to is of higher social status, BP may rise by as much as 50 per cent. This helps to explain what has been dubbed ‘white-coat hypertension’ - a rise in pressure caused by the very act of having BP measured by a doctor. Thus, BP readings taken by a doctor are inherently suspect. In fact, it has been estimated that as many as one in every five people diagnosed as hypertensive are receiving unnecessary drugs because of white-coat hypertension.

BP and stress expert Dr Paul Rosch, of New York Medical College, says things may often be worse. 'A not uncommon scenario is that, when the patient returns after the initial diagnosis of hypertension has been made and a medication has been prescribed, he or she is even more nervous, blood pressure is still high or higher and the dose is increased. This may be repeated on subsequent visits and/or additional drugs are ordered. The result may be dizziness or other side-effects that the patient attributes to a worsening of hypertension, causing even more stress.'

Another problem is with GPs themselves. One survey showed that many of them lack the proper BP equipment - particularly the appropriate measuring cuff, which goes around the arm. Too small a cuff will result in a falsely high BP reading; this is a common problem for overweight people with fat arms.

One alternative is to wear a portable BP kit, consisting of a battery-operated device worn round the waist, with a cuff attached to the upper arm. It is usually programmed to take readings every hour over a 24-hour period. All the patient has to do is note his activities during that day. This ‘ambulatory blood pressure monitoring’ is believed to provide the most accurate estimate of BP.

Another solution is to take your own BP readings. Sphygmomanometers (a blood-pressure cuff plus stethoscope) are widely available; the procedure is fairly simple and is as accurate as ambulatory monitoring (Blood Press Monit, 2000; 5: 175-80). There are also more high-tech devices that are worn around the wrist like a watch, or connected to the finger, although these have been found to overestimate BP readings.

However, the World Health Organization advises caution over relying on just a few measurements and recommends that BP monitoring take place over six months.