Health: I was feeling fine until I saw you, doctor: A visit to the GP can raise your blood pressure and lead to unnecessary treatment with hypertension drugs. Rob Stepney looks at the 'white coat effect'

DOCTORS spend much time and energy trying to reduce patients' blood pressure, but the immediate effect of their presence is to increase it dramatically. This was first shown 10 years ago by an Italian study that continously monitored blood pressure in a group of 48 hospital patients. In all but one case, the doctor's arrival at the bedside led to a rapid and prolonged rise in both heart rate and blood pressure. When the doctor approached a second time, the same increase occurred.

This study took place in hospital. But the same phenomenon, termed the 'white coat effect', has now been widely documented in GPs' surgeries in Europe, the United States and most recently in Oxford. Many people whose blood pressure is high when it is taken by their doctor have normal pressures outside the surgery.

Current health promotion targets require GPs to screen apparently well people for 'hidden' diseases such as hypertension. Faced with the evidence of the unreliability of blood pressure measurements taken in the surgery, doctors are concerned that thousands of their patients are being wrongly diagnosed as hypertensive and given unnecessary drugs.

An additional drawback can be needlessly high health- or life-insurance premiums. Richard Green is a middle-aged man in good health. But five years ago a routine check showed that his blood pressure was mildly above normal. Repeat visits to his GP confirmed the finding. Mr Green's hypertension was not enough to warrant treatment, but it was sufficiently disturbing for him to spend pounds 60 on a DIY blood pressure monitor. Measured at home, his blood pressure was consistently normal.

A year later, he applied for health insurance. The doctor asked to screen him again found a raised blood pressure, which resulted in the insurance company imposing a pounds 70 yearly excess on his policy. The GP said he was fairly sure that Mr Green was suffering only from white-coat hypertension, but that there was nothing he could do to prove it.

But the advent of a small, portable device that monitors and records blood pressure every half hour while the person goes about normal life is likely to lead to a radical reappraisal of the way doctors diagnose hypertension. This Ambulatory Blood Pressure Monitoring was the subject of a workshop for GPs held in Oxford recently. 'Many patients are right in thinking that blood pressure levels taken in what for them are the unusual circumstances of a clinic are not the real readings,' reported Dr Andrew Coats, consultant cardiologist at the Royal Brompton National Heart and Lung Institute.

Dr David Ebbs, a GP in Didcot, has used ambulatory monitoring on 100 patients found to have raised blood pressure on repeated surgery visits. On the basis of blood pressures recorded by 24-hour monitoring, Dr Ebbs and his colleagues decided to defer treatment in more than a third of cases. 'These people are perhaps not entirely normal,' he told the Oxford meeting. 'They may be at increased risk (of stroke and heart disease) and should be carefully followed. But the ambulatory pressures were low enough to give us the confidence not to treat them at an early stage. And in many cases clinic pressures eventually fell towards the lower values recorded outside the surgery.'

Dr Paul Padfield, consultant physician at Western General Hospital, Edinburgh, said that, in his experience, having a large number of measurements throughout the day and averaging them 'gives you a better feel for what the actual pressures are'.

One GP with more than three years' experience of the technique is Dr Ken Burch, from Thame, Oxfordshire. 'Provided the machine is correctly maintained, it is useful in picking up 'false' hypertensives,' he says. A quarter to a third of his patients would be given unnecessary treatment on the basis of blood pressure readings in surgery. 'Quite apart from its effect on the drug budget, excluding these people by ambulatory monitoring is clearly of benefit to the individuals concerned.'

No drug treatment is without risk of physical side-effects; and there may be long-term psychological damage in labelling well people as sick. In avoiding unnecessary medication, doctors are clearly doing patients a service. They may also be helping in other ways.

Dr Eoin O'Brien, professor of cardiovascular medicine at Beaumont Hospital in Dublin, is chairman of the British Hypertension Society's working party on blood pressure measurement. 'Young people with hypertension can be penalised when it comes to jobs or pensions or insurance,' he says. He believes that, using ambulatory monitoring, doctors can often show that these people do not really have elevated blood pressure.

But not all doctors accept the idea that ambulatory blood pressure values should be used to decide when to prescribe drugs for high blood pressure. The problem is that all the studies that show blood pressure treatment reduces the risk of stroke are based on clinic measurements and no comparisons have been made. No one is certain that using ambulatory monitoring, rather than readings taken in the clinic as the criterion for starting drug therapy would lead to the same reduction in deaths and disability. One study from the United States indicates that ambulatory monitoring is a better predictor of eventual cardiovascular disease than measurements of blood pressure made in the surgery. But the study was small.

To end the uncertainty, clinical trials now under way will compare the results of starting treatment on the basis of ambulatory monitoring rather than the clinic blood pressure test. Independent of its implications for treatment, the ability to monitor blood pressure as people go about their normal lives provides a new opportunity to study the importance of factors such as job stress and coffee drinking. The monitor has been used to show that reducing caffeine intake does not lower blood pressure in people with mild hypertension. And a recent study from New York showed that women who perceived their jobs as stressful had higher blood pressure when they were at work than women who did not report stress. When the two groups of women returned home, their blood pressure levels were the same.