Introduction

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Hypertension is perhaps the most common reason for initiation of lifelong drug treatment and ongoing management by doctors. Ambulatory blood pressures may be a much better predictor of target organ damage and subsequent adverse events than measurements made in a clinic.¹ As these results were found in research studies and mostly not in typical primary care settings, however, patients may have had a higher "alerting response" than in everyday settings with their family doctor or nurse. It is thus important to clarify whether the white coat effect applies equally outside a research study and in typical family practice settings.

Why is ambulatory monitoring not commonly used to make management decisions? The problem is not just extrapolating results from research or secondary care to routine settings but that clinic derived thresholds have been used in previous research to make treatment decisions. However, several lines of evidence show that patients with daytime ambulatory pressure lower than 135/85 mm Hg have a low risk of subsequent events.² An ambulatory pressure of 135/85 mm Hg thus represents good control and approximately corresponds to a clinic pressure of 140/90 mm Hg,² a generally accepted marker for control.³ Recent guidelines recommended ambulatory monitoring for both initial diagnosis and assessing control,⁴ although few studies have looked at the assessment role in primary care.⁵

What about other alternatives? Preliminary evidence, mostly from other settings, indicates that measurements by a nurse or technician, repeated measurements, or home measurements may be closer to ambulatory pressure.^5-12 To our knowledge, no study in a typical primary care setting has compared these methods with ambulatory monitoring. Another alternative is self measurement by patients with equipment in the clinic, which to our knowledge has never been assessed.

We set out to assess the following in typical primary care settings. (1) The white coat effect in the broad group in which decisions are usually made on clinic readings (diagnosis and monitoring control). (2) The extent of the white coat effect as a research artefact. (3) The agreement between ambulatory monitoring of blood pressure and the realistic alternatives (measurement by doctor or nurse, self measurement in surgery, home). (4) The potential implications of using alternative methods of blood pressure measurement in predicting treatment thresholds.

	Methods

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Setting Eight doctors and three practice nurses from three varied practices, each serving 8000 patients, agreed to participate. Practice nurses were trained to use the 24 hour monitor and to follow the protocol of measurements.

	Results

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Sample
Of the 200 participants, 107 (54%) were women, 63/194 (33%) were over the age of 65, 96 had newly diagnosed high or borderline high blood pressure (systolic pressure 161 (SD 16) mm Hg; diastolic pressure 95 (9) mm Hg), and 104 were treated but had poor control (systolic pressure 163 (16) mm Hg; diastolic pressure 94 (9) mm Hg). Six patients did not have ambulatory readings; 173 patients had a complete set of all readings.

Estimating agreement: main results
Overall agreement for systolic pressure had a moderate rank correlation (table 1). Readings made by the doctor were much higher than systolic ambulatory pressure (difference=18.9 mm Hg, 95% confidence interval 16.1 to 21.7), as were recent clinic readings not made in a research study (19.9 mm Hg, 17.6 to 22.1). The white coat effect applied equally for patients on established treatment with poor control (readings by doctor v ambulatory pressure, difference=21.4 mm Hg, 17.3 to 25.4). For most methods the difference from ambulatory monitoring increased with blood pressure (a positive correlation on the Bland Altman plots, see table 1). Readings by the doctor and in the clinic also ranked systolic ambulatory pressure poorly compared with other methods (table 1). Most methods were sensitive in predicting high systolic ambulatory pressure (all sensitivity >75%), but readings by the doctor and recent clinic readings were not specific, with poor likelihood ratios (table 2). Nor were readings by the doctor or recent clinic readings specific in predicting ambulatory systolic treatment thresholds, having poor likelihood ratios (table 3). Measurement by the doctor and recent clinic readings performed slightly better for diastolic pressures than for systolic pressures, although other methods still performed better, with higher likelihood ratios for a positive test.

Attenuation of white coat effect with visits to nurse
The fall in blood pressure between the two visits to the nurse was even greater for the last 70 patients, in whom no intervening measurements occurred. For these 70 patients the difference between ambulatory systolic pressure and measurements taken by the nurse was 11 mm Hg for the first visit and 2.8 mm Hg for the second visit.

Patient documentation at home compared with semiautomated sphygmomanometer memory
Agreement existed for consecutive patients between the 14 readings documented by patients and the machine memory for both systolic pressure (n=21; r=0.97; mean difference 3.5 (SD 9.3) mm Hg) and diastolic pressure (r=0.85; mean difference 1.4 (6.0) mm Hg). The differences between readings recorded by the patient and the machine reflect the fact that "practice" readings or readings taken at other times of day (for example, during work) were also recorded by the machine.

	Discussion

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Limitations of the study
Routine equipment The agreement with ambulatory pressure was similar when routine equipment was used and when well validated equipment was used. Thus the main limitation of measurement in routine clinic settings is not the type of sphygmomanometer used.

The white coat effectartefact of setting, sample, or research studies?
The estimates of the white coat effect in this study are similar to those in previous work, mostly not from typical primary care settings.^6-12 Thus the white coat effect observed in the major prognostic studies to date is not likely to be an artefact of other settings or of research studies and applies equally to new diagnosis and assessment of control.

Although debate continues about just how benign white coat hypertension is,¹⁵ such studies have been criticised as some have not measured the white coat effect in a standard way.¹⁶ Prospective studies indicate that patients with white coat hypertension are at considerably reduced risk compared with those with higher ambulatory pressures and that treatment modifies blood pressure and outcomes only in patients with high ambulatory blood pressure. ^{2 5 15} The overzealous initiation and maintenance of treatment for white coat hypertension represents an enormous opportunity cost for health professionals and for patients, in addition to the associated iatrogenesisparticularly unnecessary anxiety¹⁷ and side effects. ^{18 19}

Are the alternatives to measurements by a doctor better?
Evidence exists from prospective studies about the relation between ambulatory blood pressure and outcome.¹ Nevertheless, ambulatory monitoring equipment is expensive (the machine used in this study cost £2000), and each practice would need several machines to cope with the workload.

This study shows that repeated readings by a nurse in primary care provide a better assessment than readings by doctors, supporting research from other settings.⁸ Patients can accurately measure and record their blood pressure themselves at home, ^{8 12 20} with great potential advantages of lower equipment and staff costs compared with ambulatory pressure. Self measurement by patients in the clinic may provide similar levels of overall agreement with ambulatory pressure to home measurement. More visits to the clinic are needed than for home readings, and suitable rooms have to be made available. The estimates from this study are also less precise and need confirmation (only 52 of the 70 patients invited completed both measurements).

Conclusion
The "white coat" effect is important in diagnosing and assessing control of hypertension in primary care and is not a research artefact. If ambulatory or home measurements are not available, repeated measurements by a nurse or the patient should result in considerably less unnecessary monitoring, initiation, and changing of treatment. It is time to stop using high blood pressure readings documented by general practitioners to make decisions about treatment.