LIMITATIONS OF THE DIFFERENCE BETWEEN CLINIC AND DAYTIME BLOOD-PRESSURE AS A SURROGATE MEASURE OF THE WHITE-COAT EFFECT

Background The difference between clinic and ambulatory average daytim e blood pressures is frequently taken as a surrogate measure of the `w hite-coat effect' (i. e. the pressor reaction triggered in the patient by the physician's visit). Objective To assess the reproducibility of this difference and its relationship with clinic and average ambulato ry daytime blood pressure levels. Design and methods These issues were addressed with two large groups of subjects in whom both clinic and a mbulatory blood pressures were measured, namely 783 outpatients with s ystolic and diastolic essential hypertension [Group 1, aged 50.8 +/- 9 .4 years (mean +/- SD)], participating in standardized Italian trials of antihypertensive drugs, and 506 elderly patients (group 2, age 71 /- 7 years) with isolated systolic hypertension, participating in the European Syst-Eur trial. Results The clinic-daytime blood pressure dif ference for the essential systolic and diastolic hypertensive patients (group 1) was 13.6 +/- 14.3 mmHg for systolic and 9.1 +/- 8.6 mmHg fo r diastolic blood pressure (P always < 0.01). This difference for the elderly patients with isolated systolic hypertension (group 2) was 21. 2 +/- 16.0 mmHg for systolic and only 1.3 +/- 10.2 mmHg for diastolic blood pressure (P < 0.01 and P < 0.05, respectively). In both studies little or no systematic clinic-daytime difference could be observed fo r heart rate. The reproducibility of the clinic-daytime blood pressure difference, tested for 108 essential systolic and diastolic hypertens ive patients from group 1 and 128 isolated systolic hypertensives from group 2, was invariably lower than that both of daytime and of clinic blood pressure values. Finally, the clinic-daytime blood pressure dif ference was progressively higher for increasing levels of clinic blood pressure and progressively lower for higher levels of ambulatory dayt ime blood pressure. Conclusions Thus, the clinic-daytime blood pressur e difference has a limited reproducibility; depends not only on clinic but also on daytime average blood pressure, which means that its size is a function of the blood pressure criteria employed for selection o f the patients in a trial; and is never associated with a systematic c linic-daytime difference in heart rate, which further questions its us e as a reliable surrogate measure of the true pressor response induced in the patient by the doctor's visit. (C) 1998 Rapid Science Ltd.