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Researchers investigated the agreement between primary care and daytime ambulatory monitoring in blood pressure measurement. Study participants were patients with newly diagnosed high or borderline high blood pressure or those receiving treatment for hypertension but with poor control. In total, 179 patients were recruited from three general practices, and eight doctors were involved in measuring blood pressure. Daytime ambulatory monitoring was undertaken between 0700 and 2300 hours. 1
A significant correlation was found between the systolic blood pressure measured by the general practitioner and daytime ambulatory systolic pressure (r=0.46; P<0.05). The measurements made by the doctors exceeded those obtained by ambulatory monitoring by an average of 18.9 mm Hg. The Bland-Altman method was used to plot the difference in systolic blood pressure for each patient (GP measurement minus daytime ambulatory monitoring measurement) against the mean of the two measurements (fig 1 ). The limits of agreement are indicated by the red broken lines-that is, the interval of two standard deviations of the measurement differences either side of the mean difference.
Which of the following statements, if any, are true?
a) The significant correlation (r=0.46; P<0.05) between the systolic blood pressure measurements indicates good agreement between primary care and daytime ambulatory monitoring
b) About 95% of patients will have a difference in systolic blood pressure between the limits of agreement on the Bland-Altman plot
c) To derive the limits of agreement on the Bland-Altman plot, the differences in systolic blood pressure measurements were assumed to be normally distributed
d) The Bland-Altman plot indicates good agreement between GP and daytime ambulatory monitoring in the measurement of systolic blood pressure
Answers
Statement b is true, whereas a, c, and d are false.
The aim of the study was to assess the extent of agreement between primary care and daytime ambulatory monitoring in the measurement of blood pressure. Evidence was needed from primary care of the implications of using ambulatory monitoring, both in the initiation of treatment and the monitoring of hypertension control. Blood pressure recorded in primary care and by daytime ambulatory monitoring is unlikely to be exactly the same for all patients. If the differences were small, perhaps daytime ambulatory monitoring could replace measurement in...