Endgames

# Limits of agreement (Bland-Altman method)

BMJ 2013; 346 (Published 15 March 2013) Cite this as: BMJ 2013;346:f1630
1. Philip Sedgwick, reader in medical statistics and medical education
1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
1. p.sedgwick{at}sgul.ac.uk

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.

Fig 1 Bland-Altman plot of difference in systolic blood pressure (general practitioner measurement minus daytime ambulatory monitoring measurement) against the mean of the two measurements

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 …

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