Limits of agreement (Bland-Altman method)

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

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 …