Should patients with higher blood pressure variability be excluded from validation studies? An assessment of the '12/8' rule.

To limit the inclusion of participants with increased blood pressure (BP) variability and presumably to avoid potential bias, the International Standards Organization BP device validation standard recommends exclusion of patients with a BP variability of more than 12/8 mmHg across reference readings. This '12/8 rule' is based on expert consensus and lacks empirical justification. In a post-hoc analysis of a study comparing two types of cuff designs carried out according to the International Standards Organization standard, we divided the study sample into patients who did not have (n=79) and patients who had (n=55) more than 12/8 mmHg variability. Patients with more than 12/8 mmHg variability were older and had a higher prevalence of diabetes (41.8 vs. 22.8%; P=0.02) and hypertension (43.6 vs. 29.1%; P=0.08). The mean systolic BP differences between the two cuff designs were not significantly different in participants who did not show more than 12/8 mmHg variability versus those who did (2.2±3.5 vs. 3.1±3.4; mean difference of differences -0.9±3.4; P=0.14). Similarly, the mean diastolic BP differences were not significantly different in participants who did not have more than 12/8 mmHg variability versus those who did (1.5±2.2 vs. 1.4±2.6; mean difference of differences 0.1±2.4; P=0.82). A limitation of our analysis is that the original study data focused on a comparison of different cuff designs and not formal validation of a specific device. Therefore, replication of these findings is warranted. Nevertheless, our findings do not support the use of the 12/8 rule and indicate that this rule may be promoting unnecessarily homogenous study samples, limiting external generalizability, and needlessly increasing workload and expense.