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.