All of the 5 devices used in the study are automatic oscillometric devices for BP measurements on the upper arm. BP7450 has the preformed cuff called the ComFit Cuff, covering arm circumference range of 22–42 cm (9–17 inches); and the BP5450 has wide range D-ring cuff covering arm circumference range of 22–42 cm (9–17 inches) and a small D-Ring cuff for adult arm circumferences of 17–22 cm (7–9 inches). UA-8000WM is has a D-Ring cuff that covers arm circumference range of 23–43 cm (9–17 inches). WGNBPA-240BT has an UltraSoft™ D-Ring cuff that covers arm circumference range of 23–43 cm (9–17 inches). BP3MW1-4YCVS has a SoftFit™ D-Ring cuff that covers arm circumference range of 22–42 cm (9–17 inches). Two units of study devices were provided by study sponsor with a written statement that BP7450 and BP5450 were obtained from the sponsors’ inventory at their warehouse, whereas UA-8000WM, WGNBPA-240BT, and BP3MW1-4YCVS monitors were purchased directly from their corresponding retail stores. These three devices were selected because they are major retail store branded blood pressure monitors with more functions and features than some other store branded monitors. Therefore, consumers might view such monitors as being of above average quality. One of each monitor was randomly selected out of the 2 units and used for the entire duration of the study.

Study participants who were 18 years and older with arm circumference of 17 cm to 43 cm were recruited from the general public using IRB approved research study flyers. Staff and patients of Northwestern University and Northwestern Medicine could also participate. We also recruited from patients identified through searches of the Northwestern Medicine Electronic Data Warehouse to identify potential participants with low body mass index who would be more likely to have a small arm circumference and to identify individuals with elevated blood pressures. We screened participants who met the age, arm circumference and BP readings criteria and consented them into the study. Participants who did not meet the study inclusion criteria were excluded from participating in the study.

The study was conducted by an experienced project manager (study supervisor) and 2 observers who were all trained and certified in the basic Shared Care Research and Education, Inc. Stateline Nevada, USA blood pressure measurement training program using the American Heart Association (AHA) guidelines, with additional training on how to conduct an Association for the Advancement Medical Instrumentation (AAMI) protocol for validations of automatic BP devices.

Northwestern University’s Institutional Review Board reviewed and approved the study. After obtaining written informed consent forms, participants’ arms were measured as a reference to select a corresponding cuff size for BP measurements. Participants rested for 5 minutes before baseline BP readings were taken with the reference sphygmomanometer. Two observers listened through a connected y-tube standard mercury sphygmomanometers that was calibrated before the study began. Using a dual head teaching stethoscope, the observers recorded simultaneous reference auscultatory BP measurements blinded to the other observer’s reading.

The independent readings were handed to a third study staff member, a study supervisor who determined whether the readings were within 4 mm Hg. When the two observers’ readings differed by more than 4 mm Hg, those readings were discarded in accordance with the ISO 81060-2:2018/AMD 1:2020 validation procedure. Both observers were blinded to each other’s readings and to the results of the device under test. The test device was also used to obtain an initial reading. After both baseline readings were recorded, four reference mercury sphygmomanometer measurements were taken alternately with three test device measurements with 60 second rests observed between readings. Additional readings were taken to replace the excluded readings in order to complete three valid sets of reference, test device, reference BP comparisons. Readings were excluded for pre-specified observer differences, blood pressure reading variation, participant body movement, talking or when one or both observers were unable to hear a reading. The subject was excluded if readings were repeated for a third time and the two observer measurements were not within 4 mmHg. Furthermore, subjects with BP variability in reference BP measurements >12/8 mm Hg between any two of the four reference BP measurements were excluded with the option to include from these subjects in the analysis two (instead of the three) consecutive valid pairs of reference measurements that fulfill the requirement provided this applied to no more than 10% of subjects.

We aimed to recruit participants who had an adequate distribution of SBP, DBP, sex, and arm circumference to satisfy the requirements of the ISO 81060-2:2018/AMD 1:2020 standard. In order to reduce the burden of recruiting participants with less common characteristics (e.g., very small arm circumference, higher diastolic blood pressure) in some cases we recruited more than the 85 required participants who had eligible measurements. We used a blinded backwards removal process based solely on the participants’ initial mercury sphygmomanometer readings, sex and arm circumference—whoever was most recently recruited was examined and if they were not needed to fulfill the distribution requirements of arm circumference, SBP, DBP or sex they were removed not to be in excess of 85 subjects.

We assessed means and standard deviations of the absolute values of the differences between the SBP and DBP measurements of the devices under test and the reference standard and compared these to Criterion 1 of the ISO 81060-2:2018/AMD 1:2020 standard, namely whether or not the means were ≤5.0 mm Hg and the standard deviation was ≤8.0 mm Hg. We made Bland-Altman plots of the differences in SBP and DBP determinations by blood pressure (means of test device and observer measurements) and arm circumference values. We calculated the standard deviations of the differences of the averaged paired determinations of the device under test and the reference measurement for each subject and compared these to Criterion 2 of the ISO 81060-2:2018/AMD 1:2020 standard.