How Many Measurements Are Needed to Estimate Blood Pressure Variability Without Loss of Prognostic Information

Average real variability (ARV) is a recently proposed index to represent short-term, reading-to-reading, within-subject variability in blood pressure (BP).1 ARV attempts to correct for the limitations of the commonly used standard deviation (SD), which accounts only for the dispersion of values around the mean, and not for the order of the BP readings.1–3 Several recent studies reported on the association between cardiovascular outcome and BP variability as assessed by 24-hour ambulatory BP monitoring.1,2,4–6 Most of these studies included a small (n 70%) of valid BP readings obtained at intervals ranging from 151,5 to 304,6 minutes during daytime and from 301,5 to 604,6 minutes during nighttime. Participants with fewer than 32,6 57,1 or 595 BP readings during the 24-hour monitoring period were excluded from these studies. The study with the largest number of participants (n = 8,938), taken from the International Database on Ambulatory Blood Pressure Monitoring in Relation to Cardiovascular Outcomes (IDACO),7 excluded subjects that had less than 10 daytime or 5 nighttime BP readings or missing BP measurements during 3 consecutive hours.2 However, none of the aforementioned studies considered the minimum number of BP readings required to estimate BP variability in an accurate manner. The aim of our study was to use a dataset of 1,254 IDACO subjects with at least 80 ambulatory BP readings to determine an adequate number of BP readings needed to calculate ARV. We took advantage of the prospective design of IDACO to determine such number based on outcome data. The results were then validated using a test dataset with a larger number of subjects.