Does the number and timing of visits, or the measurement device, affect the calculated visit-to-visit variability of blood pressure?
The number and timing of visits, as well as the device used to measure blood pressure, significantly influence visit-to-visit variability and must be standardized or accounted for in study designs.
Visit-to-visit variability (VVV) of blood pressure is associated with cardiovascular disease. The authors examined the effects of visit number and timing and automated or manual measurement device on VVV in the placebo arm of the Trial of Preventing Hypertension (TROPHY) (N=225) and simulations. VVV was assessed using intra-individual standard deviation (SD), range, maximum, coefficient of variation, successive variation, and average real variability of systolic blood pressure. VVV increased with number of visits used to calculate it in the TROPHY population (P for trend <.05 for all metrics) and simulations. Using consecutive visits in TROPHY, average SD was 5.6 mm Hg from 3 visits, 6.8 mm Hg from 7 visits, and 7.7 mm Hg from 18 visits. When 7 visits were spread out across 4 years, the average SD was higher (7.5 mm Hg) than when visits were consecutive over 18 months (P<.001). SD was higher using a single blood pressure measurement per visit (compared with the mean of 3 measurements per visit P<.001) and with automated vs manual devices (P<.001). In summary, number and timing of visits and device used to measure blood pressure influence VVV and need to be considered when designing, interpreting, and comparing studies.
Levitan et al. (2012) studied this question.