Circadian and seasonal changes have a substantial impact on measurement of elevated blood pressure (BP) in practice

Abstract

Blood pressure (BP) is known to vary by time of day and day of year. It is not known whether variation is clinically meaningful. We used data from 57 Geisinger Health System community practice clinics extracted from electronic health records (EHR) over a 3 to 7 year period to evaluate the influence of temporal factors on measured BP. Relevant data on over two million patient encounters were extracted from the EHR of Geisinger's integrated health system. For processing, analysis was limited to a random sample of 203,927 BP measurements from 24,904 patients. General estimating equation (GEE) logistic regression was used to determine the effect of time of day and month of year on the probability of identifying BP values at or above four different BP levels (i.e., systolic BP>120 and >140mmHg, and diastolic BP >80 and >90mmHg). As results were similar for each criteria for elevated BP, we limit the presentation to systolic BP>140. Time of day was significantly associated with variation in the odds of measuring elevated BP, regardless of definition. The lowest odds occurred in the morning and at midday and the highest odds occurred at the end of the day. The odds ratio for the evening (7:00PM) versus midday (1:00PM) was 1.26 (p<0.001). Month of the year was also significantly associated with the odds of an elevated measurement. The lowest odds occurred during the summer months and the highest odds during the winter months. The odds ratio comparing summer and winter months was 1.24 (p<0.001). These results were not confounded by age, gender, race, history of hypertension, or treatment status. In clinical practice, measurement of an elevated BP level varies by 40% depending on the time of day and month of year. For example, the probability of a systolic BP>140 for a measurement taken in July at 1:00PM was 21.1% compared to 29.6% if the BP was taken in January at 7:00PM, a difference of 40%. The magnitude of the variability in BP measurement attributable to the combined effect of these temporal factors is clinically significant. Anticipation of changes in BP attributable to temporal factors may improve accuracy of diagnosis and precision of therapy.