Are two really better than one? Empirical examination of repeat blood pressure measurements and stroke risk in the Renfrew/Pitisley and collaborative studies

Background and Purpose-Blood pressure measured on 2 occasions in 2 large prospective cohort studies in Scotland was related to stroke, defined as stroke mortality or hospital admission for stroke. The purpose was to investigate whether 2 blood pressure readings gave a more accurate estimate of stroke risk over a long follow-up period than 1 reading. Methods-In the 1970s, the Renfrew/Paisley general population study investigated 3060 men and 3502 women and the Collaborative study investigated 2683 employed men on 2 occasions. The mean years between screening were 4 and 5, respectively. Blood pressure measured on the 2 occasions was related to stroke risk in 17-year and 21-year follow-up periods after the second screening, respectively. Results-For both systolic and diastolic blood pressure, the mean of the measures on the 2 occasions, the maximum of the 2 measures and the measure corrected for regression dilution was more strongly related to stroke over the follow-up periods than either single measure. Conclusions-Two blood pressure measurements seem better than 1 for indicating stroke risk. Underestimation using single measures will lead to both misclassification of the risk of disease for individuals and also the population-attributable risk of disease associated with elevated blood pressure.