Removing confounders from the relationship between mortality risk and systolic blood pressure at low and moderately increased systolic blood pressure.

OBJECTIVE: To assess the relationship between mortality risk and systolic blood pressure (SBP) at low and moderately increased SBP (less than the 70th percentile) before and after correcting for the regression-dilution bias and J-curve effects.
DESIGN: Cohort study.
SETTING: The First National Health and Nutrition Examination Survey Epidemiologic Follow-Up Study. PARTICIPANTS: The 6839 individuals who participated in the 1982-1984 survey for whom there were no missing data (age range 34-87 years). MAIN OUTCOME MEASURE: Cardiovascular disease mortality (n = 678) during a 9-year follow-up.
METHODS: Corrections were made for the regression-dilution bias by using average SBP during the decade before baseline as the mortality predictor, and for J-curve effects by excluding individuals who exhibited high age-stratified mortality rates and a decrease in SBP. Cox's regression was used to analyse the follow-up relationship between mortality risk and SBP.
RESULTS: The corrected relative cardiovascular disease mortality risk was 1.23 (95% confidence interval (CI), 1.16 to 1.31) for a 10 mmHg increase in SBP. The relationship was monotonically positive starting at the lowest SBP category in the analysis (< 115 mmHg), and robustly so above about the 32nd percentile (120 mmHg). The equivalent uncorrected result was 1.08 (95% CI, 1.05 to 1.13), and the relationship was J-shaped and became positive above about the 68th percentile (135 mmHg). Below the 70th percentile of SBP, individuals in the corrected analysis were 89% of all individuals.
CONCLUSION: These two corrections transformed the relationship between mortality risk and SBP at low and moderately increased SBP from no association to a robustly positive association starting at 120 mmHg, for the majority of individuals.