EFFECT OF CHANGE IN SODIUM-EXCRETION ON CHANGE IN BLOOD-PRESSURE CORRECTED FOR MEASUREMENT ERROR - THE TRIALS OF HYPERTENSION PREVENTION, PHASE-I

Intraperson variability in both blood pressure (BP) and sodium excreti on dilutes associations and leads to underestimates of the dose-respon se relation, The authors applied statistical correction techniques to data from the Trials of Hypertension Prevention (TOHP), Phase I, carri ed out 1987-1990, Men and women with high normal diastolic BP (80-89 m mHg) were randomized to sodium reduction (n = 327) or a usual care com parison group (n = 417), Regression estimates of the effects of change in sodium and sodium/potassium ratio (Na/K ratio) on blood pressure c hange in the pooled sample were corrected for both the within-person v ariance of the excretion measures and the within-person covariance wit h blood pressure using a multivariate error correction. The estimated cross-sectional reliability was 0.36 for root Na and 0.42 for root Na/ K ratio and that for change was 0.31 and 0.28, respectively, Corrected coefficients suggested a decrease of 4.4 mmHg in systolic BP (95% con fidence interval (CI) 0.1-8.8) and 2.8 mmHg in diastolic BP (95% CI -0 .2 to 5.8) per 100 mmol/24 hour reduction in sodium, and of 3.4 mmHg i n systolic BP (95% CI 0.8-6.1) and 1.7 mmHg in diastolic BP (95% CI 0. 0-3.5) per unit decrease in Na/K. These results are comparable with th ose from the Intersalt Study, and suggest that the true effect of sodi um change on blood pressure change in normotensives over 18 months is underestimated by more than half in uncorrected data.