New salt-sensitivity metrics: variability-adjusted blood pressure change and the urinary sodium-to-creatinine ratio.
METHODS
We report the results of a 24-week, placebo-controlled, two-period, crossover trial of sodium supplementation in 112 normotensive African Americans, aged 25 to 64 years, with average blood pressure (BP) of 105/70 mm Hg. Estimated 24-hour urinary sodium excretion was 133.6 mmol; the average urinary sodium-to-creatinine ratio was 0.74.


METHODS
Variability-adjusted BP change was the difference in BP level after the respective treatment periods, divided by the intra-person standard deviation of the average BP obtained at 3 consecutive screening visits during a 4-week period.


RESULTS
The urinary sodium-to-creatinine ratio and the total urine sodium content were 37.8% and 26.5% higher, respectively, at the end of the sodium treatment period. Twenty-four hour ambulatory BP change (mm Hg) (95% CI) was systolic 1.2 (0, 2.4), and diastolic 0.7 (-0.3, 1.8); cuff BP change was systolic 0.9 (-1, 2.9), and diastolic 1.4 (0.1, 2.7). Variability-adjusted BP change was systolic 0.2 (-0.4, 0.8) and diastolic 0.4 (-0.1, 0.9). Though variability-adjusted and unadjusted SBP change correlated highly (r = 0.941, P<.001), only the former correlated with body mass index (r = 0.224, P<.05), a known correlate of salt sensitivity. While total urinary sodium content in timed urine collections and urinary sodium-to-creatinine ratio correlated (r = 0.727, P<.001), neither correlated with cuff BP changes. Change in urinary sodium-to-creatinine ratios of 3 consecutive pooled overnight 8-hour urine collections correlated with changes in 24-hour ambulatory SBP (r = 0.294, P<.001) and DBP (r = 0.193, P<.05); however, change in total urinary sodium content was uncorrelated. Total urinary sodium content of these pooled collections (P = .001), but not the urinary sodium-to-creatinine ratio, was positively related to urinary creatinine excretion per kilogram of body weight, the latter being an indicator of urine collection duration.


CONCLUSIONS
The lack of effect of the duration of urine collection on the urinary sodium-to-creatinine ratio is advantageous in individuals who may report inaccurately the duration of their urine collection. Sequential regression analyses demonstrated that the urinary sodium-to-creatinine ratio conveyed all of the changes in urinary sodium excretion information contained in aggregate urinary sodium excretion-and more. Variability-adjusted BP change was the more sensitive metric of BP response to dietary sodium manipulations, than unadjusted BP change. Thus, variability-adjusted BP change and the urinary sodium-to-creatinine ratio appear to be incrementally better metrics of salt sensitivity than those traditionally used.
Flack
John M
JM
Department of Internal Medicine, Wayne State University School of Medicine and the Detroit Medical Center, Michigan 48201, USA. jflack@intmed.wayne.edu
Grimm
Richard H
RH
Staffileno
Beth A
BA
Dnsc
Elmer
Patricia
P
Yunis
Carla
C
Hedquist
Lowell
L
Dudley
Amanda
A
D000328:Adult; D044383:Black People / Q000235:genetics; D001794:Blood Pressure / Q000502:physiology; D018660:Blood Pressure Monitoring, Ambulatory; D003404:Creatinine / Q000652:urine*; D018592:Cross-Over Studies; D004311:Double-Blind Method; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D006973:Hypertension / Q000175:diagnosis* / Q000208:ethnology; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D015999:Multivariate Analysis; D009318:Natriuresis / Q000502:physiology; D011336:Probability; D012016:Reference Values; D012680:Sensitivity and Specificity; D012964:Sodium / Q000378:metabolism / Q000652:urine*; D012982:Sodium, Dietary / Q000008:administration & dosage*; D001741:Black or African American
D012982:Sodium, Dietary; D012964:Sodium; D003404:Creatinine