Abstract
BackgroundAssociation between casual and 24-h urinary sodium-to-potassium (Na/K) ratio is well recognized, although it has not been validated in diverse demographic groups. Our aim was to assess utility across and within populations of casual urine to estimate 24-h urinary Na/K ratio using data from the INTERSALT Study.MethodsThe INTERSALT Study collected cross-sectional standardized data on casual urinary sodium and potassium and also on timed 24-h urinary sodium and potassium for 10 065 individuals from 52 population samples in 32 countries (1985–87). Pearson correlation coefficients and agreement were computed for Na/K ratio of casual urine against 24-h urinary Na/K ratio both at population and individual levels.ResultsPearson correlation coefficients relating means of 24-h urine and casual urine Na/K ratio were r = 0.96 and r = 0.69 in analyses across populations and individuals, respectively. Correlations of casual urine Na/creatinine and K/creatinine ratios with 24-h urinary Na and K excretion, respectively, were lower than correlation of casual and 24-h urinary Na/K ratio in analyses across populations and individuals. The bias estimate with the Bland–Altman method, defined as the difference between Na/K ratio of 24-h urine and casual urine, was approximately 0.4 across both populations and individuals. Spread around, the mean bias was higher for individuals than populations.ConclusionWith appropriate bias correction, casual urine Na/K ratio may be a useful, low-burden alternative method to 24-h urine for estimation of population urinary Na/K ratio. It may also be applicable for assessment of the urinary Na/K ratio of individuals, with use of repeated measurements to reduce measurement error and increase precision.
Highlights
Correlations of 24-h urinary Na/K ratio and Na/K ratio of casual urine across samples ranged from r 1⁄4 0.88 to 0.96 in subgroups categorized by sex, age and across Western/ Asian populations (Table 2, Figure 1 and Supplementary Figures 1–3, available as Supplementary data at IJE online)
Main findings from the INTERSALT Study data here are that population mean 24-h urinary Na/K ratio and casual urine Na/K ratio were highly correlated across the 52 population samples allowing first-order correction for systematic differences between casual urine Na/K ratio and 24-h values at a population level
For individuals (n1⁄4 10 065), bias was similar to the population-level bias, spread around the mean bias was larger, reflecting diurnal variability and ‘measurement error’ in estimation of the Na/K ratio based on a single casual urine specimen
Summary
High dietary sodium (Na) and low dietary potassium (K) intakes are associated with adverse blood pressure (BP) levels and excess risks of cardiovascular diseases (CVD).[1,2,3,4] The gold standard for estimating individual daily sodium intake is 24-h urine collection.[5,6,7] the amount of K excreted in 24-h urine is correlated with dietary K intake.[6,7,8] 24-h urine specimens are neither easy nor practical to collect for patients at clinics or individuals at home, especially if repeated samples are required to estimate individual intake.[9]The Na/K ratio in 24-h urine is cross-sectionally associated with BP in most epidemiological studies.[10,11,12,13,14,15,16,17] The Na/K ratio has been reported to be a superior metric to either Na or K alone in relation to BP10,11,18; studies report associations between Na/K ratio and CVD.[19]. Methods: The INTERSALT Study collected cross-sectional standardized data on casual urinary sodium and potassium and on timed 24-h urinary sodium and potassium for 10 065 individuals from 52 population samples in 32 countries (1985–87). Pearson correlation coefficients and agreement were computed for Na/K ratio of casual urine against 24-h urinary Na/K ratio both at population and individual levels. Results: Pearson correlation coefficients relating means of 24-h urine and casual urine Na/K ratio were r 1⁄4 0.96 and r 1⁄4 0.69 in analyses across populations and individuals, respectively. The bias estimate with the Bland–Altman method, defined as the difference between Na/K ratio of 24-h urine and casual urine, was approximately 0.4 across both populations and individuals. Conclusion: With appropriate bias correction, casual urine Na/K ratio may be a useful, low-burden alternative method to 24-h urine for estimation of population urinary Na/K
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