Association of the spot urine sodium-to-potassium ratio with blood pressure is independent of urinary Na and K levels: The Nagahama study.

The sodium-to-potassium ratio (Na/K) of a urine sample is a simple index of salt loading. To practically use Na/K, we aimed to determine whether the Na/K value affects blood pressure (BP) at any age, irrespective of urinary Na and K levels. We analyzed a dataset of the general population (the Nagahama study), including baseline and second-visit measurements performed 5 years after the baseline. Spot urine samples were used for Na/K assessments. A total of 18,505 observations were analyzed using a linear mixed model, including the measurement term as a random effect. Urinary Na/K values showed a positive association with BP. When the highest quartile of Na/K was further divided by the urinary Na/creatinine (Cre) and K/Cre levels, the high-Na/Cre (3.58) and high-K/Cre (0.75) (Na/K = 4.80) groups, as well as the low Na/Cre (1.23) and low-K/Cre (0.26) (Na/K = 4.87) groups, exhibited similar effects on systolic BP (6.82 mmHg [95% CI: 5.72-7.92] and 6.63 mmHg [95% CI: 5.35-7.91], respectively). A similar association was observed in other Na/K quartiles. The positive association of Na/K and Na/Cre with BP was steeper in the older groups, while the inverse association of K/Cre was predominant in the younger population. The results of the multivariate analysis identified interaction terms between age and Na/K, Na/Cre and K/Cre as significant determinants for SBP. The positive association of urinary Na/K with BP was independent of the urinary Na and K levels. The association between Na/K and BP may not be uniform across ages by decade.