Does casual urine Na/K ratio predict 24 h urine Na/K ratio in treated hypertensive patients? Comparison between casual urine voided in the morning vs. 24 h urine collected on the previous day

The urine sodium-to-potassium (Na/K) ratio is associated with blood pressure and cardiovascular diseases. A single urine sample is preferable for determining the Na/K ratio in clinical practice. We evaluated whether the Na/K ratio measured using morning casual urine samples predicts the ratio measured using the preceding 24 h urine sample in patients with hypertension. The study included 187 hypertensive patients (mean age 66.1 years, 52.4% female) whose Na and K concentrations were measured both in 24 h (24Na/K) and casual urine the next morning (CNa/K). The Na/K ratios were 3.54 ± 1.5 in 24NaK and 2.63 ± 1.9 in CNa/K. The two estimates showed a significant positive correlation (r = 0.49, p < 0.0001), and (CNa/K-24Na/K)/24Na/K was −23.5 ± 44.4%. In the Bland–Altman plot, the mean difference was −0.91. When CNa/K was divided into three groups, <2 (low), 2–4 (medium), and ≥4 (high), the overall agreement with 24Na/K was 46.0% (86 of 187). The low group had 24.4% agreement and 75.6% underestimation (24Na/K > CNa/K); the medium group had 60.8% agreement, 30.5% underestimation, and 8.7% overestimation (24Na/K < CNa/K); and the high group had 71.8% agreement and 28.2% overestimation. These results indicate that CNa/K and 24Na/K were significantly correlated; however, CNa/K was generally lower than 24Na/K, particularly at Na/K levels < 2. Further efforts should be made to address the validity of using casual urine Na/K ratios in hypertension management practices.