Performance of equations for estimation of 24-hour urinary sodium from morning fasting urine samples in a multisite study in India

Background. Dietary salt intake is an important modifiable risk factor for cardiovascular diseases. Estimation of 24-hour salt intake using morning urine samples needs to be validated in the Indian context. We examined the performance of INTERSALT, Tanaka and Kawasaki equations for the estimation of 24-hour urinary sodium from morning fasting urine (MFU) samples. Methods. We enrolled 486 adults aged 18-69 years from four regions of India with equal rural/urban and sex representation to provide 24-hour urine samples. The next day, a MFU sample was obtained. Based on the volume and sodium content of the 24-hour urine sample, 24-hour sodium excretion (reference method) was calculated. Sodium levels in the MFU samples were measured along with other parameters required, and the above equations were used to estimate 24-hour urinary sodium levels. Intraclass correlation coefficient (ICC) was used to assess the degree of agreement between the estimates from the reference method and the three equations. Bland-Altman (BA) plots were used to identify systematic bias and limits of agreement. A difference of 1 g of salt (0.39 g of sodium) between the mean salt intake by 24-hour urine and as estimated by equations was considered acceptable. Results. A total of 346 participants provided both the samples. The mean (SD) daily salt intake estimated by the 24 -hour urine sample method was 9.9 (5.8) g. ICC was low for all the three equations: highest for Kawasaki (0.16; 95% CI 0.05-0.26) and least for Tanaka (0.12; 0.02-0.22). Only Tanaka equation provided estimates within 1 g of measured 24-hour salt intake (-0.36 g). BA plots showed that as the mean values increased, all the three equations provided lower estimates of salt intake. Conclusion. Tanaka equation provided acceptable values of 24-hour salt intake at the population level. However, poor performance of all the equations highlights the need to understand the reasons and develop better methods for the measurement of sodium intake at the population level.