Comparison of three spot urine formulae and their validation using 24-hour urine sodium for estimation of daily salt intake: a cross-sectional study among Bangladeshi adults

OBJECTIVE: This study aimed to assess the validity of three commonly used (Tanaka, Kawasaki, INTERSALT) methods based on spot urinary sodium excretion against the 24-hour urinary sodium excretion to estimate the dietary salt intake in Bangladesh. DESIGN: A population-based cross-sectional survey. SETTING: A cross-sectional survey was done in an urban and a rural area of Bangladesh in 2012–2013. PARTICIPANTS: 418 community living residents aged 40–59 years participated in the survey and data of 227 subjects who had complete information were analysed for this validation study. OUTCOME MEASURES: The Bland-Altman method was used to evaluate the agreement between the estimated and measured 24-hour urinary sodium. The estimated average salt intake from Tanaka, Kawasaki and INTERSALT methods were plotted against 24-hour urinary sodium excretion. RESULTS: The mean 24-hour estimated salt intake was 10.0 g/day (95% CI 9.3 to 10.6). The mean estimated urinary salt by Tanaka, Kawasaki and INTERSALT methods were 8.5 g/day (95% CI 8.2 to 8.8), 11.4 g/day (95% CI 10.8 to 12.0) and 8.8 g/day (95% CI 8.6 to 9.0), respectively. Compared with the estimated mean salt intake from 24-hour urine collection, the Bland-Altman plot indicated the mean salt intake was overestimated by the Kawasaki method and underestimated by Tanaka and INTERSALT methods. The linear regression line showed the Kawasaki method was the least biased and had the highest intraclass correlation coefficient (0.57, 95% CI 0.45 to 0.67). CONCLUSION: Tanaka, Kawasaki and INTERSALT methods were not appropriate for the estimation of 24-hour urinary sodium excretion from spot urine samples to assess dietary salt intake in Bangladesh. Among the three methods, the Kawasaki method has the highest agreement with the 24-hour urinary sodium excretion concentration in this population.