Spot urinary sodium to monitor relative changes in population salt intake during the UK salt reduction programme.

Objectives: The most accurate method to measure population salt intake is to collect the complete 24-h urinary sodium excretion (24-h UNa) but it is resource intensive and is rarely carried out frequently. We, therefore, assessed the use of spot urinary sodium concentration to monitor relative changes in population salt intake in between 24-h urine surveys.

Methods: We used 24-h (n = 2020) and spot urine (n = 21 711) samples drawn from adult participants in separate, cross-sectional, nationally representative surveys in England, repeated between 2006 and 2014.

Results: As population average 24-h UNa fell from 2006 to 2014 (from 8.7 to 7.6 g/day, i.e. by 12%) with the ongoing salt reduction programme, spot sodium concentration fell by a similar extent (from 106.1 to 93.1 mmol/l, i.e. by 13%). The regression slopes of 24-h UNa and spot sodium concentration ran parallel (P value = 0.1009) in a linear regression modelling the difference in their year-on-year changes [by regressing the 24-h UNa or sodium concentration values on time, estimation method (24-h versus spot), and their interaction term]. In contrast, when 24-h UNa was estimated by applying the Kawasaki, Tanaka, or INTERSALT formulas to spot sodium concentrations, almost no change was detected from 2006 to 2014 (±1%) and their regression slopes were significantly different from that of the measured 24-h UNa (all P values <0.0001).

Conclusion: Spot urinary sodium concentration drawn from random and representative samples of the population accurately reflected relative changes in population average 24-h UNa, and can therefore, be used in between 24-h urine surveys to monitor population salt reduction programmes. Formulas commonly used to estimate 24-h UNa were unsuitable to do so.