Monitoring population sodium intake using spot urine samples: validation in a New Zealand population

Although 24-h urine collection is widely considered the 'gold standard' for estimation of population sodium intake, spot urine collection is increasingly used as a convenient and affordable alternative. We used four published formulae to convert spot urine sodium into estimates of 24-h sodium excretion in order to establish which (if any) formula would be suitable for use in the New Zealand population. A convenience sample of 101 healthy volunteers provided two spot urine samples and a 24-h urine collection. Two formulae (one proposed by the Pan American Health Organisation (PAHO) and one derived from INTERSALT data) were most accurate, and provided estimates of key population indicators (mean, range and proportion above nutrient reference values) that were close to those of the measured 24-h urine excretion. Estimates using these formula were in closer agreement with 24-h excretion than the other formulae estimates using the Bland-Altman method. We conclude that spot urine sampling is a suitable alternative to 24-h urine collection in population surveys when spot urine sodium results are converted into estimates of 24-h sodium excretion using either the PAHO or INTERSALT formulae. However spot urine is a poor predictor of 24-h urinary sodium excretion for individual assessment.