Background: Self monitoring of salt intake may improve adherence to recommended salt intake level. The gold standard of sodium estimation in urine is 24-hour urinary excretion. However, this method is cumbersome and inconvenient for the patient. Therefore, several studies compared the accuracy of spot and overnight sampling to 24-hour urine sodium excretion as the gold standard. In this systematic review we assessed whether spot and overnight urine sampling predicts 24-hour urine sodium excretion.
Methods: The electronic databases Pubmed and Embase were searched for papers in adults using the terms ‘spot urine’ or ‘overnight urine’ in the title and 'sodium' or ‘salt’ or ‘sodium chloride’ in the abstract.
Results: Our systematic search yielded 23 papers. Nine papers fulfilled the inclusion criteria, of a comparison between spot (n = 3) or overnight urine (n = 7) with 24 h collection. These papers considered 968 white, black, Japanese, and Chinese subjects older than 16 years (616 men and 352 women, 947 healthy subjects and 21 patients with renal diseases). Subjects were reported to have a controlled salt intake in only 2 studies. Sodium excretion in spot urine correlated poorly with 24-h urine measurement, between 0.12 and 0.47 (mean 0.35). Overnight urine showed a significant correlation with 24-hour urine, between 0.60 and 0.86 (mean 0.76).
Conclusion: Overnight urine collection but not spot urine has a significant correlation with 24 h collection. Therefore, overnight urine could be a clinically useful and simple method to estimate sodium intake.