The potential of nighttime urine samples for epidemiologic research on sodium intake: Evidence from a...

The gold standard for measuring sodium intake is based on multiple 24-h urine sodium (UNa) collections, which are logistically complex and pose a high burden on study participants. Its major alternative, spot urine sampling, has been shown to lead to systematic bias in sodium intake estimation. Nighttime urine collections are a potential alternative, as they place a substantially lower burden on participants and are thus less likely to lead to underestimation of UNa compared with 24-h urinary collections.

We examined the possibility of estimating 24-h UNa excretion using nighttime collections only, both at the individual and population level.

We used the data from the Swiss kidney project on genes in the hypertension cohort, containing 1757 24-h urine collections from 1090 individuals, divided into daytime and nighttime samples. The participants were adults of European ancestry living in Switzerland. We examined the ability to predict 24-h UNa excretion based on nighttime collections using the following 3 methods for sodium estimation: 1) normalization by collection duration, 2) normalization by mean urine volume, and 3) multivariable linear regression models.

The Pearson correlation coefficient for nighttime collections compared with the 24-h measurements were 0.661, 0.651, and 0.682-0.749 (depending on the regression model specifications) using the 3 nighttime collection methods of sodium estimation, respectively. All of the estimation methods led to different probability distributions from the target distribution. Method 1 showed systematic negative bias, method 1 and method 2 showed bias with respect to diabetes and hypertension status, and method 3 showed an overestimation of low excretion and an underestimation of high excretion.

Estimating 24-h UNa excretion based on nighttime collections was found to be imprecise and/or biased at the individual level and failed to capture population-level characteristics beyond central tendency.