To assess the validity of the estimation of 24-h urinary sodium (UNa) and potassium (UK) excretion obtained through 4 formulae based on occasional urine samples.Design and method:
We analyzed 2460 individuals (50.2% females) aged 18–96 years representatives of Portuguese population (PP). Tanaka, Kawasaki, INTERSALT and NHANES formulae were used to predict 24-h UNa and UK excretions from occasional urinary samples (OUrS) and we compared it with validated real 24-h urine samples (VUrS) (average 24 h UNa = 4073 mg/d). We also derive formulas specific to the PP using Tanaka and Kawasaki's formulae with adjusted parameters, obtained through the method of least squares. All formulas were then used on data collected from 24-h urine samples to compute predictions. We compared observed vs estimated measurements by examining bias (observed minus predicted UNa/UK), the intra-class correlation (ICC) coefficients between measurements, and Bland-Altman plots categorized by hypertension and normotension diagnosis.Results:
The degree of bias of estimations based on OUrS relative to VUrS measurements ranged from -31 mg/day of sodium in Tanaka, to 1969 mg/day of UNa in Kawasaki and from -259 mg/day to 736 mg/day of UK in Tanaka. The only formula for which bias was not significant was Tanaka's for UNa prediction. All ICC were lower than 0,470 for both UNa and UK excretion estimations. The Bland-Altman plots indicated a high dispersion of the estimates biases regardless of the formulae used and the diagnosis of normotension/hypertension. Most formulas’ estimations showed a positive relationship between bias and the magnitude of the average between observed and predicted measurements. All formulas lead to a wide range of overestimation and underestimation of both 24 h UNa and UK excretion even when applied to the 24-h real samples.Conclusions:
We found a poor agreement between estimated and observed measurements of UNa and UK in our large population exhibiting a high salt intake. That suggests that any of these formulas may incur in over- or under-estimations of UNa and UK excretion that may be unreliable for clinical evaluation of individual's daily UNa and UK+ excretion.