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Formula to detect high sodium excretion from spot urine in chronic kidney disease patients.

INTRODUCTION
Excessive sodium intake is related to adverse renal and cardiovascular outcomes in patients with chronic kidney disease (CKD) and assessment of sodium intake is complex and not evaluated very often in clinical practice.


OBJECTIVE
To develop a new formula to estimate 24h sodium excretion from urine sample (second void) of patients with CKD.


METHODS
We included 51 participants with CKD who provided 24-hour urine collection and a sample of the second urine of the day to determine the sodium excretion. A formula to estimate the 24-hour sodium excretion was developed from a multivariate regression equation coefficients. The accuracy of the formula was tested by calculating the P30 (proportion of estimates within 30% of measured sodium exection) and the ability of the formula to discriminate sodium intake higher than 3.6 g/day was evaluated by ROC curve.


RESULTS
Correlation test between measured and estimated sodium was significant (r = 0.57; p < 0.001), but P30 test identified a low accuracy (61%) of the formula. Different cutoff points were tested by performance tests and a ROC curve was generated with the cutoff that showed better performance (3.6 g/day). An area under the curve of 0.69 with a sensitivity of 0.91 and specificity of 0.53 was obtained.


CONCLUSION
A simple formula with high sensitivity in detecting patients with sodium consumption higher than 3.6 g/day from isolated urine sample was developed. Studies with a higher number of participants and with different populations are necessary to test formula´s validity.