Estimation of 24-h urinary sodium excretion using lean body mass and overnight urine collected by a pipe-sampling method.

Unlabelled: BACKGROUND Although salt intake can be estimated from 24-h urinary sodium excretion (Na24 ), for a long time there has been no precise and easy method for its measurement.

Objective: To investigate the possibility of estimating Na24 using pipe-sampling of overnight urine and lean body mass.

Participants And Methods: Body height, body weight and body fat determined by bioelectrical impedance (lean body mass = body weight body fat) was measured in 351 healthy individuals (126 men, 225 women). Twenty-four-hour urine was collected and creatinine and sodium were measured. To predict 24-h urinary creatinine excretion (Cr24 ), the relationship between Cr24 and lean body mass was investigated. Both 24-h urine and overnight urine specimens were collected in 149 individuals (71 men, 78 women) using a sampling pipe (semi-automatic proportional urine sampling device; height 16 cm, width 1.5 cm). Multiple regression analysis was used to estimate Na24.

Results: The prediction of Cr24 (Pr.UCr24 ) was derived from lean body mass. Using Pr.UCr24 and the overnight urinary Na/Cr excretion ratio (Na n /Cr n ), Na24 was estimated as 0.634 (Na n /Cr n ) Pr.UCr 24 + 104.7 mmol/day for men and 0.682 (Na n /Cr n ) Pr.UCr 24 + 62.6 mmol/day for women. The correlation coefficient (r) between true Na24 and Na24 estimated by these formulae was r = 0.78 ( P<0.001; mean difference SD-0.03 39.0 mmol/day).

Conclusions: A new pipe-sampling method using overnight urine and lean body mass was easy and reliable for the estimation of Na 24. Furthermore, this method is convenient and may enable counselling on salt intake.