Evaluation of 24h Urine Volume as an Indicator of Uosm:Posm Ratio and Free Water Clearance in Pregnant Women

Abstract

Free water clearance (FWC), calculated from urine and blood measures, is often used to describe body water balance via the kidney's regulation of diuresis. The purpose of this investigation was to evaluate if 24h urine volume was a significant predictor of the urine-to-serum osmolality ratio (USR) and FWC in pregnant (PREG) and non-pregnant (CON) women. Methods: 24h urine volume (Uvol), 24h urine osmolality, and serum osmolality were measured in 18 PREG (age: 31±3y, height: 1.66±0.07m; pre-pregnancy weight: 68.08±18.92kg) women at the end of the first, second, and third trimesters, and in 18 CON (age: 29±4y; height: 1.64±0.08m; weight: 64.83±13.84kg) at similar time intervals for calculation of USR and FWC. Relationships between 24h Uvol and USR, as well as 24h Uvol and FWC, were evaluated with linear regression. Results: 24h Uvol was a significant predictor of USR (PREG: r2=0.61, p<0.0001; CON: r2=.50, p<0.0001) and FWC (PREG: r2=0.70, p<0.0001; CON: r2=.45, p<0.0001); relationships were similar between PREG and CON for 24h Uvol and USR (p=0.24) and for 24h Uvol and FWC (p=0.90). Conclusion The relationships between 24h Uvol and both USR and FWC are maintained throughout pregnancy. This suggests that 24h Uvol may provide adequate diagnostic accuracy for the evaluation of body water balance during pregnancy. Supported by Danone Research.