Urea volume of distribution exceeds total body water in patients with acute renal failure

An accurate estimate of volume of distribution of urea (Vurea) is critically important to guide the prescription of therapy and the quantification of delivered dialysis dose in patients with chronic and acute renal failure (ARF). While Vurea has been shown to be substantially the same as total body water (TBW) in other patient populations, this relationship has not been adequately studied in detail in ARF patients.

To evaluate this question, we undertook a systematic study of these parameters in a cohort of 28 patients with ARF to analyze methods of estimating Vurea and TBW using blood-based kinetic data, anthropometric data and bioelectrical impedance analysis (BIA).

The results show that Vurea estimated by double-pool Kt/V (67.9 +/- 19.2 L) and by equilibrated Kt/V (61.2 +/- 13.6 L) were statistically significantly higher than Vurea determined by single-pool Kt/V (55.3 +/- 12.9 L; difference of 16% and 11%, respectively). Determination of TBW by anthropometric measurements (Watson, 42.5 +/- 7.0 L; Hume-Weyer, 43.6 +/- 7.1 L; Chertow, 46.8 +/- 8.1 L) yielded significantly lower measures compared to TBW determined by physiological formulae and by BIA (51.1 +/- 11.6 L and 51.1 +/- 13.3 L, respectively). Most importantly, all measures of Vurea by blood-based kinetics exceeded TBW measurements by any method (7% to 50% difference).

Our results suggest that in terms of useful guidelines to prescribe a specific dose of dialysis in patients with acute renal failure, estimates of TBW cannot be used as a surrogate for Vurea in determining dialysis adequacy.