[True body water volume in patients on peritoneal dialysis]

Body water volume (BWV) equals urea distribution volume (UDV), crucial for accurate calculation of dialysis dosage from urea clearance in patients treated with peritoneal dialysis. BWV is precisely determined only by monitoring tritium--or deuterium--labeled water concentration in patient plasma. These are not routine methods, thus a number of alternative methods and anthropometric formulas have been used instead trying to determine BWV from patient body weight and body height, age and sex. These methods are relatively simple but not fully reliable. In the present study, BWV being mostly determined by use of Watson formula at the time of peritoneal dialysis and upon switching to hemodialysis or undergoing kidney transplantation, was monitored in peritoneal dialysis patients to demonstrate that it significantly exceeded the value obtained by the formula. Immediately before switching to hemodialysis or undergoing kidney transplantation, 39 patients (14 female, 25 male) without dialysate in the abdomen had a mean body weight of 74.60 +/- 12 kg and mean BWV of 37.90 +/- 5.80 L according to Watson formula. In the first month of switching to another dialysis method, all patients reduced their body weight by a mean of 3.35 +/- 2.55 kg. Now, their mean body weight was 71.25 +/- 11.45 kg and mean BWV 36.80 +/- 5.50 L. However, differences in the mean body weight and BWV did not reach statistical significance (t body weight = 1.25; t body water = 0.84; p>0.05). The body weight reduction during the period of observation could have almost certainly been ascribed to the accumulated fluid elimination. This in turn implies that immediately before switching to hemodialysis or undergoing kidney transplantation, peritoneal dialysis patients had a total body water greater by a mean of 3.35 +/- 2.55 L than the figure obtained by Watson formula. Thus, their BWV was not 37.90 +/- 5.80 L (50.80 +/- 7.75% of body weight) but 41.25 +/- 6.85 L (55.16 +/- 9.15% of body weight). According to t-test, then their true BWV statistically significantly (by 8.83%) exceeded the figure yielded by the formula (t=2.39; p<0.05), the Kt/V was falsely higher by approximately the same percentage, and the BWV reduction upon switching from peritoneal dialysis to another dialysis method or undergoing kidney transplantation was statistically significant (41.25 +/- 6.85 vs. 36.80 +/- 5.50 L; t=3.20; p<0.01). In conclusion, retrograde assessment of BWV in patients treated by peritoneal dialysis showed the BWV calculated by the most widely used anthropometric formula to be considerably underestimated and urea clearance overestimated for the same reason.