Potassium is not a good solute to estimate residual volume in CAPD.

Residual volume is defined as the volume of dialysate remaining in the peritoneal cavity after complete drainage of fluid. In this study the residual volume was estimated in 9 patients, age 42 +/- 9 years, on continuous ambulatory peritoneal dialysis (CAPD) for 14 +/- 12 months, using two different solutes: dextran 70 and potassium. The residual volume was calculated from the change induced in drain dialysate solute concentration by the known volume and solute concentration of the newly infused dialysis solution. The residual volume estimated with dextran 70 was 195 +/- 105 mL (75-375 mL) and with potassium 411 +/- 108 mL (245-596 mL) (K > D, p < 0.001). The correlation between these two measurements was close to significance (r = 0.654; p = 0.056). In conclusion, potassium overestimates the residual volume, probably because of its rapid diffusion during the infusion time.