Relationship between apparent (single-pool) and true (double-pool) urea distribution volume

Background. The volume of urea distribution (V) is usually derived from sin gle-pool variable volume urea kinetics. A theoretical analysis has shown th at modeled single-pool V (Vsp) is overestimated when the urea reduction rat io (URR) is greater than 65 to 70% and is underestimated when the URR is le ss than 65%. The "true" volume derived from double-pool kinetics (Vdp) does not exhibit this effect. An equation has been derived to adjust Vsp to the expected Vdp. Methods. To validate these theoretical predictions, we examined data from t he Hemodialysis (HEMO) Study to assess the performance of Vdp as estimated from Vsp using the previously published prediction equation. For increased precision, both Vsp and Vdp were factored by anthropometric volume (Va). Pa tients were first dialyzed with a target equilibrated dialysis dose (eKt/V) of 1.45 during a baseline period and were then randomly assigned to eKt/V targets of either 1.05 (a URR of approximately 67%) or 1.45 (a URR of appro ximately 75%). A brood sample was obtained one hour after starting dialysis during one dialysis in each patient. Results. Vsp/Va was (mean +/- SD) 1.014 +/- 0.127 in 795 patients during th e baseline period when the URR was approximately 1.45. During the first mod eled dialysis after randomization, the Vsp/Va fell to 0.961 +/- 0.138 in th e group with an eKt/V target of 1.05, but did not change significantly unde r the high eKt/V goal. The correction of Vsp to Vdp using the prediction eq uation resulted in a Vdp/Va ratio of 0.96 to 0.98 in all three circumstance s without significant differences. When a blood sample was drawn one hour a fter starting dialysis, the apparent Vsp/Va ratio at one hour was much lowe r at 0.708 +/- 0.139. However, the mean Vdp/Va ratio, computed using the co rrection equation, was 0.968 +/- 0.322, which was similar to the Vdp/Va rat io calculated from the postdialysis blood urea nitrogen. Conclusions. These data suggest that the previously derived formula for adj usted Vsp is valid experimentally. The Vsp/Vdp correction should be useful for prescribing hemodialysis with either a very low Kt/V (for example, dail y and early incremental dialysis) or a very high Kt/V.