Imprecision of the hemodialysis dose when measured directly from urea removal

Background. The postdialysis blood urea nitrogen (BUN; C₁) is a pivotal parameter for assessing hemodialysis adequacy by conventional blood-side methods, but C(t) is relatively unstable because of hemodialysis-induced disequilibrium. The uncertainty associated with this method is potentially reduced or eliminated by measuring urea removed on the dialysate side, a more direct approach that can determine adequacy from the fraction of urea removed and by substituting an estimate of the equilibrated postdialysis BUN (C(eq)) for C₁. For a patient with a known urea volume (V), C(eq), the equilibrated Kt/V (eKt/V), and the solute removal index (SRI) can be calculated from the predialysis BUN (C₀), total urea nitrogen removed (A), and V from simple mass balance calculations (dialysate/volume method). However, a theoretical error analysis showed that relatively small errors in A, C₀, or V are magnified when SRI or eKt/V is calculated using this method, especially at higher eKt/V values (for example, if eKt/V = 1.4 per dialysis, a 7% dialysate collection error causes a 20% error in eKt/V). Methods. During three to four baseline dialyses in each of 39 patients enrolled in the pilot phase of the HEMO Study, 'A' was measured using an instrument that sampled dialysate frequently (Biostat®), and V was calculated from A, C₀, and C(eq) (median CV for V = 5.6%). The mean V was then applied to the dialysate/volume method to estimate eKt/V and SRI during two to five subsequent dialyses per patient (comparison dialyses). The accuracy and precision of these estimates were assessed by comparing them with eKt/V and SRI derived from a direct measurement of C(eq) drawn 30 minutes after dialysis (reference method), from mathematical curve-fitting of sequential dialysate urea concentrations (dialysate curve-fit method), and from another blood-side method that estimates eKt/V from single pool Kt/V and the fractional rate of solute removal (rate method): eKt/V = spKt/V - 0.6 · K/V + 0.03. Results. During 128 comparison dialyses, median absolute errors for calculated eKt/V compared with the reference method were 0.169, 0.061, and 0.071 for the dialysate/volume method, the rate method, and the dialysate curve-fitting method, respectively. The corresponding correlation coefficients were 0.47, 0.88, and 0.81. For SRI, median absolute errors were 0.044, 0.018, and 0.027, and the correlation coefficients were 0.54, 0.85, and 0.74 for the three methods. Conclusions. The precision of eKt/V and SRI measurements was significantly lower for the dialysate/volume method compared with the blood- side methods. Inclusion of the dialysate curve analysis provided by the Biostat® restored precision to the dialysate method to a level comparable to that of the bloodside methods. New techniques employing dialysate urea analysis should include a concentration profile to avoid these inherent methodological errors and assure the accuracy of eKt/V and SRI.