Imprecision of the hemodialysis dose when measured directly from urea removal

Background. The postdialysis blood urea nitrogen (BUN; C-t) is a pivotal pa rameter for assessing hemodialysis adequacy by conventional blood-side meth ods, but C-t is relatively unstable because of hemodialysis-induced disequi librium. The uncertainty associated with this method is potentially reduced or eliminated by measuring urea removed on the dialysate side, a more dire ct approach that can determine adequacy from the fraction of urea removed a nd by substituting an estimate of the equilibrated postdialysis BUN (C-eq) for C-t. 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 th e predialysis BUN (C-0), total urea nitrogen removed (A), and V from simple mass balance calculations (dialysate/volume method). However, a theoretica l error analysis showed that relatively small errors in A, C-0, or V are ma gnified when SRI or eKt/V is calculated using this method, especially at hi gher 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 enro lled in the pilot phase of the HEMO Study, "A" was measured using an instru ment that sampled dialysate frequently (Biostat(R)), and V was calculated f rom A, Co, and C-eq (median CV for V = 5.6%). The mean V was then applied t o the dialysate/volume method to estimate eKt/V and SRI during two to five subsequent dialyses per patient (comparison dialyses). The accuracy and pre cision of these estimates were assessed by comparing them with eKt/V and SR I derived from a direct measurement of C-eq drawn 30 minutes after dialysis (reference method), from mathematical curve-fitting of sequential dialysat e urea concentrations (dialysate curve-fit method), and from another blood- side method that estimates eKt/V from single pool Kt/V and the fractional r ate of solute removal (rate method): eKt/V = spKt/V - 0.6 K/V + 0.03. Results. During: 125 comparison dialyses, median absolute errors for calcul ated 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-f itting method, respectively. The corresponding correlation coefficients wer e 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 t he 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(R) resto red precision to the dialysate method to a level comparable to that of the blood-side methods. New techniques employing dialysate urea analysis should include a concentration profile to avoid these inherent methodological err ors and assure the accuracy of eKt/V and SRI.