Assessing dialysis adequacy and dietary intake in the individual hemodialysis patient

Background Urea kinetic modeling (UKM) and food records are widely used to assess the dialysis adequacy. Clinicians use these methods in individual patients to decide whether the dialysis prescription should be adjusted. We determined the variation in UKM parameters and dietary intake within individual patients in order to determine the required number of UKM measurements, and the number of food recording days to assess dialysis adequacy and dietary intake reliably. Methods. Session-to-session variation in urea reduction ratio (URR), Kt/V, urea distribution volume (UDVDDQ), and protein catabolic rate (PCR) was determined during three midweek dialysis sessions in 50 stable hemodialysis patients on three-times per week hemodialysis with a Kt/V of 0.98 +/- 0.13 (mean +/- SD). The dialysis prescription was kept constant. The day-to-day Variation in dietary protein intake (DPI) and dietary energy intake (DEI) was determined from seven-day food records. The 90th percentile value of the coefficient of variation (CV) was used to determine the number of measurements. Results. The variation in URR [CV, 2.4% (0.3 to 9.5) median (range)] and in Kt/V [CV, 4.0% (0.6 to 11.6)] was small in the majority of the patients. The variation in UDVDDQ [CV, 4.9% (0.3 to 25.7)] and PCR [CV, 9.3% (0.8 to 28.5)] was considerably larger. The variation in DPI [CV, 17.3% (8.4 to 64.0)] was larger than that in DEI [CV, 12.9% (5.0 to 33.0)]. To assess the URR within +/-10% of its true value, the average of two measurements was required. Reliable assessment of Kt/V required three measurements. URR and Kt/V could be assessed reliably from a single measurement in 86 and 66% of the patients, but we were not able to distinguish these patients beforehand. Reliable estimation of UDVDDQ required six measurements. The required number of measurements for PCR, DPI, and DEI was determined using a precision of +/- 20%. To assess PCR reliably, three measurements were needed. Estimation of DPI and DEI required seven and five food recording days, respectively. Conclusions. The session-to-session variation in URR and Kt/V is small in stable hemodialysis patients. Nevertheless, the averaged value of two to three measurements is required to assess the dose of dialysis reliably. Assessment of dietary intake requires at least three PCR measurements or food records for at least one week. Basing clinical decisions on a single dialysis adequacy assessment is an unjustified practice that should be abandoned.