Comparison of a modified urea kinetic model direct dialysis quantification and classic urea kinetic modeling

In the classic urea kinetic model (UkM), the measurement of clearance (K) as a model input is onerous, and its result is higher than actual body clearance because of urea disequilibrium. Urea kinetics performed by direct dialysis quantification (DDQ), with an equilibrated post rebound sample, gives reliable results but is routinely impractical. The modified UKM (mUKM) method is based on the input of urea distribution volume (V), easily obtainable by several methods and more stable than clearance, and uses simple formulae derived from urea mass balance, avoiding iterative computation. Compared with DDQ, mUKM results are identical, provided that the V value obtained by DDQ is used as input. Compared with UKM, mUKM urea clearances are significantly lower (158.1 +/- 20.5 versus 180.7 +/- 26.8 ml/min), as are urea nitrogen generation rates (5.85 +/- 2.0 versus 6.41 +/- 2.14 mg/min), whereas V results are close (32,444 +/- 6301 versus 32,141 +/- 5293 ml): consequently, mUKM Kt/V is significantly lower than UKM Kt/V (1.07 +/- 0.17 versus 1.22 +/- 0.18). This is because both mUKM and DDQ take account of post dialysis urea rebound. Therefore, if UKM formulae are used with the DDQ body clearance and the equilibrated post rebound blood urea nitrogen value as input data, their results strictly approximate the reference DDQ (V = 32,457 +/- 6274; Kt/V = 1.07 +/- 0.17; G = 5.85 +/- 2.0), demonstrating the common mathematical basis of DDQ and UKM. Therefore, reliable results may be obtained by DDQ, by mUKM, and by "equilibrated" UKM; we suggest that the mUKM model, less cumbersome than DDQ, and more reliable than UKM because it avoids the clearance measurement and the need for iterative computation, could be useful for routine kinetics.