New formulas for calculating dialysis dose in incremental haemodialysis

Interest in incremental haemodialysis (HD) is growing. The urea-based adequacy criterion of incremental HD is founded on two equivalent continuous clearances (ECC), i.e. the standard KtV (stdKt/V), which suggests a constant target (2.3 volumes/week), and the equivalent renal urea clearance (EKRU, mL/min), which suggests a variable target as a function of residual kidney urea clearance (KRU): EKRUN = 12 - KRUN, where EKRUN and KRUN are the normalized values of EKRU and KRU, expressed in units of mL/min per 35 L of urea distribution volume (V). The aim of the study was to develop new formulas that provide the dose of dialysis (eKt/V) to be prescribed to achieve the targets of the two ECCs in the incremental HD setting and validate them in a historical cohort of 202 consecutive incident HD patients.

Firstly, two formulas to calculate the two ECCs (either EKRUN or stdKt/V) were obtained from computer simulation of the dialysis sessions by means of the regression analysis: ECC = a + b × KRUN + c × eKt/V + d × WFLN, where WFLN is the normalized weekly fluid loss, and a, b, c and d are the coefficients of the two formulas that predict EKRUN and stdKt/V. The coefficients change according to the treatment schedule and the ECC under evaluation. Then, the two formulas were solved to calculate the eKt/V to be prescribed to get the stdKt/V and EKRUN targets: eKt/V = (ECC target - a - b × KRUN - d × WFLN/c.

The stdKtV and EKRUN values estimated with the new formulas were compared with the paired ones obtained using the Solute-solver software, recommended by the Kidney Disease Outcomes Quality Initiative (KDOQI) clinical practice guidelines for HD adequacy. There was an excellent correlation between the paired values, with the regression line virtually coincident with the identity line with a squared Pearson coefficient (R2) >0.97.

The new formulas can facilitate the implementation of incremental HD. However, it must be acknowledged that the assessment of HD adequacy cannot be based only on a single depurative index. The dialysis prescription must also take into account the clinical and metabolic status of the patient.