Dialysis adequacy indices that are applied for the evaluation of the efficiency of urea removal include fractional water volume cleared from urea during dialysis (KT T/V), fractional solute removal (FSR), and equivalent urea clearance (EKR). Using a constant-volume, one-compartment urea kinetic model for an anuric patient, the FSR and EKR are shown to depend on only three nondimensional parameters: (i) KT/V, where K is the dialyzer clearance for hemodialysis (HD) or peritoneal mass transport coefficient for peritoneal dialysis (PD), T is the time period of dialysis, and V is urea distribution volume; (ii) T/Tc, where Tc is the length of treatment cycle; and (iii) VD/V, where VD is the volume of dialysis fluid applied. In particular, analytical formulas for FSR and EKR, valid for HD as well as for PD, were derived as functions of these three parameters. Numerical simulations, performed using a two-compartment urea kinetic model, showed that the analytical formulas are valid also for the two-compartment model, except for short, highly effective HD, where the overestimation of FSR and EKR using the analytical formulas is however, not higher than 20 and 16%, respectively. KT T/V is equal to KT/V for HD and FSR for PD. Thus, our formulas provide an integrative description of the relationships between dialysis efficiency indices and operational dialysis parameters that is valid for all modalities and schedules of dialysis. They may be applied not only for standard HD and continuous ambulatory PD, but also for HD with circulating dialysis fluid or intermittent forms of PD.