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Improving the “second generation Daugirdas equation” to estimate Kt/V on the once-weekly haemodialysis schedule



The haemodialysis (HD) dose, as expressed by Kt/V urea, is currently routinely estimated with the second generation Daugirdas (D2) equation (Daugirdas in J Am Soc Nephrol 4:1205–1213, 1993). This equation, initially devised for a thrice-weekly schedule, was modified to be used for all dialysis schedules (Daugirdas et al. in Nephrol Dial Transplant 28:2156–2160, 2013), by adopting a variable factor that adjusts for the urea generation (GFAC) over the preceding inter-dialysis interval (PIDI, days). This factor was set at 0.008 for the mid-week session of the standard thrice-weekly HD schedule. In theory, by setting PIDI = 7, one could get GFAC = 0.0025, to be used in patients on the once-weekly (1HD/wk) schedule, but actually this has never been tested. Moreover, GFAC was derived not taking into account the residual kidney urea clearance (Kru). Aim of the present study was to provide a specific value of GFAC for patients on  a once-weekly hemodialysis schedule.

Subjects and methods

The equation to predict GFAC (GFAC-1) in the 1HD/wk schedule was established in a group of 80 historical Italian patients (group 1) and validated in a group of 100 historical Spanish patients (group 2), by comparing the Kt/V computed using GFAC-1 (Kt/VGFAC-1) with the reference Kt/V (Kt/VSS) values, as computed with the web-based Solute-Solver software (SS) (Daugirdas et al. in Am J Kidney Dis 54:798–809, 2009). Three more sets of Kt/V (Kt/V0.008, Kt/V0.0025 and Kt/V0.0035) values were computed using the GFAC of the original D2 equation (0.008), the GFAC predicted by PIDI/7 (0.0025) and the mean observed GFAC-1 (0.0035), respectively. They were compared with the reference Kt/VSS values.


The predicting equation obtained from group 1 was: GFAC-1 = 0.0022 + 0.0105 × Kru/V (R2 = 0.93). Mean Kt/VSS in the group 2 was 1.54 ± 0.29 SD (N = 500 HD sessions). The mean percent differences for Kt/V0.008, Kt/V0.0025, Kt/VGFAC-1, and Kt/V0.0035 were 5.1 ± 1.0%, − 1.4 ± 0.7%, 0.0 ± 0.3%, − 0.3 ± 0.7%, respectively. No statistically significant difference was found between Kt/V values, except for Kt/V0.008.


A linear relationship was found between GFAC and Kru/V in patients on the 1HD/wk schedule. Such a relationship is able to improve the “second generation Daugirdas equation” for an accurate estimate of the single pool Kt/V in this setting. However, a simple replacement in the D2 equation of 0.008 with the mean observed GFAC (0.0035) could suffice in the clinical practice.

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Fig. 1
Fig. 2



Blood urea nitrogen, plasma concentration (mg/dL)


Post-dialysis body weight (kg)


Second generation Daugirdas equation


Urea generation rate (mg/min)


G-Factor, accounting for urea generation over the inter-dialysis interval prior to the studied HD session


GFAC obtained from the Eq. 3




Number of HD sessions per week: 1HD/wk = once-weekly HD schedule; 3HD/wk = thrice-weekly HD schedule


Residual kidney urea clearance (mL/min)

Kru/V ratio:

Ratio of Kru to V


Fractional urea clearance = Kd × t/V (dimensionless),

Kt/V0.0025 :

Single pool Kt/V computed using the D2 equation with GFAC = 0.0025

Kt/V0.0035 :

Single pool Kt/V computed using the D2 equation with GFAC = 0.0035

Kt/V0.008 :

Single pool Kt/V computed using the D2 equation with GFAC = 0.008

Kt/VGFAC-1 :

Single pool Kt/V computed the D2 equation with GFAC = GFAC-1, as predicted by Eq. 3


Residual kidney function


Solute Solver: web-based software (available at that allows single pool and double pool urea kinetic modelling


Duration of the dialysis session (in hours with the D2 equation)


Urea kinetic model


Post-dialysis urea distribution volume (L)


Variable-volume single pool


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Casino, F.G., Deira, J., Suárez, M.A. et al. Improving the “second generation Daugirdas equation” to estimate Kt/V on the once-weekly haemodialysis schedule. J Nephrol 34, 907–912 (2021).

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  • Incremental haemodialysis
  • Kidney urea clearance
  • Kt/V
  • Once-weekly haemodialysis
  • Urea kinetic modelling