During continuous renal replacement therapy (CRRT), a high net ultrafiltration rate (NUF) may worsen the decrease in urine output (UO) associated with starting CRRT. However, fluid balance (FB) may modulate this association. We aimed to examine the relationship between NUF, UO and FB at the start of CRRT.

A retrospective cohort study of 1030 CRRT-treated patients admitted to two tertiary ICUs.

Median age was 60 years (IQR, 48–70), median APACHE III was 94 (IQR, 76–114) and median NUF rate was 0.7 mL/kg/h. In the 24 h after CRRT started, the mean hourly UO decreased from 25.5 mL to 11.9 mL (P < 0.001). Moreover, after adjusting for multiple confounders on multivariable analysis, a higher NUF was not significantly associated with a lower UO (−1.5 mL/kg for every 1 mL/kg/h increase in NUF; 95% CI -3.1 to 0.04; p = 0.064). In addition, pre-CRRT FB did not modulate the above relationship between higher NUF and lower UO.

A higher NUF rate was not significantly associated with a greater immediate and sustained reduction in UO after CRRT commencement. FB before CRRT was also not associated with a greater reduction in UO. These findings do not provide evidence for an effect of NUF on renal function.