Introduction: Continuous renal replacement therapy (CRRT) is common in the intensive care unit (ICU) but a high net ultrafiltration rate (UFNET) calculated with daily data may increase mortality. We aimed to study early UFNET practice using minute-by-minute CRRT machine recordings and to assess its association with admission diagnosis and mortality. Methods: We studied CRRT treatments in three adult ICUs over 7 years. We calculated early UFNET rates minute-by-minute and categorized UFNET into tertiles of mean UFNET in the first 72 h and admission diagnosis. We applied Cox-proportional hazards modelling with censoring of patients who died within 72 h. Results: We studied 1,218 patients, 154,712 h, and 9,282,729 min of CRRT (5,702 circuits). Mean early UFNET was 1.52 (1.46–1.57) mL/kg/h. Early UFNET tertiles were similar to, but somewhat higher than, previously reported values at 0.00–1.20 mL/kg/h, 1.21–1.93 mL/kg/h, and >1.93 mL/kg/h. UFNET values were similar whether evaluated at 24 or 72 h or for the entire duration of CRRT. There was, however, significant variation in UFNET practice by admission diagnosis: higher in respiratory diseases (pneumonia p = 0.01, other p < 0.0001) and cardiovascular disease (p = 0.005) but lower in cardiothoracic surgery (p = 0.04), renal (p = 0.0003) and toxicology-associated diagnoses (p = 0.01). Higher UFNET was associated with an increased hazard of death, HR 1.24 (1.13–1.37), independent of admission diagnosis, weight, age, sex, presence of end-stage kidney disease, and severity of illness. Conclusion: Early UFNET practice varies significantly by admission diagnosis. Higher early UFNET is independently associated with mortality. Impacts of UFNET on mortality may vary by admission diagnosis. Further work is required to elucidate the nature and mechanisms responsible for this association.

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