Urine output before the initiation of CRRT and mortality

The incidence of acute kidney injury (AKI) is still increasing, and severe AKI requiring renal replacement therapy is associated with high mortality rates despite advances in medical treatment. In hemodynamically unstable patients, continuous renal replacement therapy (CRRT) is tolerated better than intermittent hemodialysis; however, the mortality rate has not been reduced. Therefore predicting the prognosis of AKI patients requiring CRRT is important, as are efforts to reduce mortality rates in AKI patients with multiorgan failure and septic shock. The prognostic factors associated with mortality from severe AKI have been identified as the severity of comorbid conditions, hemodynamic instability, and the time of initiation of CRRT [1–4]. Oh et al [5] retrospectively analyzed predictors of 90-day mortality in 67 hemodynamically unstable patients with AKI who required CRRT. They demonstrated that a urine output of less than 500 mL for 12 hours before the initiation of CRRT was the only independent predictor. This is consistent with a previous report that showed low urine output before the initiation of CRRT to be associated with mortality [2]. As the urine output during a certain period (6 or 12 hours) reflects the severity of the patient’s condition more accurately than blood pressure or laboratory parameters at one time point, We agree with the authors that urine output during a certain period can be a superior predictor to previously known factors. Moreover, urine output during the first several hours may also reflect whether the patient has responded to fluid therapy before the initiation of CRRT, and therefore may also reflect the recovery of the hemodynamic instability and kidney injury. However, we have several concerns about this study. First, the authors defined hypotension as an initial mean arterial pressure less than 60 mmHg (about 80/50 mmHg), which is stricter than the definition of hypotension generally used—less than 100/60 mmHg. In clinical practice, CRRT cannot be started in patients with a blood pressure less than 80/50 mmHg or in those who do not respond to vasopressor treatment. Therefore factors that reflect the response to fluid therapy in the early period of CRRT treatment, such as urine output, may be more reliable than blood pressure at one time point. The authors did not analyze whether the initial blood pressure was different between the survivors and nonsurvivors, probably because all the patients were hemodynamically unstable. We suggest that the mean blood pressure