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Comparison of RIFLE with and without urine output criteria for acute kidney injury in critically ill patients: a task still not concluded!

In a recent issue of Critical Care, we read with interest the article by Wlodzimirow and colleagues [1], who prospectively studied the Risk Injury Failure Loss Endstage renal disease (RIFLE) [2] classification with serum creatinine (SCr) and urine output (UO) (RIFLESCr+UO) and without UO criteria (RIFLESCr) for acute kidney injury (AKI) in 260 critically ill patients. RIFLESCr significantly underestimated the presence of AKI on admission and during the first week in the intensive care unit and significantly delayed AKI diagnosis. Those are important findings that corroborate the utility of simultaneously using both criteria as proposed by the Acute Dialysis Quality Initiative workgroup [2]. The authors also found that RIFLESCr was associated with higher mortality than RIFLESCr+UO. This observation should be interpreted with extreme caution, as this association has not been tested by multivariate analysis. Data regarding the impact on mortality of RIFLE defined by SCr and UO or by SCr are not conclusive. For example, in a systematic review, the relative risk for death among studies that used RIFLESCr+UO was lower than in those using RIFLESCr [3]. Previously, however, we did not find any difference in terms of mortality for RIFLESCr+UO (Risk, odds ratio (OR) 2.69; Injury, OR 2.01; Failure, OR 3.59; AKI of any category, 2.78; area under the receiver operator characteristic (AUROC), 0.733) or for RIFLESCr (Risk, OR 2.63; Injury, OR 2.12; Failure, OR 3.2; AKI of any category, 2.68; AUROC, 0.729) [4]. Therefore, prospective studies with a large number of patients are still needed to better determine the impact on mortality of RIFLE defined by SCr+UO criteria or by SCr criteria.

Authors' response

Ameen Abu-Hanna, Kama A Wlodzimirow, Marcus Schultz and Catherine SC Bouman

We agree with Lopes and Jorge that multivariate analysis should be attempted when testing whether RIFLESCr is associated with higher mortality than RIFLESCr+UO. Essentially the question is whether the group (hereafter G1) of patients with AKI based on the RIFLESCr criteria (regardless of UO) is at higher risk of death than the group (hereafter G2) classified as having AKI based on the UO criteria only. Additional analysis, not reported in [1], shows that out of admission type, age, gender, weight, Acute Physiology and Chronic Health Evaluation (APACHE) score, Simplified Acute Physiology Score (SAPS), cardiopulmonary resuscitation, and length of stay, only SAPS was a confounder. Before adjustment for SAPS, patients in G1 had 1.64 times the odds of dying than those in G2. After adjustment for SAPS, the OR was reduced to 1.45 (P = 0.0004), still confirming our findings, which are in agreement with those of the other study [3].

The seeming contradiction between our findings and those of Lopes and colleagues [4] is easily explained by the significant differences in case mix. In our study, 48.6% of the RIFLESCr+UO AKI patients were classified as having AKI on the basis of the UO criteria only [1] versus 5.6% in the study by Lopes and colleagues [4]. Differences in case mix may be attributable to the different inclusion criteria, the Modification of Diet in Renal Disease (MDRD)-based estimation of baseline SCr in all patients in the previous study [4], which may overestimate AKI based on SCr [5], and the outcome definition. All of these are important factors to consider when comparing studies.



acute kidney injury


area under the receiver operator characteristic


odds ratio


Risk Injury Failure Loss End-stage renal disease


RIFLE criteria based on the serum creatinine criteria only


RIFLE criteria based on serum creatinine and urine output criteria


Simplified Acute Physiology Score


serum creatinine


urine output.


  1. 1.

    Wlodzimirow KA, Abu-Hanna A, Slabbekoorn M, Chamuleau RA, Schultz MJ, Bouman CS: A comparison of RIFLE with and without urine output criteria for acute kidney injury in critically ills. Crit Care 2012, 16: R200. 10.1186/cc11808

  2. 2.

    Bellomo R, Ronco C, Kellum JA, Mehta RL, Palevsky P, the ADQI workgroup: Acute renal failure - definition, outcome measures, animal models, fluid therapy and information technology needs: the Second International Consensus Conference of the Acute Dialysis Quality Initiative (ADQI) Group. Crit Care 2004, 8: R204. 10.1186/cc2872

  3. 3.

    Ricci Z, Cruz D, Ronco C: The RIFLE criteria and mortality in acute kidney injury: a systematic review. Kidney Int 2008, 73: 538-546. 10.1038/

  4. 4.

    Lopes JA, Fernandes P, Jorge S, Gonçalves S, Alvarez A, Costa e Silva Z, França C, Martins Prata M: Acute kidney injury in intensive care unit patients: a comparison between the RIFLE and the Acute Kidney Injury Network classifications. Crit Care 2008, 12: R110. 10.1186/cc6997

  5. 5.

    Bagshaw SM, Uchino S, Cruz D, Bellomo R, Morimatsu H, Morgera S, Schetz M, Tan I, Bouman C, Macedo E, Gibney N, Tolwani A, Oudemans-van Straaten HM, Ronco C, Kellum JA: A comparison of observed versus estimated baseline creatinine for determination of RIFLE class in patients with acute kidney injury. Nephrol Dial Transplant 2009, 24: 2739-2744. 10.1093/ndt/gfp159

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Correspondence to José António Lopes.

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Competing interests

The authors declare that they have no competing interests.

Authors' contributions

JAL and SJ drafted the letter, revised it critically for important intellectual content, and gave final approval of the version to be published.

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Lopes, J.A., Jorge, S. Comparison of RIFLE with and without urine output criteria for acute kidney injury in critically ill patients: a task still not concluded!. Crit Care 17, 408 (2013).

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  • Acute Kidney Injury
  • Urine Output
  • Risk Injury
  • Simplify Acute Physiology Score
  • Chronic Health Evaluation