Unifying Concepts

Definition and Classification/Staging System for Acute Kidney Injury (AKI)

In 2004, the ADQI group and representatives from three nephrology societies established the Acute Kidney Injury Network (AKIN). Its intentions are to facilitate international, interdisciplinary and intersocietal collaborations and to ensure progress in the field of AKI, including the development of uniform standards for the definition and classification of AKI. As part of this process, the RIFLE nomenclature and classification was modified to a staging/classification system differentiating between AKI stage I, II and III. In addition, a 48-hour time window for the diagnosis of AKI was introduced to ensure that the process was acute.

AKI stage Creatinine criteria Urine output criteria
stage I
Increase of serum creatinine by ≥ 0.3 mg/dl (≥ 26.4 umol/L)
increase to ≥ 150% – 200% from baseline
Urine output < 0.5 ml/kg/hour for > 6 hours
stage II
Increase of serum creatinine to > 200% – 300% from baseline Urine output < 0.5 ml/kg/hour for > 12 hours
stage III
Increase of serum creatinine to > 300% from baseline
serum creatinine ≥ 4.0 mg/dl (≥ 354 µmol/L) after a rise of at least 44 µmol/L
treatment with renal replacement therapy
Urine output < 0.3 ml/kg/hour for > 24 hours
anuria for 12 hours

Diagnostic criteria for AKI includes an abrupt (within 48 hours) reduction in kidney function defined as an absolute increase in serum creatinine of either 0.3 mg/dl or more (≥ 26.4 umol/L) or a percentage increase of 50% or more (1.5 fold from baseline) or a reduction in urine output.



  1. Ostermann M, Chang R; Riyadh ICU Program Users Group. Correlation between the AKI classification and outcome. Crit Care. 2008;12(6):R144. [Medline]
  2. Mehta RL, Kellum JA, Shah SV, Molitoris BA, Ronco C, Warnock DG, Levin A; Acute Kidney Injury Network. Acute Kidney Injury Network: report of an initiative to improve outcomes in acute kidney injury. Crit Care. 2007;11(2):R31. [Medline]


Created: 09 Dec, 2009.

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