Classification Systems for Acute Kidney Injury 

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Acute kidney injury (AKI), formerly called acute renal failure (ARF), is commonly defined as an abrupt decline in renal function, clinically manifesting as a reversible acute increase in nitrogen waste products—measured by blood urea nitrogen (BUN) and serum creatinine levels—over the course of hours to weeks. The vague nature of this definition has historically made it difficult to compare between scholarly works and to generalize findings on epidemiologic studies of AKI to patient populations. Several classification systems have been developed to streamline research and clinical practice with respect to AKI. ^{[1, 2, 3, 4]}  Despite a broad consensus of support, however, the nephrology community continues to point out the shortcomings of these classification systems. ^[5]

For more information, see Acute Kidney Injury and Acute Tubular Necrosis

In 2002, the Acute Dialysis Quality Initiative (ADQI) was created with the primary goal of developing consensus and evidence-based guidelines for the treatment and prevention of acute kidney injury (AKI). The first order of business was to create a uniform, accepted definition of AKI; hence, the RIFLE criteria were born (see the table below). RIFLE is an acronym of Risk, Injury, and Failure; and Loss; and End-stage kidney disease.Table 1. RIFLE Classification System for Acute Kidney Injury ^[6]

Table. (Open Table in a new window)

Stage

GFR^a Criteria

UO^b Criteria

Risk

SCr^c increased 1.5-2 times baseline

or

GFR decreased >25%

UO < 0.5 mL/kg/h < 6 h

Injury

SCr increased 2-3 times baseline

or

GFR decreased >50%

UO < 0.5 mL/kg/h >12 h

Failure

SCr increased >3 times baseline

or

GFR decreased 75%

or

SCr ≥4 mg/dL; acute rise ≥0.5 mg/dL

UO < 0.3 mL/kg/h 24 h

(oliguria)

or

anuria 12 h

Loss of function

Persistent acute renal failure: complete loss of kidney function >4 wk (requiring dialysis)

ESRD^d

Complete loss of kidney function >3 mo (requiring dialysis)

^a GFR = glomerular filtration rate.

^b UO = urine output.

^c SCr = serum creatinine.

^d ESRD = end-stage renal disease.

Note: Patients can be classified either by GFR criteria or by UO criteria. The criteria that support the most severe classification should be used. The superimposition of acute on chronic failure is indicated with the designation RIFLE-F_C; failure is present in such cases even if the increase in SCr is less than 3-fold, provided that the new SCr is greater than 4 mg/dL (350 μmol/L) and results from an acute increase of at least 0.5 mg/dL (44 μmol/L).

When the failure classification is achieved by UO criteria, the designation of RIFLE-F_O is used to denote oliguria. The initial stage, “risk,” has high sensitivity; more patients are classified in this mild category, including some who do not actually have renal failure. Progression through the increasingly severe stages of RIFLE is marked by decreasing sensitivity and increasing specificity.

A recent study to assess RIFLE urine output criteria based on creatinine concentrations for the development of contrast- induced acute kidney injury (CA-AKI) in intensive care (ICU) patients found low predictive value. The authors concluded that there was limited use for assessing the effects of therapeutic interventions on the development and progression of AKI in this population. ^[7]

In September 2004, the Acute Kidney Injury Network (AKIN) was formed. AKIN advised that the term acute kidney injury (AKI) be used to represent the full spectrum of renal injury, from mild to severe, with the latter having increased likelihood for unfavorable outcomes (eg, loss of function and end-stage renal disease [ESRD]). ^[8]

A report by the AKIN proposed the following criteria for AKI ^{[8, 9]} :

Abrupt (within 48 h) reduction in kidney function currently defined as an absolute increase in serum creatinine of 0.3 mg/dL or more (≥26.4 μmol/L) or

A percentage increase in serum creatinine of 50% or more (1.5-fold from baseline) or

A reduction in urine output (documented oliguria of < 0.5 mL/kg/h for >6 h)

The AKIN criteria differ from the RIFLE criteria in several ways. The RIFLE criteria are defined as changes within 7 days, while the AKIN criteria suggest using 48 hours. The AKIN classification includes less severe injury in the criteria and AKIN also avoids using the glomerular filtration rate as a marker in AKI, as there is no dependable way to measure glomerular filtration rate and estimated glomerular filtration rate are unreliable in AKI.

AKIN notes that the diagnostic criteria proposed only after volume status has been optimized and urinary tract obstructions must be excluded when using oliguria as diagnostic criteria.

Fujii and colleagues assessed the three systems discriminative ability based on serum creatinine for the prediction of hospital mortaliy and found the AKIN classification system to be inferior to the RIFLE and KDIGO systems. ^[10]

In 2012 the Kidney Disease Improving Global Outcomes (KDIGO) released their clinical practice guidelines for acute kidney injury (AKI), which build off of the RIFLE criteria and the AKIN criteria. ^[11]

KDIGO defines AKI as any of the following:

Increase in serum creatinine by 0.3mg/dL or more within 48 hours or

Increase in serum creatinine to 1.5 times baseline or more within the last 7 days or

Urine output less than 0.5 mL/kg/h for 6 hours

The KDIGO has also recommended a staging system for the severity of the AKI.

The KDIGO consensus classification has yet to be validated.

Table 2. KDIGO Staging for AKI Severity (Open Table in a new window)

Stage

Serum Creatinine

Urine Output

1

1.5-1.9 times baseline

or

≥0.3 mg/dL increase

< 0.5 mL/kg/h for 6 h

2

2-2.9 times baseline

< 0.5 mL/kg/h for 12 h

3

3 times baseline

or

Increase in serum creatinine to ≥4 mg/dL

or

Initiation of renal replacement therapy

< 0.3 mL/kg/h for 24 h

or

Anuria for ≥12 h

In a cohort of 14,795 hospitalized children, 7712 children were diagnosed with AKI according to at least one of the three definitions. A total of 5406 (70%) children were diagnosed by all three definitions. Differences in the definitions resulted in the following variances ^[12] :

Overview

What is acute kidney injury (AKI)?

What are the RIFLE criteria for acute kidney injury (AKI)?

What is the AKIN classification system for acute kidney injury (AKI)?

What are the AKIN criteria for acute kidney injury (AKI)?

How do the AKIN criteria differ from the RIFLE criteria for acute kidney injury (AKI)?

What are the KDIGO criteria of acute kidney injury (AKI)?

How does the incidence of acute kidney injury (AKI) vary between the three main classification systems?

Roy AK, Mc Gorrian C, Treacy C, Kavanaugh E, Brennan A, Mahon NG, et al. A Comparison of Traditional and Novel Definitions (RIFLE, AKIN, and KDIGO) of Acute Kidney Injury for the Prediction of Outcomes in Acute Decompensated Heart Failure. Cardiorenal Med. 2013 Apr. 3(1):26-37. [Medline]. [Full Text].

Hui WF, Chan WK, Miu TY. Acute kidney injury in the paediatric intensive care unit: identification by modified RIFLE criteria. Hong Kong Med J. 2013 Feb. 19(1):13-9. [Medline].

Ratanarat R, Skulratanasak P, Tangkawattanakul N, Hantaweepant C. Clinical accuracy of RIFLE and Acute Kidney Injury Network (AKIN) criteria for predicting hospital mortality in critically ill patients with multi-organ dysfunction syndrome. J Med Assoc Thai. 2013 Feb. 96 Suppl 2:S224-31. [Medline].

Ricci Z, Ronco C. Neonatal RIFLE. Nephrol Dial Transplant. 2013 Apr 25. [Medline].

Barasch J, Zager R, Bonventre JV. Acute kidney injury: a problem of definition. Lancet. 2017 Feb 25. 389 (10071):779-781. [Medline]. [Full Text].

Bellomo R, Ronco C, Kellum JA, Mehta RL, Palevsky P. 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 Aug. 8(4):R204-12. [Medline]. [Full Text].

Hocine A, Defrance P, Lalmand J, Delcour C, Biston P, Piagnerelli M. Predictive value of the RIFLE urine output criteria on contrast-induced nephropathy in critically ill patients. BMC Nephrol. 2016 Mar 28. 17:36. [Medline]. [Full Text].

Mehta RL, Kellum JA, Shah SV, Molitoris BA, Ronco C, Warnock DG, et al. Acute Kidney Injury Network: report of an initiative to improve outcomes in acute kidney injury. Crit Care. 2007. 11(2):R31. [Medline]. [Full Text].

Mehta R, Kellum JA, Levin A. From acute renal failure to acute kidney injury: what’s changed?. Nephrology Self-Assessment Program. 2007. 6(5):281.

Fujii T, Uchino S, Takinami M, Bellomo R. Validation of the Kidney Disease Improving Global Outcomes criteria for AKI and comparison of three criteria in hospitalized patients. Clin J Am Soc Nephrol. 2014 May. 9 (5):848-54. [Medline]. [Full Text].

Acute Kidney Injury Work Group. Kidney Disease: Improving Global Outcomes (KDIGO) – Clinical Practice Guideline for Acute Kidney Injury. Kidney Inter. 2012. 2:1-138.

Sutherland SM, Byrnes JJ, Kothari M, Longhurst CA, Dutta S, Garcia P, et al. AKI in hospitalized children: comparing the pRIFLE, AKIN, and KDIGO definitions. Clin J Am Soc Nephrol. 2015 Apr 7. 10 (4):554-61. [Medline]. [Full Text].

Stage

GFR^a Criteria

UO^b Criteria

Risk

SCr^c increased 1.5-2 times baseline

or

GFR decreased >25%

UO < 0.5 mL/kg/h < 6 h

Injury

SCr increased 2-3 times baseline

or

GFR decreased >50%

UO < 0.5 mL/kg/h >12 h

Failure

SCr increased >3 times baseline

or

GFR decreased 75%

or

SCr ≥4 mg/dL; acute rise ≥0.5 mg/dL

UO < 0.3 mL/kg/h 24 h

(oliguria)

or

anuria 12 h

Loss of function

Persistent acute renal failure: complete loss of kidney function >4 wk (requiring dialysis)

ESRD^d

Complete loss of kidney function >3 mo (requiring dialysis)

^a GFR = glomerular filtration rate.

^b UO = urine output.

^c SCr = serum creatinine.

^d ESRD = end-stage renal disease.

Note: Patients can be classified either by GFR criteria or by UO criteria. The criteria that support the most severe classification should be used. The superimposition of acute on chronic failure is indicated with the designation RIFLE-F_C; failure is present in such cases even if the increase in SCr is less than 3-fold, provided that the new SCr is greater than 4 mg/dL (350 μmol/L) and results from an acute increase of at least 0.5 mg/dL (44 μmol/L).

When the failure classification is achieved by UO criteria, the designation of RIFLE-F_O is used to denote oliguria. The initial stage, “risk,” has high sensitivity; more patients are classified in this mild category, including some who do not actually have renal failure. Progression through the increasingly severe stages of RIFLE is marked by decreasing sensitivity and increasing specificity.

Stage

Serum Creatinine

Urine Output

1

1.5-1.9 times baseline

or

≥0.3 mg/dL increase

< 0.5 mL/kg/h for 6 h

2

2-2.9 times baseline

< 0.5 mL/kg/h for 12 h

3

3 times baseline

or

Increase in serum creatinine to ≥4 mg/dL

or

Initiation of renal replacement therapy

< 0.3 mL/kg/h for 24 h

or

Anuria for ≥12 h

Piper Julie Hughes, MD, MS Resident Physician, Department of Internal Medicine, Vidant Medical Center

Piper Julie Hughes, MD, MS is a member of the following medical societies: American College of Physicians, American Medical Association

Disclosure: Nothing to disclose.

Tejas Desai, MD Staff Nephrologist, WG (Bill) Hefner VA Medical Center

Tejas Desai, MD is a member of the following medical societies: American College of Physicians, American Society of Nephrology

Disclosure: Nothing to disclose.

Edgar V Lerma, MD, FACP, FASN, FAHA, FASH, FNLA, FNKF Clinical Professor of Medicine, Section of Nephrology, Department of Medicine, University of Illinois at Chicago College of Medicine; Research Director, Internal Medicine Training Program, Advocate Christ Medical Center; Consulting Staff, Associates in Nephrology, SC

Edgar V Lerma, MD, FACP, FASN, FAHA, FASH, FNLA, FNKF is a member of the following medical societies: American Heart Association, American Medical Association, American Society of Hypertension, American Society of Nephrology, Chicago Medical Society, Illinois State Medical Society, National Kidney Foundation, Society of General Internal Medicine

Disclosure: Author for: UpToDate, ACP Smart Medicine, Elsevier, McGraw-Hill, Wolters Kluwer.

Vecihi Batuman, MD, FASN Huberwald Professor of Medicine, Section of Nephrology-Hypertension, Tulane University School of Medicine; Chief, Renal Section, Southeast Louisiana Veterans Health Care System

Vecihi Batuman, MD, FASN is a member of the following medical societies: American College of Physicians, American Society of Hypertension, American Society of Nephrology, International Society of Nephrology, Southern Society for Clinical Investigation

Disclosure: Nothing to disclose.

Mahendra Agraharkar, MD, MBBS, FACP, FASN Clinical Associate Professor of Medicine, Baylor College of Medicine; President and CEO, Space City Associates of Nephrology

Mahendra Agraharkar, MD, MBBS, FACP, FASN is a member of the following medical societies: American College of Physicians, American Society of Nephrology, and National Kidney Foundation

Disclosure: South Shore DaVita Dialysis Center Ownership interest/Medical Directorship Other; Space City Dialysis /American Renal Associates Ownership/Medical Directorship Same; US Renal Care Ownership interest Other

George R Aronoff, MD Director, Professor, Departments of Internal Medicine and Pharmacology, Section of Nephrology, Kidney Disease Program, University of Louisville School of Medicine

George R Aronoff, MD is a member of the following medical societies: American Federation for Medical Research, American Society of Nephrology, Kentucky Medical Association, and National Kidney Foundation

Disclosure: Nothing to disclose.

F John Gennari, MD Associate Chair for Academic Affairs, Robert F and Genevieve B Patrick Professor, Department of Medicine, University of Vermont College of Medicine

F John Gennari, MD is a member of the following medical societies: Alpha Omega Alpha, American College of Physicians-American Society of Internal Medicine, American Federation for Medical Research, American Heart Association, American Physiological Society, American Society for Clinical Investigation, American Society of Nephrology, and International Society of Nephrology

Disclosure: Nothing to disclose.

Brent Kelly MD Assistant Professor, Department of Dermatology, University of Texas Medical Branch, Galveston, Texas

Brent Kelly is a member of the following medical societies: Alpha Omega Alpha and American Medical Association

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Reference Salary Employment

Classification Systems for Acute Kidney Injury 

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