Acute Kidney Injury Definition and Diagnosis

Core Diagnostic Criteria

The Kidney Disease: Improving Global Outcomes (KDIGO) criteria define acute kidney injury (AKI) as an abrupt decrease in kidney function occurring over 7 days or less, characterized by a rise in serum creatinine ≥0.3 mg/dL within 48 hours, or a ≥50% increase in serum creatinine from baseline within 7 days, or urine output <0.5 mL/kg/h for 6 consecutive hours, according to the KDIGO guideline 1
The American College of Physicians recommends using the KDIGO criteria to diagnose AKI, which requires meeting any single criterion from the following: serum creatinine increase ≥0.3 mg/dL within 48 hours, serum creatinine increase to ≥1.5 times baseline within 7 days, or urine output <0.5 mL/kg/h for ≥6 consecutive hours 1, 3

The KDIGO criteria predict a fourfold increase in hospital mortality, even at the smallest threshold (≥0.3 mg/dL creatinine rise), emphasizing the importance of early detection and intervention, as stated by the KDIGO guideline 1, 3

The KDIGO guideline classifies AKI severity into three stages based on the most severe criterion met (either creatinine or urine output), with Stage 1 being the least severe and Stage 3 being the most severe 3
The National Kidney Foundation recommends using the KDIGO staging system to classify AKI severity, which includes:

Progression through higher stages strongly correlates with increased mortality, emphasizing the importance of early detection and intervention, as stated by the KDIGO guideline 2, 3

The ADQI consensus framework distinguishes AKI from related entities based on duration, with AKI defined as an abrupt kidney function decrease occurring over ≤7 days, according to the ADQI consensus framework 4, 1
The National Kidney Foundation recommends using the ADQI consensus framework to distinguish AKI from related entities, which includes:

Using known creatinine values is superior to imputation methods when establishing baseline, as stated by the KDIGO guideline 1
The KDIGO guideline recommends using the lowest creatinine value available during hospitalization if a known baseline is not available 1
Serum creatinine has inherent limitations that can lead to misclassification, including muscle wasting, volume expansion, hyperbilirubinemia, and increased tubular secretion of creatinine in chronic kidney disease, according to the KDIGO guideline 1, 2
The National Kidney Foundation recommends considering urine output criteria in addition to serum creatinine, as urine output criteria can identify cases of AKI that may be missed by serum creatinine alone 1, 2
However, urine output criteria are unreliable in specific populations, such as cirrhotic patients with ascites and patients on diuretic therapy, as stated by the KDIGO guideline 1, 2

Detection must occur in real-time based on initial marker changes rather than waiting for retrospective confirmation, as emphasized by the KDIGO guideline 2
The staging system is applied retrospectively when the episode is complete, classifying according to the most severe criterion met, according to the KDIGO guideline 2
AKI often presents without specific symptoms, making systematic laboratory monitoring essential in at-risk patients, as stated by the KDIGO guideline 3