Acute Kidney Injury Diagnosis and Classification

Key Clinical Features and Diagnostic Criteria

The American College of Radiology suggests that AKI often presents without specific symptoms, making laboratory monitoring and clinical context essential for detection 1
Oliguria (urine output <0.5 mL/kg/h for ≥6 hours) is a cardinal sign of AKI, though it can occur with normal urine output 1, 2
Volume overload, manifesting as peripheral edema, pulmonary edema, or jugular venous distension, is a clinical manifestation of AKI 3
The KDIGO definition of AKI includes an increase in serum creatinine ≥0.3 mg/dL within 48 hours, or an increase in serum creatinine to ≥1.5 times baseline, or urine volume <0.5 mL/kg/h for 6 consecutive hours 1, 2
Even small creatinine increases (≥0.3 mg/dL) are independently associated with approximately fourfold increase in hospital mortality 6

The KDIGO classification stages AKI severity from 1 to 3 based on the most severe criterion met, with stage 1 being the least severe and stage 3 being the most severe 1, 2, 5
Stage 1 AKI is characterized by a creatinine increase of 1.5-1.9 times baseline or an increase of ≥0.3 mg/dL, or urine output <0.5 mL/kg/h for 6-12 hours 5, 7
Stage 2 AKI is characterized by a creatinine increase of 2.0-2.9 times baseline, or urine output <0.5 mL/kg/h for ≥12 hours 5, 7
Stage 3 AKI is characterized by a creatinine increase of ≥3.0 times baseline or an increase to ≥4.0 mg/dL, or initiation of renal replacement therapy, or urine output <0.3 mL/kg/h for ≥24 hours or anuria for ≥12 hours 5, 7

The BMJ suggests that patient-related risk factors for AKI include age ≥65 years, chronic kidney disease, diabetes mellitus, heart failure, and liver disease/cirrhosis 3
Exposure-related risk factors for AKI include nephrotoxic medications, diuretic use, recent surgery, and critical illness requiring ICU admission 1, 3, 6
Clinical context risk factors for AKI include hospital-acquired AKI, cardiogenic shock, acute heart failure, and COVID-19 1, 3