Definition and Staging of AKI
Acute kidney injury (AKI) is defined by the KDIGO (Kidney Disease: Improving Global Outcomes) guidelines as any of the following: rise in serum creatinine by ≥26.5 μmol/L within 48 hours; rise in creatinine to ≥1.5 times baseline within 7 days; or urine output <0.5 mL/kg/hour for ≥6 hours. AKI is then staged 1–3 based on the degree of creatinine rise or urine output reduction, with Stage 3 representing the most severe injury (creatinine ≥3 times baseline, or anuria for ≥12 hours, or requiring RRT).
AKI occurs in approximately 50% of ICU patients and is independently associated with increased mortality, prolonged ICU stay, and risk of chronic kidney disease. It is not a single disease but a syndrome with multiple causes that share a common clinical presentation of declining renal function.
| Stage | Creatinine Criterion | Urine Output Criterion |
| Stage 1 | Rise ≥26.5 μmol/L in 48h OR 1.5–1.9× baseline | <0.5 mL/kg/h for 6–12 hours |
| Stage 2 | 2.0–2.9× baseline creatinine | <0.5 mL/kg/h for ≥12 hours |
| Stage 3 | ≥3× baseline, or ≥354 μmol/L, or commencing RRT | <0.3 mL/kg/h for ≥24h, or anuria ≥12h |