What are the stages of Acute Kidney Injury (AKI)?

Stages of Acute Kidney Injury (AKI)

Acute Kidney Injury is classified into three stages based on serum creatinine rise or urine output reduction according to the KDIGO (Kidney Disease Improving Global Outcomes) criteria, which is the current standard for AKI diagnosis and staging. 1

Diagnostic Criteria for AKI

AKI is diagnosed when any of the following criteria are met:

• Increase in serum creatinine ≥0.3 mg/dl (≥26 μmol/l) within 48 hours 1, 2
• Increase in serum creatinine ≥50% from baseline within 7 days 1, 2
• Urine output <0.5 ml/kg/h for 6 hours or more 1, 2

AKI Staging System

Stage 1

• Creatinine criteria: Rise of >26 μmol/l (0.3 mg/dl) within 48 hours OR 50-99% rise from baseline within 7 days 1, 2
• Urine output criteria: <0.5 ml/kg/h for more than 6 hours 1, 2

Stage 2

• Creatinine criteria: 100-199% rise from baseline within 7 days 1, 2
• Urine output criteria: <0.5 ml/kg/h for more than 12 hours 1, 2

Stage 3

• Creatinine criteria: ≥200% rise from baseline within 7 days OR creatinine ≥354 μmol/l (4.0 mg/dl) with either rise of >26 μmol/l (0.3 mg/dl) within 48 hours or >50% rise from baseline within 7 days OR any requirement for renal replacement therapy 1, 2
• Urine output criteria: <0.3 ml/kg/h for 24 hours or anuria for 12 hours 1, 2

Important Clinical Considerations

• Patients are classified according to the highest possible stage where criteria are met, either by creatinine rise or by urine output 1
• Initial diagnosis of AKI is based on meeting any of the criteria for stage 1, while staging is carried out retrospectively when the episode is complete 1
• Even small rises in creatinine (≥0.3 mg/dl) are independently associated with approximately fourfold increase in hospital mortality 1
• Progression through AKI stages strongly correlates with increased mortality risk 2

Recent Developments and Nuances

• Some researchers have proposed subdividing Stage 1 AKI into two substages (1a and 1b) based on whether the diagnosis was made by absolute creatinine increase (1a) or relative percentage increase (1b), as these subgroups show significant differences in outcomes 3
• The term "Acute Kidney Disease" (AKD) has been proposed for kidney damage present for <3 months, with AKI being a subset where the disease process occurred within 7 days 1
• AKD staging has been proposed to monitor patients after an AKI episode, with stages 0A, 0B, 0C, and stages 1-3 that parallel AKI staging 1

Clinical Pitfalls to Avoid

• Failing to recognize that transient AKI (recovery within 3 days) is not benign and still carries significant mortality risk (15% vs. 4% for no AKI) 1
• Overlooking AKI in patients with chronic kidney disease (CKD), where the absolute rise in creatinine may be more important than percentage change 1
• Relying solely on creatinine measurements in critically ill patients, where creatinine rise may be delayed, potentially postponing AKI diagnosis 1
• Neglecting to follow up patients after an AKI episode, as they remain at increased risk for chronic kidney disease, cardiovascular events, and mortality even after apparent recovery 1