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

From the Guidelines

Acute Kidney Injury (AKI) is defined as an increase in serum creatinine by ≥0.3 mg/dL within 48 hours, an increase in serum creatinine to ≥1.5 times baseline within 7 days, or urine output less than 0.5 mL/kg/hour for 6 hours, as per the KDIGO criteria 1. The KDIGO criteria are the most widely used and provide staging for AKI severity, which is crucial for early recognition of kidney dysfunction and prompt intervention to prevent progression to more severe kidney damage and improve patient outcomes. Some key points to consider when defining AKI include:

• The increase in serum creatinine should be abrupt, occurring within 48 hours, and sustained for at least 24 hours 1.
• The KDIGO criteria also provide staging for AKI severity, with Stage 1 involving a 1.5-1.9 times increase in creatinine or ≥0.3 mg/dL rise, Stage 2 involving a 2.0-2.9 times increase, and Stage 3 involving a 3.0 times increase, creatinine ≥4.0 mg/dL, initiation of renal replacement therapy, or decreased eGFR to <35 mL/min in patients under 18 1.
• Other classification systems, such as the RIFLE criteria and AKIN criteria, use similar parameters but with slight differences in thresholds and timeframes 1.
• It is essential to distinguish between AKI and other conditions, such as chronic kidney disease (CKD), which is defined as an abnormality of kidney structure or function present for >3 months, with health consequences 1.
• The diagnosis of AKI is critical, as it is associated with substantial morbidity and mortality, and timely identification and treatment can improve patient outcomes 1.

From the Research

Definitions of Acute Kidney Injury (AKI)

• Acute kidney injury (AKI) is defined as a decrease in kidney function within hours, which encompasses both injury and impairment of renal function 2.
• AKI is also defined by a sudden loss of excretory kidney function 3.
• Another definition of AKI is an abrupt decline in the glomerular filtration rate with accumulation of nitrogenous waste products and the inability to maintain fluid and electrolyte homeostasis 4.
• AKI can be defined by the rate of rise of serum creatinine and a decline in urine output 5.
• The international guideline group Kidney Disease: Improving Global Outcomes (KDIGO) has defined AKI according to rises in serum creatinine and/or reductions in urine output 6.

Key Characteristics of AKI

• AKI is not considered a pathological condition of single organ failure, but a syndrome in which the kidney plays an active role in the progression of multi-organ dysfunction 2.
• AKI is part of a range of conditions summarized as acute kidney diseases and disorders (AKD), in which slow deterioration of kidney function or persistent kidney dysfunction is associated with an irreversible loss of kidney cells and nephrons, which can lead to chronic kidney disease (CKD) 3.
• The pathophysiology of AKI is diverse and requires a careful and systematic assessment of predisposing factors and localization of site of injury 5.

Clinical Implications of AKI

• AKI increases hospital mortality and has a poor prognosis in critically ill patients 2, 3, 4.
• Long-term consequences of AKI and AKD include CKD and cardiovascular morbidity 3.
• Early detection of AKI is necessary for preventing progression and starting renal replacement therapy at adjusted doses that reflect metabolic requirements 4, 5, 6.