AKI stands for Acute Kidney Injury, a rapid decline in kidney function that can arise in hours to days. It is diagnosed by a rise in serum creatinine and/or a drop in urine production and typically signals an acute problem that needs urgent evaluation.
Definition and what it implies
AKI is a clinical syndrome rather than a single disease. It reflects a sudden impairment of the kidneys' ability to filter waste, regulate fluid balance, and maintain electrolyte and acid-base homeostasis. The consequences can range from mild to life-threatening, depending on severity and underlying cause.
How AKI is classified
KDIGO staging overview
The common framework used by clinicians to gauge severity and guide treatment is KDIGO staging. The following criteria define each stage by changes in creatinine and urine output.
- Stage 1: Increase in serum creatinine by ≥0.3 mg/dL within 48 hours or an increase to 1.5–1.9 times baseline, or urine output <0.5 mL/kg/h for 6–12 hours.
- Stage 2: Increase in creatinine to 2.0–2.9 times baseline, or urine output <0.5 mL/kg/h for ≥12 hours.
- Stage 3: Increase in creatinine to ≥3.0 times baseline or 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.
Note on baseline creatinine: in some cases the patient’s baseline kidney function is unknown. When baseline values are not available, clinicians use the most recent stable reading or interpret the KDIGO criteria with clinical judgment to determine AKI.
Common causes of AKI
Categories of AKI
AKI typically arises from one of three broad categories, reflecting changes in kidney perfusion, direct kidney injury, or urinary tract obstruction.
- Prerenal AKI: reduced blood flow to the kidneys due to dehydration, bleeding, heart failure, or severe illness, which can be reversible with fluid resuscitation or hemodynamic optimization.
- Intrinsic (renal) AKI: damage within the kidneys themselves from toxins, inflammatory processes, infections, or conditions such as acute tubular necrosis or glomerulonephritis.
- Postrenal AKI: obstruction of urine flow from the kidneys downward, which can be caused by stones, tumors, or enlarged prostate and is relieved by removal of the obstruction.
Understanding the underlying cause is essential because treatments differ markedly depending on whether the problem is perfusion-related, intrinsic kidney injury, or obstruction.
Risk factors and diagnostic approach
Who is most at risk and how AKI is detected
Certain patients carry a higher risk of AKI, and early recognition improves outcomes. The following factors and tests help clinicians identify at-risk individuals and confirm the diagnosis.
- Age and preexisting kidney disease
- Recent major surgery, sepsis, or critical illness
- Exposure to nephrotoxic drugs (for example certain antibiotics, NSAIDs, and contrast agents used in imaging)
- Severe dehydration, low urine output, or electrolyte disturbances
- Laboratory and imaging tests: serum creatinine, urine output monitoring, complete blood count, electrolytes, and, when indicated, imaging to assess obstruction or structural kidney disease
Timely diagnosis enables targeted management and reduces the risk of progression to chronic kidney disease or kidney failure requiring long-term dialysis.
Treatment, prognosis and what to expect
Managing AKI in practice
Treatment focuses on identifying and addressing the underlying cause, supporting kidney function, and preventing complications. Key steps include optimizing blood pressure and circulation, ensuring appropriate fluid balance, avoiding nephrotoxic exposures, and initiating renal replacement therapy if needed.
- Supportive care: careful fluid management, electrolyte balance, and hemodynamic support.
- Remove or limit nephrotoxins when possible (drugs, contrast agents).
- Renal replacement therapy: dialysis or analogous therapies when there is severe AKI with complications or when kidney function fails to recover.
Outcomes vary: AKI increases short-term mortality risk, length of hospital stay, and can lead to longer-term kidney problems in some patients. Early, multidisciplinary care improves recovery prospects.
Summary
AKI is a rapid, potentially reversible decline in kidney function due to a range of insults. Its detection relies on changes in creatinine and urine output, and management hinges on treating the underlying cause, supporting organ systems, and avoiding further kidney injury. With prompt recognition and appropriate care, many people recover kidney function, though some may experience lasting effects or progression to chronic kidney disease.