Acute Kidney Injury I: Introduction

Acute kidney injury (AKI) is a rapid decline in kidney function that develops over hours to days, characterized by the kidneys' failure to remove waste products from the bloodstream. Common causes include dehydration, sepsis, or exposure to nephrotoxic substances like NSAIDs and contrast agents. AKI is potentially reversible but carries a high mortality rate, especially in hospitalized patients.

AKI is classified using the RIFLE system, which comprises three severity stages: Risk, Injury, and Failure, followed by two outcome stages: Loss and End-Stage Kidney Disease. The Risk stage involves a 1.5-fold increase in serum creatinine or 25% GFR decrease. Injury involves a 2-fold increase or 50% GFR reduction. Failure involves a 3-fold increase or greater than 75% GFR reduction.

AKI is identified by an increase of 0.3 mg/dL or more in serum creatinine within 48 hours or a 1.5-fold rise in baseline creatinine over seven days. Reduced urine output of less than 0.5 mL/kg/hour for six hours is also a key diagnostic marker. These measurements help guide acute kidney injury diagnostic studies and prevention strategies.

Untreated AKI leads to dangerous complications including metabolic acidosis, fluid overload, and electrolyte imbalances such as hyperkalemia, which can cause life-threatening arrhythmias. If left unmanaged, AKI can progress to chronic kidney disease or End-Stage Kidney Disease requiring long-term dialysis or kidney transplantation for survival.

Loss refers to complete loss of kidney function persisting for more than four weeks, often requiring renal replacement therapy. End-Stage Kidney Disease (ESKD) occurs when kidney failure persists for more than three months and represents irreversible damage requiring long-term dialysis or transplantation. Both stages describe outcomes of severe, sustained kidney dysfunction.

Early detection using the RIFLE classification system is crucial to improving patient outcomes and preventing progression to chronic kidney disease or ESKD. Identifying AKI through serum creatinine levels and urine output measurements allows clinicians to intervene promptly. Timely recognition enables appropriate acute kidney injury interprofessional care and management strategies.

At the Risk stage, urine output decreases to less than 0.5 mL/kg/hour for six hours. During Injury, output falls below 0.5 mL/kg/hour for 12 hours with rising BUN and serum creatinine. At Failure, output may be less than 0.3 mL/kg/hour for 24 hours or anuria (no urine) may occur for 12 hours, indicating severe kidney dysfunction.