---
title: Acute Kidney Injury Classification
description: A comprehensive clinical reference on the diagnosis, staging, and management of acute kidney injury.
keywords: [acute kidney injury, AKI, KDIGO, creatinine, oliguria, renal replacement therapy]
category: clinical
---

## Introduction to Acute Kidney Injury

Acute kidney injury (AKI) is a sudden and often reversible decline in renal function, leading to the retention of waste products and dysregulation of fluid and electrolyte balance. AKI is a common and serious condition, affecting up to 20% of hospitalized patients and carrying a high risk of morbidity and mortality. Prompt recognition and appropriate management of AKI is critical to improving patient outcomes.

## Diagnostic Criteria for AKI

The Kidney Disease: Improving Global Outcomes (KDIGO) clinical practice guidelines define AKI as any of the following:

1. Increase in serum creatinine by ≥0.3 mg/dL (≥26.5 μmol/L) within 48 hours
2. Increase in serum creatinine to ≥1.5 times baseline, which is known or presumed to have occurred within the prior 7 days
3. Urine volume <0.5 mL/kg/h for 6 hours

The KDIGO criteria allow for the staging of AKI severity based on the degree of creatinine elevation and urine output reduction:

- Stage 1: Creatinine 1.5-1.9 times baseline or ≥0.3 mg/dL increase; urine output <0.5 mL/kg/h for 6-12 hours
- Stage 2: Creatinine 2.0-2.9 times baseline; urine output <0.5 mL/kg/h for ≥12 hours
- Stage 3: Creatinine ≥3.0 times baseline or increase to ≥4.0 mg/dL; urine output <0.3 mL/kg/h for ≥24 hours or anuria for ≥12 hours

## Etiology and Risk Factors for AKI

The most common causes of AKI include:
- Prerenal: Hypovolemia, shock, heart failure, liver disease
- Intrinsic: Acute tubular necrosis, glomerulonephritis, acute interstitial nephritis
- Postrenal: Urinary tract obstruction, neurogenic bladder

Major risk factors for developing AKI include:
- Advanced age
- Chronic kidney disease
- Diabetes mellitus
- Hypertension
- Sepsis
- Exposure to nephrotoxic medications (e.g., NSAIDs, aminoglycosides, contrast dye)
- Cardiac surgery

## Evaluation and Diagnostic Workup

The initial evaluation of AKI should include a thorough history, physical exam, and laboratory testing. Key elements include:
- Urine output monitoring
- Serum creatinine and estimated glomerular filtration rate (eGFR)
- Urine dipstick and microscopy
- Fractional excretion of sodium (FENa) and urea (FeUrea)
- Renal ultrasound to evaluate for obstruction

Additional testing, such as urine biomarkers (NGAL, KIM-1) or a kidney biopsy, may be warranted in select cases to determine the underlying etiology.

## Management and Treatment of AKI

The management of AKI is focused on identifying and treating the underlying cause, optimizing hemodynamics, avoiding further kidney injury, and providing supportive care.

### Hemodynamic Management
- Fluid resuscitation with crystalloids (e.g., normal saline, lactated Ringer's) to maintain euvolemia
- Vasopressor therapy (e.g., norepinephrine, dopamine) for shock states
- Avoidance of nephrotoxic medications

### Supportive Care
- Monitoring of fluid balance, electrolytes, and acid-base status
- Dietary protein restriction and management of uremia
- Initiation of renal replacement therapy (RRT) for severe AKI or life-threatening complications

### Specific Interventions
- Treatment of the underlying cause (e.g., antibiotics for sepsis, relief of obstructive uropathy)
- Discontinuation of offending medications
- Avoidance of contrast dye in high-risk patients

The decision to initiate RRT, such as hemodialysis or continuous renal replacement therapy (CRRT), is based on the severity of AKI, presence of complications (e.g., hyperkalemia, acidosis, volume overload), and the patient's clinical status. The KDIGO guidelines recommend considering RRT when any of the following criteria are met:
- Refractory fluid overload
- Hyperkalemia (K+ >6.0 mmol/L)
- Metabolic acidosis (pH <7.15)
- Blood urea nitrogen (BUN) >100 mg/dL (35.7 mmol/L)
- Symptoms of uremia (e.g., pericarditis, encephalopathy)

## Prognosis and Outcomes in AKI

The prognosis of AKI depends on the underlying cause, severity, and presence of comorbidities. Overall, AKI is associated with a high risk of morbidity and mortality, particularly in the setting of severe or progressive disease.

Key prognostic factors include:
- Severity of AKI (KDIGO stage)
- Need for RRT
- Development of chronic kidney disease
- Presence of multiorgan dysfunction
- Underlying comorbidities (e.g., diabetes, cardiovascular disease)

Patients who develop AKI have an increased risk of short-term complications, such as fluid overload, electrolyte disturbances, and metabolic acidosis. Long-term consequences may include the development of chronic kidney disease, increased risk of cardiovascular events, and higher mortality rates.

## Conclusion

Acute kidney injury is a common and serious condition that requires prompt recognition and appropriate management. The KDIGO criteria provide a standardized approach to the diagnosis and staging of AKI, which is essential for guiding treatment and predicting outcomes. A thorough understanding of the underlying causes, risk factors, and supportive care measures is crucial for optimizing patient care and improving clinical outcomes.