Diagnostic Evaluation and Initial Management of Acute Kidney Injury
The essential laboratory workup for acute kidney injury (AKI) includes serum creatinine, blood urea nitrogen, complete blood count, urinalysis with microscopy, urine sodium, fractional excretion of sodium, and renal ultrasound, while initial management requires discontinuation of nephrotoxic medications, fluid resuscitation, and correction of electrolyte imbalances. 1
Definition and Diagnosis of AKI
AKI is defined according to the Kidney Disease: Improving Global Outcomes (KDIGO) criteria as:
- Increase in serum creatinine by ≥0.3 mg/dL within 48 hours, OR
- Increase in serum creatinine to ≥1.5 times baseline within 7 days, OR
- Urine volume <0.5 mL/kg/h for 6 hours 1, 2
AKI Staging
| Stage | Serum Creatinine | Urine Output |
|---|---|---|
| 1 | 1.5-1.9× baseline or ≥0.3 mg/dL increase | <0.5 mL/kg/h for 6-12h |
| 2 | 2.0-2.9× baseline | <0.5 mL/kg/h for ≥12h |
| 3 | 3.0× baseline or increase to ≥4.0 mg/dL or initiation of RRT | <0.3 mL/kg/h for ≥24h or anuria for ≥12h |
Essential Laboratory Evaluation
Initial Laboratory Tests
- Serum creatinine and blood urea nitrogen (BUN)
- Complete blood count (CBC)
- Comprehensive metabolic panel
- Urinalysis with microscopic examination
- Urine sodium and calculation of fractional excretion of sodium (FENa) 1, 3
Additional Tests Based on Clinical Context
- Urine microscopy for sediment analysis (casts, cells) 1
- Urine protein-to-creatinine ratio
- Serum and urine electrolytes
- Biomarkers (when available): neutrophil gelatinase-associated lipocalin (NGAL), kidney injury molecule-1 (KIM-1), interleukin-18 (IL-18) 1
Imaging Studies
- Renal ultrasound: Recommended for most patients with AKI to rule out obstruction, especially in older men 1, 3
- Consider Doppler ultrasound to assess renal blood flow in suspected vascular causes 1
- Advanced imaging (CT, MRI) only when indicated based on clinical suspicion 1
Diagnostic Approach by Etiology
1. Prerenal AKI (60-70% of cases)
- BUN/creatinine ratio >20:1
- FENa <1% (if not on diuretics)
- Urine sodium <20 mEq/L
- Concentrated urine (specific gravity >1.020)
- Minimal sediment on urinalysis 3, 4
2. Intrinsic Renal AKI
- FENa >2%
- Urine sodium >40 mEq/L
- Urinalysis: hematuria, proteinuria, casts (granular, epithelial, RBC)
- Consider kidney biopsy when glomerular disease is suspected 1, 3
3. Postrenal AKI
- Hydronephrosis on ultrasound
- Variable urinalysis findings
- May have anuria or fluctuating urine output 1
Initial Management
Immediate Interventions
Discontinue nephrotoxic medications:
Fluid management:
- Isotonic crystalloids for volume expansion in hypovolemia
- Target euvolemia - avoid both hypovolemia and fluid overload
- Maintain mean arterial pressure ≥65 mmHg to ensure adequate renal perfusion 2
Electrolyte management:
- Correct hyperkalemia, hypocalcemia, hyperphosphatemia
- Monitor electrolytes daily 2
Consider Furosemide Stress Test (FST):
- Useful to identify patients likely to progress to severe AKI
- Administer 1.0-1.5 mg/kg IV furosemide (or 2.0-2.5 mg/kg if previously on loop diuretics)
- Poor response (<200 mL urine output in 2 hours) predicts progression 1
Indications for Renal Replacement Therapy
- Refractory hyperkalemia
- Volume overload unresponsive to diuretics
- Severe metabolic acidosis
- Uremic symptoms (encephalopathy, pericarditis, pleuritis)
- Certain toxin ingestions 3
Follow-up and Monitoring
Short-term Monitoring
- Daily serum creatinine and electrolytes
- Strict fluid balance monitoring
- Daily weight measurements
- Reassessment of medication dosages 1
Long-term Follow-up
- Follow-up within 1 month of AKI diagnosis
- Serial measurements of serum creatinine and proteinuria
- Monitoring for development of chronic kidney disease
- Cardiovascular risk assessment 1, 5
Common Pitfalls to Avoid
Delayed recognition: Relying solely on serum creatinine may delay diagnosis; monitor urine output and trends in creatinine 1
Indiscriminate fluid administration: Avoid fluid overload, which can worsen outcomes 1
Continued nephrotoxin exposure: Thoroughly review all medications and discontinue potential nephrotoxins 3
Inadequate follow-up: AKI increases long-term risk of chronic kidney disease, cardiovascular events, and mortality; ensure appropriate follow-up 1, 5
Misclassification: The traditional approach of classifying AKI as pre-renal, renal, and post-renal may lead to inappropriate management; consider the specific mechanisms of injury 1
AKI remains a significant cause of morbidity and mortality, with up to 20% of critically ill patients affected. Early recognition, prompt management, and appropriate follow-up are essential to improve outcomes and prevent progression to chronic kidney disease.