What labs should be ordered for a patient with suspected acute kidney injury (AKI)?

Laboratory Tests for Suspected Acute Kidney Injury (AKI)

For patients with suspected acute kidney injury, essential laboratory tests should include serum creatinine, blood urea nitrogen, complete blood count, urinalysis, urine sodium, fractional excretion of sodium, and renal ultrasound to determine the cause and severity of kidney injury. 1

Core Laboratory Tests for AKI Diagnosis

Initial Laboratory Panel

• Serum creatinine: Critical for diagnosis and staging according to KDIGO criteria 1
  • Increase ≥0.3 mg/dL within 48 hours, or
  • Increase to ≥1.5 times baseline within 7 days
• Blood urea nitrogen (BUN): Helps assess severity and distinguish between prerenal, intrinsic, and postrenal causes 2
• Complete blood count (CBC): Identifies anemia, infection, or hemolysis that may contribute to AKI 1
• Basic metabolic panel: Evaluates electrolyte abnormalities (potassium, sodium, bicarbonate) 3
• Urinalysis with microscopy: Detects casts, cells, protein, and blood that help determine etiology 1, 2

Urine Studies

• Urine sodium and fractional excretion of sodium (FENa): Helps differentiate prerenal (FENa <1%) from intrinsic renal causes (FENa >2%) 3, 2
• Urine osmolality: Aids in distinguishing prerenal (high osmolality) from intrinsic renal causes (isosthenuric) 3
• Urine protein-to-creatinine ratio: Quantifies proteinuria to assess glomerular damage 4

Additional Tests Based on Clinical Context

For Suspected Specific Etiologies

• Serologic studies when systemic disease is suspected:
  • Complement levels (C3, C4)
  • Anti-nuclear antibody (ANA)
  • Anti-neutrophil cytoplasmic antibody (ANCA)
  • Anti-glomerular basement membrane antibody
  • Hepatitis panel 2

For Monitoring and Risk Assessment

• Phosphorus and parathyroid hormone (PTH): For patients with underlying CKD or severe AKI 4
• Novel biomarkers (where available):
  • Neutrophil gelatinase-associated lipocalin (NGAL)
  • Kidney injury molecule-1 (KIM-1)
  • Interleukin-18 (IL-18)
  • Liver-type fatty acid binding protein (L-FABP) 1, 5

Imaging Studies

• Renal ultrasound: Should be performed in most patients to rule out obstruction, particularly in older men 2
• Doppler studies: When renovascular disease is suspected 2

AKI Staging and Follow-up Testing

Staging Based on Laboratory Results

AKI should be staged according to KDIGO criteria to guide management 1:

• Stage 1: Increase in serum creatinine 1.5-1.9 times baseline or ≥0.3 mg/dL
• Stage 2: Increase in serum creatinine 2.0-2.9 times baseline
• Stage 3: Increase in serum creatinine ≥3.0 times baseline or to ≥4.0 mg/dL

Follow-up Testing

• Serial creatinine measurements: Daily during hospitalization 3
• Electrolytes: Monitor daily, particularly potassium in severe AKI 3
• Post-discharge monitoring: Serum creatinine every 2-4 weeks for 3 months 3, 4
• Long-term follow-up: Evaluate patients 3 months after AKI for resolution or progression to CKD 1

Important Considerations

Timing of Laboratory Tests

• Obtain baseline and serial measurements to track progression
• Compare current values to previous results when available
• Consider time course of injury when interpreting results 1

Common Pitfalls to Avoid

1. Relying solely on serum creatinine: May delay diagnosis as it rises 24-48 hours after injury
2. Failing to obtain baseline values: Makes staging difficult
3. Neglecting urine studies: Provides valuable diagnostic information
4. Missing post-discharge follow-up: Critical for detecting progression to CKD
5. Overlooking medication review: Essential to identify nephrotoxic agents 3, 2

By systematically ordering these laboratory tests, clinicians can promptly diagnose AKI, determine its etiology, assess its severity, and monitor response to treatment, ultimately improving patient outcomes.