What are the initial lab tests for a patient with suspected acute kidney injury (AKI)?

Initial Laboratory Workup for Acute Kidney Injury

The initial laboratory workup for suspected acute kidney injury (AKI) should include serum creatinine, blood urea nitrogen (BUN), complete blood count with differential, serum electrolytes with calculated anion gap, urinalysis with microscopy, and urine chemistry. 1

Core Laboratory Tests

Immediate Blood Tests

• Serum creatinine - Essential for diagnosis and staging of AKI per KDIGO criteria 1
  • AKI is defined as:
    • Rise of ≥0.3 mg/dL (≥26 μmol/L) within 48 hours, OR
    • ≥50% rise from baseline within 7 days 1
• Blood urea nitrogen (BUN) - Helps assess severity and distinguish prerenal from intrinsic causes 1
• Complete blood count with differential - Evaluates for anemia, infection, or hemolysis 1
• Serum electrolytes - Sodium, potassium, chloride, bicarbonate 1
• Calculated anion gap - Helps identify specific causes of metabolic acidosis 1
• Serum osmolality - Particularly useful in hyperglycemic states 1

Immediate Urine Tests

• Urinalysis with microscopy - Evaluates for:
  • Proteinuria (>500 mg/day) 2
  • Hematuria (>50 RBCs/HPF) 2
  • Casts (granular, epithelial, RBC) 1
  • Crystals (oxalate, urate) 1
• Urine chemistry:
  • Fractional excretion of sodium (FENa) - Helps differentiate prerenal from intrinsic causes 3
  • Urine protein-to-creatinine ratio - If proteinuria is suspected 2

Additional First-Line Tests

• Arterial blood gases - For patients with severe AKI or suspected metabolic acidosis 1
• Serum ketones - Particularly in diabetic patients to rule out diabetic ketoacidosis 1
• Bacterial cultures (urine, blood, throat) - If infection is suspected 1
• Electrocardiogram - To evaluate for cardiac causes or complications 1

Imaging

• Renal ultrasound - Should be performed in most patients with AKI to rule out obstruction and assess kidney size/echogenicity 1, 2
  • Particularly important in older men due to higher risk of obstruction 3

Follow-Up Testing

• Serial serum creatinine measurements - To track progression or resolution 2
• Electrolyte monitoring - Every 6-12 hours in severe cases 2
• Daily weights and strict intake/output monitoring - For volume status assessment 2

Common Pitfalls to Avoid

1. Relying solely on serum creatinine - Can be insensitive in early AKI and affected by muscle mass, age, and hydration status 4
2. Missing urine microscopy - Critical for differentiating causes of AKI 3
3. Delaying ultrasound - Can miss obstructive causes requiring urgent intervention 1
4. Forgetting to establish baseline creatinine - Essential for proper AKI staging and prognosis 1
5. Inadequate follow-up testing - Post-AKI monitoring is often neglected but crucial for detecting progression to chronic kidney disease 5

Special Considerations

• In patients with diabetes, check blood glucose and consider diabetic ketoacidosis workup 1
• In patients with suspected glomerulonephritis, consider additional serologic testing (complement levels, ANA, ANCA) 3
• In patients with recent contrast exposure, timing of creatinine rise is critical for diagnosis 2

By systematically ordering and interpreting these laboratory tests, clinicians can rapidly diagnose AKI, determine its etiology, assess its severity, and guide appropriate management to improve patient outcomes.