What laboratory tests 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 include serum creatinine, blood urea nitrogen, complete metabolic panel, urinalysis, and urine output monitoring to establish diagnosis, determine severity, identify etiology, and guide management decisions. 1, 2

Core Diagnostic Tests

• Serum creatinine measurements - Serial measurements are essential to detect the ≥0.3 mg/dL increase within 48 hours or ≥50% increase within 7 days that define AKI according to KDIGO criteria 2, 3
• Urine output monitoring - Quantify to detect oliguria (<0.5 mL/kg/h for ≥6 hours) which is a diagnostic criterion for AKI 2, 3
• Complete metabolic panel - Includes electrolytes, BUN, creatinine, and glucose to assess overall metabolic status and identify complications 1
• Urinalysis with microscopy - Essential for identifying hematuria, pyuria, casts, or crystals that may suggest specific etiologies of AKI 3, 4
• Urine sodium concentration - Helps differentiate prerenal (typically <20 mEq/L) from intrarenal causes (typically >40 mEq/L) with high specificity 5, 4

Additional Tests Based on Clinical Context

• Urine protein quantification - Spot urine protein-to-creatinine ratio to assess glomerular damage; particularly important in patients with suspected glomerulonephritis 1, 6
• Fractional excretion of sodium (FENa) - Helps distinguish prerenal (<1%) from intrarenal (>2%) causes, though may be affected by diuretics 5, 4
• Renal failure index (RFI) - Provides high specificity for differentiating prerenal from intrarenal AKI 5
• Urine specific gravity - Helps assess concentrating ability; typically elevated (>1.020) in prerenal AKI 5, 4

Specialized Tests for Specific Scenarios

• Imaging studies - Renal ultrasonography is the first-line imaging study when obstructive causes are suspected 3
• Novel biomarkers - In specific high-risk settings, biomarkers like TIMP-2 × IGFBP7 may help with early detection, though their routine use remains investigational 1
• Phosphorus and parathyroid hormone (PTH) - Consider in patients with pre-existing CKD or severe/prolonged AKI 1, 7

Staging Laboratory Monitoring

• Acute phase (first 48-72 hours):

  • Daily serum creatinine and electrolytes 1, 2
  • Strict urine output monitoring 2

• Recovery phase:

  • Serum creatinine monitoring until stabilization 1, 7
  • Follow-up creatinine at 90 days to assess for progression to chronic kidney disease 1, 7

Common Pitfalls to Avoid

• Relying solely on creatinine - May delay diagnosis as creatinine rises 24-48 hours after injury 8, 4
• Ignoring urine output criteria - Oliguria may precede serum creatinine elevation 2, 3
• Failure to obtain baseline creatinine - Essential for accurate diagnosis and staging 1, 2
• Overlooking post-discharge monitoring - Only 12% of AKI patients receive recommended proteinuria assessment within one year 7
• Misinterpreting absence of hydronephrosis - Does not completely rule out obstruction, particularly in early cases (<12 hours) 3

Risk Assessment for Contrast-Induced AKI

• Pre-procedure risk stratification using factors like:
  • Pre-existing renal dysfunction (serum creatinine >1.5 mg/dL)
  • Age >75 years
  • Diabetes
  • Heart failure
  • Hypotension
  • Anemia 1

By systematically ordering and interpreting these laboratory tests, clinicians can promptly diagnose AKI, determine its severity, identify the underlying cause, and guide appropriate management to improve patient outcomes.