What is the workup for Acute Kidney Injury (AKI) vs Chronic Kidney Disease (CKD)?

Workup for Acute Kidney Injury (AKI) vs Chronic Kidney Disease (CKD)

The initial diagnostic approach to differentiate AKI from CKD should focus on ultrasound evaluation of kidney size and structure, along with assessment of serum creatinine trends and specific markers of kidney damage. 1

Definitions and Time Frames

• Acute Kidney Injury (AKI): Abrupt decrease in kidney function occurring over hours to days (less than 7 days)

  • Stage 1: Increase in serum creatinine by ≥0.3 mg/dL within 48h or 1.5-1.9× baseline
  • Stage 2: Increase in serum creatinine to 2.0-2.9× baseline
  • Stage 3: Increase in serum creatinine to ≥3.0× baseline or ≥4.0 mg/dL or initiation of renal replacement therapy 1, 2

• Acute Kidney Disease (AKD): Kidney damage or decreased function persisting between 7 days and 3 months 1

• Chronic Kidney Disease (CKD): Abnormalities in kidney structure or function persisting for >3 months 1

Diagnostic Algorithm

Step 1: Imaging Assessment

• Renal Ultrasound: First-line imaging modality
  • AKI findings: Normal to increased kidney size, normal cortical thickness
  • CKD findings: Small kidneys (<9 cm in adults), cortical thinning, increased echogenicity 1
  • Important caveat: Normal-sized kidneys do not exclude CKD, especially in diabetic nephropathy or infiltrative disorders 1

Step 2: Laboratory Evaluation

• Serum Creatinine Pattern:

  • AKI: Rapid rise over hours to days with known baseline
  • CKD: Stable elevation or gradual increase over months
  • Unknown duration: More frequent measurements to detect rate of change 1

• Urinalysis:

  • AKI: Muddy brown casts (acute tubular necrosis), RBC casts (glomerulonephritis), WBC casts (pyelonephritis)
  • CKD: Broad waxy casts, oval fat bodies, persistent proteinuria 1

• Urine Protein Quantification:

  • Protein-to-creatinine ratio or albumin-to-creatinine ratio
  • Persistent albuminuria >3 months suggests CKD 1

Step 3: Historical and Clinical Context

• Prior Laboratory Values: Review previous creatinine measurements
• Risk Factors:
  • AKI: Recent surgery, contrast exposure, sepsis, nephrotoxic medications
  • CKD: Diabetes, hypertension, age >65, family history of kidney disease 2

Step 4: Additional Testing for Uncertain Cases

• Kidney Biopsy: Consider when diagnosis remains unclear despite above workup
• Biomarkers: Novel markers (NGAL, KIM-1) may help identify ongoing kidney damage even when creatinine returns to baseline 1

Special Considerations

AKI on CKD

• Many patients have underlying CKD with superimposed AKI
• Look for:
  • Baseline abnormal kidney function with acute deterioration
  • Small kidneys on imaging with rapid creatinine rise
  • Higher risk for progression to end-stage renal disease 3, 4

Acute Kidney Disease (AKD)

• Represents the transition period between AKI and potential CKD
• Critical period for intervention to prevent progression to CKD
• Requires close monitoring for 3 months after AKI episode 1

Follow-up After AKI

• Mild, resolved AKI: Primary care follow-up
• Prolonged AKI: Laboratory tests within days of discharge
• AKI stage 2-3: Nephrology follow-up within 1-2 weeks 2
• Long-term monitoring: All AKI survivors require monitoring for CKD development, even those with apparent complete recovery 5, 6

Pitfalls to Avoid

• Assuming normal-sized kidneys rule out CKD: Some forms of CKD present with normal or enlarged kidneys
• Relying solely on creatinine: Muscle mass changes can confound interpretation
• Missing AKI on CKD: Patients with CKD are at higher risk for AKI episodes
• Inadequate follow-up: Even seemingly resolved AKI carries risk for future CKD development 7

By systematically applying this approach, clinicians can effectively differentiate between AKI and CKD, identify patients with AKI on CKD, and implement appropriate monitoring and treatment strategies to improve outcomes.