Is calculating the stage of estimated Glomerular Filtration Rate (eGFR) appropriate in patients with acute kidney injury (Impaired renal function)?

Staging eGFR in Acute Kidney Injury: Not Recommended

eGFR staging should not be used for patients with acute kidney injury (AKI) as it is not validated in this setting and may lead to inaccurate assessment of kidney function. 1

Why eGFR Staging Is Not Appropriate in AKI

AKI and chronic kidney disease (CKD) use fundamentally different staging systems based on different principles:

• AKI staging: Based on relative changes in serum creatinine or urine output over hours to days
• CKD staging: Based on absolute levels of GFR and albuminuria that are stable over months 1

The Kidney Disease: Improving Global Outcomes (KDIGO) consensus specifically notes that when GFR is changing rapidly during AKI, attribution of stage based on eGFR is not possible 1. This is because:

• eGFR equations were developed and validated in patients with stable kidney function
• During AKI, serum creatinine has not reached steady state
• The relationship between serum creatinine and GFR becomes unpredictable during acute changes 2

Appropriate Assessment in AKI

Instead of using eGFR staging, AKI should be assessed using established criteria:

1. KDIGO AKI Staging:

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

2. Urine output criteria should be used in conjunction with creatinine criteria:

   • Stage 1: <0.5 mL/kg/h for 6-12h
   • Stage 2: <0.5 mL/kg/h for ≥12h
   • Stage 3: <0.3 mL/kg/h for ≥24h or anuria for ≥12h 3

When to Consider eGFR Assessment

eGFR staging becomes appropriate only after AKI has stabilized:

• After the acute phase of AKI has resolved (typically >7 days)
• When serum creatinine has stabilized
• When transitioning from AKI management to potential CKD management 1

Evidence Against Using eGFR in AKI

Research has demonstrated that eGFR equations perform poorly in AKI:

• A study comparing eGFR equations to measured 4-hour creatinine clearance in critically ill patients with AKI found unacceptable bias and percentage error 2
• Adding cystatin C to estimates did not improve accuracy 2
• None of the eGFR equations met acceptable standards of accuracy that are quoted for patients with CKD 2

Clinical Implications and Recommendations

1. For medication dosing in AKI: Use measured creatinine clearance or specific AKI-based dosing adjustments rather than eGFR-based adjustments 4, 5

2. For monitoring kidney function in AKI: Track absolute and relative changes in serum creatinine and urine output rather than eGFR 3

3. For post-AKI follow-up: Schedule follow-up within 3 months to assess for development of CKD, at which point eGFR staging becomes appropriate 3

4. For research purposes: Use standardized AKI definitions (KDIGO, AKIN, RIFLE) rather than eGFR-based classifications 6

Common Pitfalls to Avoid

• Using eGFR to determine medication dosing in AKI can lead to inappropriate dosing
• Relying on eGFR to assess severity of AKI may underestimate or overestimate kidney dysfunction
• Failing to recognize that AKI can progress to acute kidney disease (AKD) and eventually CKD, requiring different assessment approaches at each stage 1
• Overlooking that patients with pre-existing CKD who develop AKI have worse outcomes and require closer monitoring 7

By using appropriate AKI staging criteria rather than eGFR staging, clinicians can more accurately assess kidney function, guide management decisions, and improve patient outcomes in the setting of acute kidney injury.