Why is it ideal to assess both creatinine and estimated Glomerular Filtration Rate (eGFR) when evaluating renal function, rather than just eGFR?

Why Both Creatinine and eGFR Are Essential for Renal Function Assessment

When assessing renal function, it is essential to evaluate both serum creatinine and eGFR rather than relying on eGFR alone because this combined approach provides a more comprehensive assessment of kidney function and helps identify potential sources of error in GFR estimation. 1

Limitations of Using eGFR Alone

• Non-GFR determinants affect accuracy: eGFR calculations are influenced by factors that don't reflect actual kidney function:

  • Muscle mass variations (extremely high or low muscle mass)
  • Dietary intake (high protein or creatine supplements)
  • Medications affecting tubular secretion of creatinine
  • Age, gender, and race-related differences 1, 2

• Clinical context may be missed: eGFR without creatinine values may obscure important clinical information about:

  • Acute changes in kidney function
  • Potential laboratory errors
  • Specific patient factors affecting creatinine generation 1

Benefits of Evaluating Both Parameters

1. Identification of Discrepancies

• Comparing raw creatinine with eGFR helps identify situations where eGFR may be inaccurate
• Large discrepancies between creatinine trends and eGFR changes warrant further investigation 2

2. Better Clinical Decision-Making

• KDIGO guidelines recommend using serum creatinine with an estimating equation for initial assessment of GFR 1
• Understanding both values helps clinicians interpret results in context of the patient's clinical situation 1

3. Detection of Non-GFR Influences

• Unusual creatinine values relative to eGFR may indicate:
  • Changes in muscle mass
  • Dietary influences
  • Medication effects
  • Laboratory errors 2

4. Improved Monitoring of Disease Progression

• Trends in both parameters provide better insight into kidney disease progression
• Helps distinguish true GFR changes from variations in creatinine production 1

When Additional Testing Is Needed

When discrepancies between creatinine and eGFR are identified, consider:

1. Combined creatinine-cystatin C estimation (eGFRcr-cys):

   • Recommended when eGFRcr is less accurate and GFR affects clinical decision-making 1
   • More accurate than either creatinine or cystatin C alone when discordances exist 3

2. Measured GFR (mGFR):

   • Consider when highest accuracy is needed for critical clinical decisions
   • Uses plasma or urinary clearance of exogenous filtration markers 1

Clinical Algorithm for Renal Function Assessment

1. Initial Assessment: Measure serum creatinine and calculate eGFR
2. Evaluate Consistency: Check if creatinine level aligns with expected eGFR
3. If Discrepancy Detected:
   • Consider non-GFR determinants affecting creatinine
   • Measure cystatin C and calculate eGFRcr-cys
4. For Critical Decisions: Consider measured GFR when highest accuracy is needed

Common Pitfalls to Avoid

• Relying solely on eGFR: May miss important clinical context provided by raw creatinine values
• Ignoring trends: Single measurements of either parameter can be misleading
• Failing to consider non-GFR determinants: Diet, medications, and muscle mass significantly impact creatinine levels
• Not adjusting for body surface area: For medication dosing, eGFR may need adjustment for patients with body surface areas significantly different from 1.73 m² 4

By evaluating both serum creatinine and eGFR together, clinicians can make more informed decisions about kidney function, detect potential sources of error, and provide more personalized care to patients with kidney disease.