How do I calculate age-related estimated glomerular filtration rate (eGFR) using serum creatinine?

Calculating Age-Related eGFR Using Serum Creatinine

Use the 2021 CKD-EPI equation without race for calculating eGFR from serum creatinine in adults, which accounts for age and sex but removes the race coefficient. 1

Recommended Equation by Age Group

Adults (≥18 years)

• Primary recommendation: Use the CKD-EPI 2021 equation (creatinine-based, without race variable) 2, 1
  • Variables: serum creatinine, age, sex
  • Race should NOT be included in the computation 1
  • Report eGFR rounded to the nearest whole number in ml/min per 1.73 m² 1
  • Flag values <60 ml/min per 1.73 m² as low 1

Children and Adolescents (≤18 years)

• Use validated pediatric equations developed for comparable populations 1
• The Full Age Spectrum (FAS) equation can be used across all ages including children 3
• Flag eGFR <90 ml/min per 1.73 m² as "low" in children over age 2 years 1

Older Adults (≥70 years)

• Continue using CKD-EPI 2021 equation 1
• Important caveat: In patients with sarcopenia or low muscle mass, creatinine-based eGFR may overestimate kidney function 4
• Consider adding cystatin C for more accurate assessment in older adults, particularly those with suspected sarcopenia 4

Critical Implementation Requirements

Laboratory standards that must be met 1:

• Use enzymatic creatinine assay (preferred over Jaffe method)
• Assay precision: coefficient of variation <2.3% for creatinine
• Calibration traceable to international standard reference materials
• Desirable bias <3.7% compared with reference methodology
• Report creatinine rounded to nearest 0.01 mg/dl (or nearest whole number in µmol/L)
• Separate serum from red blood cells within 12 hours of venipuncture

When Creatinine-Based eGFR Is Inadequate

Consider alternative approaches in these situations:

1. Extremes of body composition (sarcopenia, obesity, amputation): Creatinine-based equations perform poorly 4, 5

2. Type 2 diabetes with preserved renal function: Creatinine-based equations substantially underestimate measured GFR and lack precision 5

3. High-consequence decisions (kidney donation approval, chemotherapy dosing): Consider measured GFR using exogenous markers 6

4. When eGFRcr seems inaccurate: Add cystatin C to calculate eGFRcr-cys (combined equation) for improved accuracy 1, 3

   • The combined CKD-EPI creatinine-cystatin C equation shows superior performance across all age groups 3

Common Pitfalls to Avoid

• Do not rely on serum creatinine alone without using an equation—this fails to account for age, sex, and muscle mass differences 1
• Do not use race coefficients—current guidelines explicitly recommend against this 2, 1
• Do not assume accuracy in healthy individuals—MDRD and older equations systematically underestimate GFR in people with normal kidney function 7
• Do not use the same thresholds for children—pediatric reference ranges differ from adults 1
• Avoid Jaffe creatinine assays in children—higher relative contribution of non-creatinine chromogens causes inaccuracy, especially in neonates 1

Regional Consistency

Use the same equation within your geographical region (continent, country, or region) to ensure consistency in clinical decision-making, though equations may appropriately differ between adults and children 1