How do I calculate my estimated Glomerular Filtration Rate (eGFR)?

How to Calculate Estimated Glomerular Filtration Rate (eGFR)

Use a validated GFR estimating equation with serum creatinine rather than relying on creatinine levels alone, with the 2021 race-free CKD-EPI creatinine equation being the preferred formula for most adults. 1

Initial Assessment Method

• Obtain serum creatinine measurement using an enzymatic assay calibrated to international reference standards with minimal bias compared to isotope-dilution mass spectrometry 1
• Apply the 2021 CKD-EPI creatinine equation (eGFRcr) to calculate eGFR from serum creatinine, incorporating age and sex without race 1
• Clinical laboratories should automatically report eGFR alongside serum creatinine values, rounded to the nearest whole number and expressed as mL/min/1.73 m² 1

The 2021 race-free CKD-EPI equation is currently recommended over older formulas like MDRD or the race-based CKD-EPI equations 1. The European Kidney Function Consortium (EKFC) equations are acceptable alternatives if shown to improve accuracy 1.

When Standard eGFRcr Calculation is Unreliable

Recognize clinical situations where creatinine-based eGFR is inaccurate and requires alternative approaches 1, 2:

• Extremes of muscle mass (very high or very low) alter creatinine generation independent of kidney function 1, 2
• Class III obesity (BMI >40 kg/m²) reduces eGFRcr accuracy 2
• Advanced cirrhosis or cancer with high cell turnover creates high catabolism/inflammation states 1, 2
• Severe malnutrition or muscle wasting conditions compromise accuracy 2
• High protein or meat intake affects creatinine levels and requires interpretation consideration 1, 2

Enhanced Calculation for Improved Accuracy

When eGFRcr is expected to be inaccurate and GFR affects clinical decision-making, measure cystatin C and calculate eGFRcr-cys (combined creatinine-cystatin C equation) 1, 2:

• The combined CKD-EPI creatinine-cystatin C equation yields more accurate GFR estimates overall, particularly in persons with large discordances between creatinine-based and cystatin C-based estimates 1
• Combined assays demonstrate coefficients of variation below 10% and biases below 0.9% with improved agreement across manufacturers 1
• This approach is specifically recommended for CKD screening or confirmation when conditions affect creatinine generation or tubular secretion 1

Most Precise Measurement Methods

For critical clinical decisions requiring precise GFR (such as kidney-cleared chemotherapy dosing), measure GFR using plasma or urinary clearance of exogenous filtration markers 1, 2:

• Measured GFR (mGFR) using exogenous markers provides the gold standard when eGFRcr-cys remains potentially inaccurate 1
• Timed urine collections for measured creatinine clearance can be considered if mGFR is unavailable and eGFRcr-cys is thought inaccurate 1
• These methods are particularly important in patients with radical differences in body composition or critical dosing decisions 1

Practical Implementation Steps

1. Start with serum creatinine and eGFRcr calculation for initial kidney function assessment in most adults 1
2. Evaluate clinical context including muscle mass, body composition, dietary intake, inflammatory states, and medication use 1, 2
3. Add cystatin C measurement if eGFRcr is likely inaccurate based on clinical characteristics and GFR impacts treatment decisions 1
4. Consider measured GFR when even combined equations may be unreliable or when precise measurement is essential for clinical decision-making 1

Common Pitfalls to Avoid

• Do not rely on serum creatinine concentration alone without applying a validated estimating equation 1
• Avoid assuming eGFRcr accuracy when clinical context suggests altered creatinine generation (extremes of muscle mass, dietary intake, catabolic states) 2
• Do not ignore potential for error when assessing GFR changes over time, as biological and analytical variability exists 1, 2
• Avoid using race in eGFR computation, as current guidelines recommend race-free equations 1