eGFR Calculator (CKD-EPI 2021) With Notes for Athletes

Creatinine-based eGFR is estimated from blood creatinine, and creatinine comes from muscle.3 That is why a muscular or recently training person can get a low creatinine-based eGFR; a low result alone does not show chronic kidney disease, but it does not rule out a real kidney problem either.5,6 This calculator uses the race-free CKD-EPI 2021 creatinine equation,2 and lists the athlete-specific factors next to the result.

Creatinine
Unit
Age (years)
Sex

The equation

CKD-EPI 2021 (without race): eGFR = 142 Γ— min(Scr/ΞΊ, 1)^Ξ± Γ— max(Scr/ΞΊ, 1)^βˆ’1.200 Γ— 0.9938^age Γ— 1.012 [if female], where Scr is serum creatinine in mg/dL, ΞΊ is 0.7 for women and 0.9 for men, and Ξ± is βˆ’0.241 for women and βˆ’0.302 for men.2 Creatinine in Β΅mol/L is divided by 88.4 to get mg/dL.1

Why the result can mislead in athletes

Creatinine-based estimates are affected by non-kidney factors including muscle mass, physical activity, protein intake, age and sex.3 In a muscular person, a creatinine within the reference range can still produce an eGFR flagged as low.6 Cystatin C is much less dependent on muscle mass; measuring it, alone or combined with creatinine in one equation, can help clarify the picture.1,7

References

Every figure and statement in this tool is linked to a source: the original study where my article cites one, otherwise my own article on the topic. The small numbers in the text refer to this list.

  1. Stevens et al. 2024. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International.
  2. Inker et al. 2021. New Creatinine- and Cystatin C–Based Equations to Estimate GFR without Race. New England Journal of Medicine.
  3. Chen et al. 2022. Advantages, Limitations, and Clinical Considerations in Using Cystatin C to Estimate GFR. Kidney360.
  4. Dr. Antti Rintanen: Creatinine in Athletes: When Kidney Function Tests Mislead
  5. Hodgson et al. 2017. Acute kidney injury associated with endurance events-is it a cause for concern? A systematic review. BMJ Open Sport Exerc Med.
  6. Dr. Antti Rintanen: eGFR in Athletes: Why Your Result May Be Misleading
  7. Dr. Antti Rintanen: Cystatin C in Athletes: Why the Standard Kidney Test Misses the Mark
  8. Davani-Davari et al. 2018. Potential Adverse Effects of Creatine Supplement on the Kidney in Athletes and Bodybuilders. Iran J Kidney Dis.
  9. Tidmas et al. 2022. Ultra-Endurance Participation and Acute Kidney Injury: A Narrative Review. Int J Environ Res Public Health.

Sources

This tool is based on the articles below and the studies cited in them:

This tool is for general education. It does not interpret your own results, give personal recommendations or replace an assessment by your doctor.