- Enter age (18 years or older for the side-by-side comparison) and sex.
- Enter a standardized (IDMS-traceable) serum creatinine, a standardized cystatin C, or both. Results update as you type.
- Optionally add height and weight to see absolute GFR in mL/min (de-indexed from 1.73 m²) for drug dosing.
All computation runs in your browser; no values are stored or transmitted beyond your own device.
When to Use
Use in a stable adult when you want to know whether the estimating equation itself changes the KDIGO G category: at a referral or drug-dosing threshold (60, 45, 30, 15 mL/min/1.73 m²), when a cystatin C result is available, or when explaining why a European laboratory report differs from a CKD-EPI report.
When NOT to rely on it
No static equation is valid in acute kidney injury (AKI) or any non-steady state, on dialysis, in pregnancy, or at extremes of muscle mass, diet, or illness. Use a kinetic eGFR in AKI and a measured GFR (iohexol, iothalamate, or radionuclide clearance) when a high-stakes decision depends on the number.
Pearls & Pitfalls
How EKFC is built
EKFC divides the biomarker by Q, the median level in healthy people of that age and sex, then applies one curve: GFR is stable while creatinine rises before age 40, and GFR declines while creatinine is flat after 40. Q for creatinine is 80 µmol/L (0.90 mg/dL) in men and 62 µmol/L (0.70 mg/dL) in women from age 25. Q for cystatin C is the same in men and women (0.83 mg/L below age 50), which makes EKFC cystatin C race- and sex-free (Pottel et al., 2021, 2023).
The 2026 sex-specific shift
In three European cohorts (more than 9000 people), switching from CKD-EPI to EKFC creatinine moved men and women alike toward more severe categories. Switching to EKFC cystatin C or combined moved women, much more than men, to milder categories: women in G3a were 1.6 to 2.7 times more likely than men to shift to G2 in the CKD cohorts, and up to 5.7 times in the population cohort. Kidney failure prediction was equivalent once age, sex, and albuminuria were in the model (Schultheiss et al., 2026).
Pitfalls
(1) Better outcome prediction does not prove a more accurate GFR: dialysis start is a clinical decision, not a filtration measurement (Delanaye & Agarwal, 2026). (2) Cystatin C is falsely high (eGFR falsely low) with systemic glucocorticoids, obesity, inflammation, smoking, and thyroid disease; creatinine misleads with low or high muscle mass and with drugs that block its secretion (trimethoprim, cimetidine, dolutegravir, cobicistat). (3) Even combined equations place only about 85% to 90% of estimates within 30% of measured GFR.
EKFC and CKD-EPI eGFR Comparison
Next Steps
- Equations agree on the category: stage and act on the combined value when you have both markers (KDIGO 2024 favors eGFR from creatinine and cystatin C when confirmation matters).
- Equations straddle a threshold that changes management (referral, metformin or SGLT2 inhibitor initiation, drug dose band): use the combined estimate, repeat the tests, and consider a measured GFR before acting.
- Creatinine and cystatin C disagree by more than 30%: look for a non-GFR cause (muscle mass, steroids, thyroid disease, inflammation, interfering drugs); the combined equation is the least biased option.
- Drug dosing: use the absolute GFR (mL/min) at extremes of body size, as in the Carboplatin Calvert calculator; compare with the Cockcroft-Gault calculator when a label specifies creatinine clearance.
- For single-equation pages see CKD-EPI 2021 creatinine and CKD-EPI creatinine–cystatin C; for which test to order, the Which Kidney Test helper.
Evidence
EKFC equation (creatinine or cystatin C)
eGFR = 107.3 × (biomarker/Q)−α × 0.990(age − 40) [age factor only when age > 40], with α = 0.322 when biomarker/Q < 1 and α = 1.132 when biomarker/Q ≥ 1.
| Q value | Men | Women |
|---|---|---|
| Creatinine, age ≥ 25 | 80 µmol/L (0.90 mg/dL) | 62 µmol/L (0.70 mg/dL) |
| Creatinine, age 18–24 | Age-specific polynomial: ln Q = 3.200 + 0.259·age − 0.543·ln(age) − 0.00763·age² + 0.0000790·age³ (men); 3.080 + 0.177·age − 0.223·ln(age) − 0.00596·age² + 0.0000686·age³ (women); Q in µmol/L | |
| Cystatin C | 0.83 mg/L below age 50; 0.83 + 0.005 × (age − 50) from age 50 (same for both sexes) | |
EKFC combined = arithmetic mean of EKFC creatinine and EKFC cystatin C, which was more accurate than either alone (Pottel et al., 2023).
CKD-EPI comparators
Creatinine: CKD-EPI 2021 race-free. Cystatin C alone: CKD-EPI 2012. Combined: CKD-EPI 2021 creatinine–cystatin C (Inker et al., 2021).
Reclassification (Schultheiss et al., 2026)
| Cohort | Switch to | Women vs men shifting to a milder category |
|---|---|---|
| NURTuRE-CKD (n = 2589) | EKFC combined | G3a→G2: 13% vs 8% (ratio 1.62); G3b→G3a: 18% vs 10% (1.8) |
| NURTuRE-CKD | EKFC cystatin C | G3a→G2 ratio 2.7; G3b→G3a ratio 3.0 |
| RENIS-3 (population, measured GFR) | EKFC cystatin C | G3a→G2 ratio 5.7; G3b→G3a ratio 5 |
| All three cohorts | EKFC creatinine | No sex-preferential shift |
In the German CKD study (n = 5085, 8.5 years), the hazard of kidney failure per 10 mL/min/1.73 m² lower eGFR was 2.14 with CKD-EPI combined and 2.26 with EKFC combined after adjustment for age, sex, and urine albumin-to-creatinine ratio (UACR).
