- Select creatinine units (mg/dL or µmol/L) to match your lab report. Switching units clears the creatinine field.
- Enter the patient's age in whole years (18–110) and select sex. Race is not required.
- Enter serum creatinine from the most recent blood draw.
- Enter cystatin C in mg/L. Normal range is approximately 0.5–1.0 mg/L; values above 1.0 suggest reduced GFR.
- Results update automatically: combined eGFR (mL/min/1.73 m²), KDIGO CKD stage, a plain-language interpretation, and a stage-specific action recommendation.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use the CKD-EPI 2021 creatinine–cystatin C equation whenever you need the most accurate eGFR from routine bloodwork in an adult (age ≥18). Combining two independent filtration markers substantially reduces the error that affects each marker alone. This calculator is particularly valuable when creatinine-only eGFR is suspected to be unreliable.
Best situations to use this combined equation
- Adults with unusually high or low muscle mass: very muscular athletes, bodybuilders, sarcopenic or frail elderly, amputees
- Patients with neuromuscular diseases, prolonged immobility, or severe malnutrition — conditions where creatinine production is atypically low
- Confirming a borderline creatinine-only eGFR (e.g., 58–72 mL/min) before a CKD diagnosis is assigned
- Pre-transplant donor or recipient evaluation requiring the most accurate GFR estimate short of a measured GFR
- Any setting where race-based correction is not desired — the 2021 equation is fully race-free
When NOT to use this calculator
Do not apply in acute kidney injury (AKI) — both creatinine and cystatin C lag behind rapidly changing GFR and will systematically underestimate true function. Do not use in children under 18; use the CKiD U25 equation for ages 1–25 instead. A single eGFR value, no matter how accurate, cannot establish CKD — KDIGO requires evidence of structural or functional abnormality persisting for more than three months.
Pearls & Pitfalls
Use the combined equation when creatinine is unreliable
When muscle mass deviates significantly from average — sarcopenic elderly, amputees, bodybuilders, or patients with neuromuscular disease — the combined creatinine + cystatin C eGFR is substantially more accurate than creatinine alone. Adding cystatin C to the workup costs little and eliminates much of the ambiguity in borderline GFR estimates.
Cystatin C can be elevated for non-renal reasons
High-dose corticosteroids increase cystatin C production independently of GFR, causing the equation to overestimate renal impairment. Hyperthyroidism also raises cystatin C. When a patient is on systemic steroids or has thyroid disease, interpret the combined eGFR cautiously and consider measured GFR if precision matters.
Pitfalls
(1) Neither creatinine nor cystatin C tracks acute changes in GFR quickly — both lag by 24–48 hours during AKI or rapid recovery. In AKI, use clinical judgement and serial trends, not eGFR equations. (2) A single eGFR, however accurate, cannot establish CKD — KDIGO requires abnormalities to persist for more than 3 months. (3) Cystatin C assays must be calibrated to the international IFCC reference material (ERM-DA471/IFCC) for the 2021 equation to apply; older assays may produce systematically different values.
Why Use It
Creatinine is generated by muscle breakdown at a fairly constant rate, making it a reasonable filtration marker in average adults. However, its production varies widely with muscle mass, dietary protein, and certain medications — so creatinine alone can overestimate GFR in highly muscular individuals and underestimate it in the sarcopenic or malnourished.
Cystatin C is produced at a constant rate by all nucleated cells regardless of muscle mass, making it a more reliable filtration marker in extremes of body composition. Its main limitation is that its production is elevated by corticosteroids, thyroid dysfunction, and malignancy — factors that can cause it to overestimate renal impairment.
The CKD-EPI 2021 combined equation exploits the strengths of both markers while partially canceling out each one's weaknesses. In the NEJM 2021 derivation and validation study, the combined equation had a mean bias of approximately 0 mL/min/1.73 m² and the tightest P30 accuracy (percentage of estimates within 30% of measured GFR) of any equation tested — outperforming creatinine alone and cystatin C alone across all race groups when race was not included as a variable.
eGFR — CKD-EPI 2021 Creatinine–Cystatin C
Enter the patient's age, sex, serum creatinine, and cystatin C to compute the combined eGFR and CKD stage. Race is not used in this equation.
⚕ Estimate only — interpret in full clinical context. Equation: Inker LA, Eneanya ND, Coresh J, et al. New Creatinine- and Cystatin C–Based Equations to Estimate GFR without Race. N Engl J Med 2021;385:1737–1749. eGFR requires ≥3 months of stability to define chronic CKD; a single value is not a diagnosis. Discuss results with your physician.
Next Steps
Use the result to support — not replace — clinical judgment.
- Interpret the value against the targets shown in the calculator and the Evidence section below, in the context of the full clinical picture.
- Trend serial measurements rather than acting on a single result; confirm abnormal or unexpected values before changing management.
- Apply the relevant KDIGO / specialty-guideline threshold and document the indication.
- Escalate or refer to nephrology when results are out of range, rapidly changing, or discordant with the clinical picture — and discuss the implications with the patient.
Evidence & References
Formula & Equations
The CKD-EPI 2021 creatinine–cystatin C equation (race-free) is:
| Component | Value / Expression |
|---|---|
| Full equation | eGFR = 135 × min(Scr/κ, 1)α × max(Scr/κ, 1)−0.544 × min(CysC/0.8, 1)−0.323 × max(CysC/0.8, 1)−0.778 × 0.9961Age × (0.963 if female) |
| κ (kappa) | 0.7 for females; 0.9 for males |
| α (alpha) | −0.219 for females; −0.144 for males |
| Creatinine units | mg/dL (µmol/L ÷ 88.4 to convert) |
| Cystatin C units | mg/L |
| Race coefficient | Not included (race-free, 2021 revision) |
KDIGO CKD Stage Reference
| Stage | eGFR (mL/min/1.73 m²) | Description |
|---|---|---|
| G1 | ≥90 | Normal or high — CKD only if kidney damage markers present ≥3 months |
| G2 | 60–89 | Mildly decreased — CKD only if kidney damage markers present ≥3 months |
| G3a | 45–59 | Mild-to-moderate decrease |
| G3b | 30–44 | Moderate-to-severe decrease |
| G4 | 15–29 | Severely decreased — prepare for kidney replacement therapy |
| G5 | <15 | Kidney failure |
Evidence & References
The CKD-EPI 2021 creatinine–cystatin C equation was derived and validated in a pooled analysis of 12 studies involving 5,017 participants with measured GFR. The combined equation achieved the highest overall accuracy (P30 = 85–91%) across subgroups defined by race, sex, age, and diabetes status — outperforming both single-marker equations. The 2021 update removed race as a variable following evidence that the race coefficient reflected structural inequity in health-care access rather than a true biological difference in creatinine generation.
- Inker LA, Eneanya ND, Coresh J, et al. New Creatinine- and Cystatin C–Based Equations to Estimate GFR without Race. N Engl J Med. 2021;385(19):1737–1749.
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int. 2024.
- Levey AS, Stevens LA, Schmid CH, et al. A new equation to estimate glomerular filtration rate. Ann Intern Med. 2009;150(9):604–612. [Original CKD-EPI creatinine equation]
- Grubb A, Blirup-Jensen S, Lindstrom V, et al. First certified reference material for cystatin C in human serum. Clin Chem Lab Med. 2010;48(11):1619–1621.
