CKD-EPI Cystatin C eGFR Calculator
CKD-EPI Cystatin C eGFR Calculator
Estimate GFR from cystatin C alone, independent of muscle mass — useful when creatinine cannot be trusted.
CKD-EPI Cystatin C eGFR
Cystatin C onlyMale, age 62, cystatin C 1.3 mg/L
Formula
- Scys
- serum cystatin C in mg/L, standardised to the ERM-DA471/IFCC reference material
- 0.8
- the inflection point, analogous to κ in the creatinine equation
- equation vintage
- this is the 2012 CKD-EPI cystatin C equation — the 2021 revision changed the creatinine and combined equations only
Worked example
Male, age 62, cystatin C 1.3 mg/L
Scys/0.8 = 1.3 ÷ 0.8 = 1.625
133 × 1 × 1.625^−1.328 × 0.996^62 = 54 mL/min/1.73 m²
When cystatin C helps
| Situation | Why creatinine misleads |
|---|---|
| Cirrhosis, cachexia, amputation | Low muscle mass gives a falsely low creatinine and falsely high eGFR |
| Bodybuilders, high muscle mass | Falsely high creatinine and low eGFR |
| Paraplegia, neuromuscular disease | Reduced creatinine generation |
| Vegetarian or vegan diet | Lower creatinine generation from reduced meat intake |
| Trimethoprim, cimetidine, dolutegravir | Block tubular creatinine secretion without changing GFR |
Where cystatin C earns its place
Cystatin C is a small protein produced at a fairly constant rate by all nucleated cells, filtered freely at the glomerulus and then reabsorbed and catabolised by the proximal tubule rather than being excreted. It is not appreciably affected by muscle mass, diet or tubular secretion, which is precisely where creatinine fails.
That makes it the test of choice when creatinine cannot be trusted: cirrhosis, cachexia, amputation, paraplegia, an unusually muscular patient, a vegan diet, or a drug such as trimethoprim or dolutegravir that blocks creatinine secretion and raises the measured value without touching real GFR. In each case the creatinine-based estimate is wrong in a predictable direction, and cystatin C is not.
It is not a universally better test. Corticosteroids, thyroid dysfunction, obesity, smoking and systemic inflammation all move cystatin C independently of GFR, so it has its own confounders — they are simply different ones. Assay standardisation was also a historical problem, now largely resolved by the ERM-DA471/IFCC reference material, but results from before standardisation are not comparable.
KDIGO recommends cystatin C to confirm a creatinine-based eGFR when the result will drive a significant decision. Where both are available, the combined equation outperforms either alone and is the better choice.
Frequently asked questions
Is this the 2021 cystatin C equation?
No — there is no 2021 cystatin-C-only equation. The 2021 CKD-EPI revision changed the creatinine and the combined creatinine-cystatin C equations. The cystatin C equation in use is the 2012 one, which never contained a race term.
When should cystatin C be used instead of creatinine?
When muscle mass is atypical — cirrhosis, cachexia, amputation, paraplegia or high muscle bulk — or when a drug such as trimethoprim, cimetidine or dolutegravir is blocking creatinine secretion.
What raises cystatin C independently of kidney function?
Corticosteroids, hyperthyroidism, obesity, smoking and systemic inflammation. These are its own confounders, distinct from those affecting creatinine.
Is cystatin C better than creatinine?
Not universally. It is better where creatinine is unreliable and worse where its own confounders are present. The combined equation using both outperforms either alone.
Related calculators
References
- Inker LA et al. Estimating glomerular filtration rate from serum creatinine and cystatin C. N Engl J Med. 2012;367(1):20–29.
- KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.