Why use cystatin C instead of creatinine?

Creatinine depends on muscle mass, so it can mislead in people who are very muscular, frail, amputees, or have liver disease. Cystatin C is produced more uniformly by cells and is less affected by muscle, giving a more reliable GFR estimate in those situations.

Which equations does this use?

It uses the 2021 race-free CKD-EPI cystatin C equation and the 2021 race-free combined creatinine-cystatin C equation, both published by Inker and colleagues. Neither includes a race coefficient, in line with current guidance.

Is the combined equation more accurate?

Yes. Averaging the information from both markers, the combined creatinine-cystatin C equation is the most accurate of the three CKD-EPI estimates and is recommended to confirm a reduced GFR before major decisions such as drug dosing or transplant work-up.

What units does the calculator expect?

Cystatin C is entered in mg per litre. Creatinine can be entered in mg per dL or micromoles per litre, and the tool converts micromoles by dividing by 88.42 before applying the equation. The result is in mL per minute per 1.73 square metres.

Can I use this for chemotherapy or transplant dosing directly?

Use it as an input, not a final answer. eGFR equations estimate a population average and carry uncertainty in any individual. For carboplatin dosing or transplant decisions, follow the protocol's specified GFR method and confirm with a specialist.

Cystatin C-Based eGFR Calculator (CKD-EPI Cys)

Cystatin C is a small protein filtered by the kidneys whose blood level reflects glomerular filtration with much less dependence on muscle mass than creatinine. That makes cystatin C based eGFR especially useful when creatinine is likely to mislead, and the combined equation is the most accurate estimate available without a measured GFR study.

How it works

Both equations are the 2021 race-free CKD-EPI forms. The cystatin C only equation scales a base rate by how far cystatin C sits above or below a reference of 0.8 mg/L, then applies age and sex factors:

eGFR(cys) = 133 x min(Scys/0.8, 1)^-0.499
                x max(Scys/0.8, 1)^-1.328
                x 0.996^age
                x (0.932 if female)

The combined equation uses both markers, with creatinine handled through sex-specific kappa and alpha constants, and is the most accurate of the three.

Staging and when to prefer cystatin C

The result is mapped to KDIGO GFR categories: G1 at 90 or above, G2 from 60 to 89, G3a from 45 to 59, G3b from 30 to 44, G4 from 15 to 29, and G5 below 15. Reach for cystatin C or the combined equation when creatinine alone is suspect, for example in someone with very high or very low muscle mass, before high-stakes drug dosing, or to confirm chronic kidney disease found on a creatinine estimate. The combined equation is the recommended confirmatory tool because it draws on both markers at once.