What is eGFR from cystatin C?
Every estimate of your kidney function works the same way: take a substance the body makes at a fixed pace and the kidneys filter away, and calculate how much filtration is needed to explain the measured amount in your blood. The ordinary eGFR uses creatinine for that, the waste of your muscles. This variant uses cystatin C, a small protein that almost all cells with a nucleus make at a fixed pace and that the kidneys filter away completely. The pace of production does not depend on how much muscle you have, not on what you eat and not on creatine. That is the whole point. In an average body the two estimates come out at about the same. In a bodybuilder or a creatine user creatinine gives a too low eGFR while the kidneys are healthy, and in a slight older person or someone who has been ill for long a too high one while the kidneys already do less; cystatin C sits closer in both cases. Cystatin C has its own confounders, an overactive thyroid, prednisone, smoking and excess weight lift it, but those are different from creatinine's, and that is why the two complement each other. Your report usually does not print this estimate; you calculate it with the formula on this page, from cystatin C, your age and your sex. The result is in the same unit as your ordinary eGFR, millilitres per minute for a body of standard size, and above 60 is healthy.
Why is eGFR from cystatin C relevant?
For most people this estimate adds little: in an average body creatinine says the same and the ordinary eGFR is enough. It counts for anyone the ordinary eGFR misleads. Anyone with a lot of muscle or using creatine who sees an eGFR just below 60 wants to know whether that is the kidneys or the muscles, and cystatin C gives that answer without having to stop training or creatine. Anyone slight, older, or who has been ill for long and sees a reassuring eGFR wants to know whether that reassurance is real, and there cystatin C sometimes shows a kidney function that sits lower. The second reason is medication. Many medicines are excreted by the kidneys and dosed on the eGFR, and if that estimate is wrong for you, the dose is wrong too. In case of doubt a prescriber or pharmacist may use an estimate from cystatin C instead of the lab value, and then notes that on the prescription. For anyone receiving chemotherapy, antibiotics or anticoagulants dosed on the kidney, that difference can count. The honest limitation is that both estimates calculate with a standard body. With a body surface far from 1.73 square metres, roughly below 1.50 or above 2.00, they both deviate the same way: too low in a large body, too high in a small one. Cystatin C solves the muscle problem, not the build problem.
How eGFR from cystatin C is worked out
What counts beside the number:
- Your ordinary eGFR from the same draw, because the difference between the two is the information
- How much muscle you have and whether you use creatine, because that distorts the ordinary eGFR and not this one
- Your thyroid, prednisone, smoking, excess weight and inflammation, because those distort cystatin C
- Your height and weight, because far from an average build both estimates are off the same way
- Protein in the urine, which with real kidney damage responds earlier than any estimate
- Which medicines you take that are dosed on kidney function
No lab prints a reference value for this estimate, and that is deliberate: the stage of your kidney function is read from the estimate from creatinine, or from the combined estimate from creatinine and cystatin C together. What you do with this number is set it beside your ordinary eGFR. The same limit does apply: above 60 is healthy, and the difference between the two estimates is what counts.
eGFR = 133 × min(cystatin C / 0.8, 1)^−0.499 × max(cystatin C / 0.8, 1)^−1.328 × 0.996^age × 0.932 (if female)
National Kidney FoundationCystatin C in mg/l, age in years, result in ml/min per 1.73 m². The two exponents make the curve run differently below and above 0.8; the factor of 0.996 per year of age and the factor for women sit in the equation itself. No creatinine enters it, and therefore no muscle mass either. That is why the calculator below asks for your sex alongside your cystatin C and your age: it changes the result, not just how you read it.
ml/min/1.73 m²
133 × min( ÷ 0,8; 1)^−0,499 × max( ÷ 0,8; 1)^−1,328 × 0,996^ × 0,932 =
eGFR from cystatin C high or low: what it means
A lower estimate means less is being filtered, or the input distorts. With reduced kidney function both estimates fall, and then the question belongs with it whether it lasts three months and whether there is protein in the urine. If this estimate sits below your ordinary eGFR, look at what lifts cystatin C: an overactive thyroid, prednisone, smoking, excess weight and inflammation; and at a body much larger than average, because then the formula estimates too low. A higher estimate belongs with brisk filtration, or with the same distortion the other way. If this estimate sits above your ordinary eGFR, that is usually the muscle story: a lot of muscle or creatine lowered the ordinary eGFR, and cystatin C shows the kidneys as they are. An underactive thyroid lowers cystatin C and lifts this estimate, and a body much smaller than average does the same. What you do about it: if the two estimates sit close together, they confirm each other and the picture is simple. If they diverge, that difference is the signal, and the question is which input distorts in your case. For the kidneys themselves the levers are the same as with the ordinary eGFR: blood pressure, blood sugar, weight, smoking, salt and going easy on anti-inflammatory painkillers. If the estimate you use deviates from the reported value, that should be noted on the prescription with medication, and whoever prescribes arranges that. Read this estimate beside your ordinary eGFR, because the comparison is the whole point, beside cystatin C itself and its confounders, and beside creatinine and your muscle mass. Do not read a stage from it: the classes belong to the estimate from creatinine, or to the combined estimate from creatinine and cystatin C together.
What lowers it
Less is being filtered, or the input distorts.
| Cause | How often |
|---|---|
| Reduced kidney functionThe reason this estimate exists; then the ordinary eGFR falls too, and protein in the urine belongs with it | Often |
| A markedly different body surfaceWith a body markedly larger than the standard of 1.73 square metres the formula estimates the kidney function too low | Sometimes |
What raises it
Brisk filtration, or the same distortion the other way.
| Cause | How often |
|---|---|
| A generous filtration rateWhat the kidney filters away does not stay behind in the blood; with a lot of muscle or creatine this estimate shows the kidneys as they are | Often |
| A markedly different body surfaceWith a body markedly smaller than the standard the formula estimates the kidney function too high | Sometimes |
How does an eGFR from cystatin C blood test work?
- Referral
- Not needed. The estimate follows from a cystatin C, and you can have that measured yourself; not every lab carries the test, so ask.
- Fasting
- Not needed; cystatin C does not respond to food, nor to a meat meal or training, which is the difference from creatinine.
- When
- At the same moment as your creatinine, because the comparison is the point. Not during a course of prednisone or an infection, because those lift cystatin C.
- The draw
- No extra tube on top of the cystatin C; the estimate is a calculation on that value, with your age and sex, which you make with the formula on this page.
- Repeating
- As often as your ordinary eGFR, and together with it. Anyone receiving medicines dosed on the kidney whose two estimates diverge has it included at every check.
- Where to draw
- You can draw at 250+ locations near you, no referral needed. See the locations
Frequently asked questions
What is the eGFR from cystatin C?
An estimate of how much blood your kidneys filter per minute, calculated from cystatin C instead of from creatinine. Cystatin C is a small protein almost all your cells make at a fixed pace and that the kidneys filter away completely, and that pace does not depend on your muscle mass, your diet or creatine. The result is in the same unit as your ordinary eGFR, so you can set the two side by side.
Can I read my kidney function stage from this?
No. The stages belong to the estimate from creatinine, or to the combined estimate from creatinine and cystatin C together once that exists. This estimate on its own is meant as a second reading: if it sits close to your ordinary eGFR, they confirm each other; if it diverges, that difference is the signal and the question is which input distorts for you.
What does this estimate add to my ordinary eGFR?
Independence from your muscles. Creatinine comes from muscle tissue, so in a bodybuilder or creatine user the ordinary eGFR comes out too low and in a slight older person too high, while the kidneys do the same. Cystatin C sits closer in both cases. It has its own confounders, an overactive thyroid, prednisone, smoking and excess weight, but those are different from creatinine's, and that is why the two complement each other.
When is this estimate useful?
If you have a lot of muscle or use creatine and see an eGFR just below 60, because then cystatin C says whether that is the kidneys or the muscles. If you are slight, older or have been ill for long and see a reassuring eGFR, because then cystatin C sometimes shows a lower kidney function. And with medicines dosed on kidney function, if the ordinary estimate does not seem right for you.
What if the two estimates diverge?
Then the difference is the information. If the estimate from cystatin C sits higher, muscle or creatine probably lowered your ordinary eGFR and the kidneys are better than it showed. If it sits lower, look at what lifts cystatin C: an overactive thyroid, prednisone, smoking, excess weight or inflammation, and at a build far from average. In case of doubt, protein in the urine is the value that says whether there is real kidney damage.
Why is there a body surface in the formula?
Because the estimate is converted to a body of standard size, with a surface of 1.73 square metres, so that a small woman and a large man can be compared. If your body surface sits far from that, roughly below 1.50 or above 2.00 square metres, both estimates are off the same way: too low in a large body, too high in a small one. For the dosing of medicines a pharmacist converts that back where needed.
Does this affect my medication?
It can. Many medicines are excreted by the kidneys and dosed on the eGFR. If a prescriber or pharmacist judges that the ordinary eGFR does not reflect your kidney function well, an estimate from cystatin C may be used, and then the prescription notes that it deviates from the lab value. For anyone receiving chemotherapy, antibiotics or anticoagulants dosed on the kidney, that difference can count.
Can I calculate the estimate myself?
Yes, with the formula on this page, from your cystatin C in mg/L, your age and your sex. The two exponents in the formula make the curve run differently below and above a cystatin C of 0.8, the factor per year of age and the factor for women sit in the equation itself, and no creatinine enters it. Use a cystatin C from the same draw as your creatinine, so the two estimates are about the same moment.
Which tests include eGFR from cystatin C?
eGFR from cystatin C is part of Advanced, the broader panel. It is not orderable yet.
Related biomarkers
Sources
- 1.KDIGO, Clinical Practice Guideline for the Evaluation and Management of CKD. 2024. kdigo.org
- 2.National Kidney Foundation, the CKD-EPI cystatin C equation (2012). 2012. kidney.org
- 3.NHG guideline on chronic kidney disease (M109), Dutch College of General Practitioners. 2026. richtlijnen.nhg.org
- 4.Unilabs, Bepalingenklapper: cystatine C. 2026. bepalingenklapper.nl
- 5.Star-shl, Labbepalingen: cystatine C. 2026. star-shl.nl
Educational information only, not medical advice. Consult a healthcare professional for clinical decisions.
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Core membership
eGFR from cystatin C is part of Advanced, the broader panel. It is not orderable yet.
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