Cystatin C alone (CKD-EPI 2012): eGFR = 133 × min(Scys/0.8, 1)^−0.499 × max(Scys/0.8, 1)^−1.328 × 0.996^age × 0.932 if female. With creatinine (CKD-EPI 2021): eGFR = 135 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^−0.544 × min(Scys/0.8, 1)^−0.323 × max(Scys/0.8, 1)^−0.778 × 0.9961^age × 0.963 if female; κ = 0.7 (female) or 0.9 (male), α = −0.219 (female) or −0.144 (male).
The result uses the same kidney guideline bands as any eGFR, all in mL/min/1.73m². G1 is 90 or above, G2 is 60 to 89, G3a is 45 to 59, G3b is 30 to 44, G4 is 15 to 29 and G5 is below 15. The bands describe filtration and are not a label on their own.
Chronic kidney disease is defined only when an eGFR below 60, or another sign of kidney damage such as albumin in the urine, lasts more than three months. Kidney guidelines treat the combined creatinine and cystatin C estimate as the more accurate figure when the two markers disagree.
Cystatin C results tend to run lower than creatinine results across a population. In general population studies, 13.7% had an eGFR below 60 with cystatin C, against 9.7% with creatinine, and the cystatin C based results tracked more closely with the risk of death and kidney failure.
Cystatin C is made by nearly every cell at a fairly steady rate, so it depends far less on muscle than creatinine does. That is the main reason to test it, although it is not free of other influences. Because muscle barely enters the picture, it is most useful in people whose muscle mass is far from average, such as strength athletes, people who lost muscle during illness, or older adults who have lost weight.
In a study of more than 8,000 adults, older age, male sex, greater weight and height, current smoking and higher CRP were each linked to higher cystatin C after allowing for measured clearance. Inflammation, smoking and body size can therefore lower the cystatin C estimate without any change in filtration.
Kidney guidelines also list thyroid problems and steroid medicines such as glucocorticoids among the factors that can shift cystatin C. When creatinine is added, everything that moves creatinine, from large meat meals to creatine supplements and very high or low muscle mass, enters the combined result too.
Cystatin C is often ordered to check a borderline creatinine result, so it is most useful drawn at the same time as creatinine, when both can go into the combined equation. A repeat is more informative once any recent infection or inflammatory flare has settled, since these can raise cystatin C.
As with any eGFR, a low result is usually confirmed over three months before chronic change is assumed. A sudden large drop, or a low result with swelling, a big change in urine output or feeling unwell, is worth raising with a clinician promptly.
When cystatin C and creatinine estimates differ widely, guidelines lean on the combined figure, and it is worth looking for reasons behind the gap, such as unusual muscle mass, a recent illness or a medicine that affects one marker more than the other. A large gap that persists is itself worth discussing with a clinician.
Both equations assume stable kidney function and were developed in adults, so they are less reliable during acute illness, in children and in pregnancy. Mean measured filtration in the 2012 development and validation groups was 68 and 70, and accuracy at the extremes is less well studied.
Cystatin C alone was not more accurate than creatinine alone in the 2012 work. In validation, 14.1% of cystatin C only estimates were more than 30% off measured filtration, against 12.8% for creatinine. The gain came from combining them, which brought that figure down to 8.5%.
The equations were developed with cystatin C and creatinine assays traceable to reference materials. Results from older or non-standardized methods may not fit the equations, so it helps to know your lab follows the international standard. Cystatin C is also not part of a standard metabolic panel, so it usually has to be requested specifically.
Take a 55 year old man with a cystatin C of 1.2 mg/L. Divided by 0.8 that is 1.5, above 1, so the max term applies. 1.5 raised to the power of minus 1.328 gives 0.584, and 0.996 to the power of 55 gives 0.802. Multiplying 133 by 0.584 and 0.802 gives about 62 mL/min/1.73m².
Now add a creatinine of 1.1 mg/dL. Divided by 0.9 that is 1.22, which gives a factor of 0.897. The cystatin C factor is 1.5 to the power of minus 0.778, or 0.729, and the age factor is 0.9961 to the power of 55, or 0.807. 135 times those three numbers gives about 71 mL/min/1.73m².
For the same man, creatinine alone in the 2021 equation gives about 79. All three results sit in the G2 band, but they spread across 17 points. The combined figure of 71 is the one guidelines lean on, and it is read with urine albumin and earlier results.
Creatinine eGFR is the routine test, cheap and on every metabolic panel. Its weakness is muscle, because very muscular people can read low and frail people can read high. Cystatin C sidesteps most of that, at the cost of its own sensitivity to inflammation, smoking and body size.
The 2021 combined equation without race was more accurate than the new creatinine only or cystatin C only equations, and it narrowed the accuracy gap between Black and non-Black participants. In the 2012 work, among people with a creatinine eGFR of 45 to 59, the combined equation correctly reclassified 16.9% as filtering at 60 or higher.
Cockcroft-Gault creatinine clearance is a different measure in mL/min that uses weight and is not scaled to body size. A measured GFR with a filtered tracer remains the reference when an exact figure is needed, but it is rarely done outside specialist care.
The Kidney Function panel measures the values this calculator uses. Related: cystatin c, creatinine, egfr.
A GFR calculator using cystatin C estimates how much blood your kidneys filter each minute from one blood protein, cystatin C, together with your age and sex. The calculator on this page runs the CKD-EPI 2012 cystatin C equation, plus a combined creatinine and cystatin C version, and returns an estimated GFR in mL/min/1.73 m².
The result is an estimate, not a measurement. Its value is that cystatin C behaves differently in the body from creatinine, so reading the two estimates together usually says more than either one alone.
The calculator converts a lab value into an estimated glomerular filtration rate using a published regression equation, not a direct measurement of filtration. In cystatin C only mode it needs three things.
In combined mode it also takes serum creatinine in mg/dL. Both equations share the same shape. Each marker has a hinge point, 0.8 mg/L for cystatin C and 0.7 or 0.9 mg/dL for creatinine depending on sex, and the equation applies a gentler slope below that hinge and a steeper one above it. That two-slope design exists because the relationship between a filtration marker and true GFR is not a straight line. A small rise at the low end of the scale reflects a large drop in filtration, while changes deep in the abnormal range move the estimate less.
Age enters as a mild decay factor, reflecting the gradual population-level decline in filtration with age. The output is indexed to a body surface area of 1.73 m², which is what makes one person's eGFR comparable to another's.
Two practical conditions sit behind the arithmetic. The cystatin C assay should be standardised to the international certified reference material, because unstandardised assays shift results, and the blood sample should be drawn at a steady state rather than during a rapidly changing illness. Published comparisons of creatinine, cystatin C and the equations built on them note that estimating equations perform best when assay calibration and clinical stability are both in place 1.
Creatinine is a breakdown product of creatine in skeletal muscle, so how much of it you produce depends on how much muscle you carry, what you eat and how you train. Cystatin C is a small protein produced by essentially all nucleated cells at a relatively steady rate, filtered freely at the glomerulus, then reabsorbed and broken down in the tubules rather than returned to the blood 2. That production pattern is what makes it far less sensitive to body composition 3.
The practical consequence shows up in the people whose creatinine was never going to be a fair reading of their kidneys. A heavily muscled lifter can produce enough creatinine to look like reduced filtration on paper. An older adult with low muscle mass, a person recovering from prolonged illness, someone with limb loss, or a long-term vegetarian may produce so little creatinine that an impairment goes unnoticed. Creatine-containing supplements and a recent high-meat meal can also nudge creatinine upward without any change in filtration.
This is not a case of one marker being correct and the other wrong. A long-standing literature review on the question concluded that cystatin C performs at least comparably to creatinine as a filtration marker, and it is best read as a complement rather than a replacement 4.
The combined creatinine and cystatin C equation is generally the most accurate of the three, which is why it is commonly used as a confirmatory step rather than a first-line screen. Kidney guidance has long recommended cystatin C testing when a creatinine-based estimate sits near a decision point and the stakes of getting it wrong are real, for example when eligibility for a procedure, a clinical trial or a medication adjustment depends on the number 3.
Several clinical situations push clinicians toward the combined version.
The transplant literature also illustrates that guideline recommendations are debated and refined over time rather than fixed, including discussion of how a cystatin C based recommendation should be applied in transplant care 6.
An eGFR result lands in one of the widely used kidney function categories, all expressed in mL/min/1.73 m². These are descriptive bands, not diagnoses.
90 and above. Normal or high filtration for most adults. Taken alone this says filtration looks preserved on the day of the draw. It does not rule out kidney disease, because protein in the urine can be present with a normal eGFR.
60 to 89. Mildly reduced filtration. In many adults, particularly with age, this is unremarkable. It is worth tracking over time rather than reacting to once.
45 to 59. Mildly to moderately reduced. This is the band where a cystatin C based or combined estimate most often changes the interpretation of a creatinine result, and it is worth discussing with a clinician alongside a urine albumin measurement.
30 to 44. Moderately to severely reduced, and a suboptimal result that warrants clinical assessment rather than watchful waiting.
15 to 29. Severely reduced filtration. Worth discussing with a clinician promptly.
Below 15. Kidney failure category. Worth discussing with a clinician promptly.
Two caveats matter more than the bands themselves. First, a chronic kidney disease label requires an abnormality that persists for at least three months, so one result is a snapshot, not a conclusion. Second, filtration categories are usually read alongside urine albumin, because the two together describe risk far better than either alone. Research suggests that cystatin C based estimates of filtration are at least as informative as creatinine based estimates for predicting adverse outcomes including death and progression to kidney failure 7.
Cystatin C is less dependent on muscle than creatinine, but it is not immune to influences outside the kidney. Factors described in the clinical literature as capable of shifting cystatin C independently of filtration include thyroid status, with hyperthyroidism tending to raise levels and hypothyroidism tending to lower them, glucocorticoid exposure, systemic inflammation, obesity, smoking and some malignancies 3. An estimate taken during an acute illness, when the marker has not yet reached a steady state, can also lag behind what the kidneys are actually doing.
There is also a described pattern in which cystatin C based eGFR sits markedly below creatinine based eGFR because of altered glomerular sieving of small proteins rather than a difference in filtration alone, studied under the name shrunken pore syndrome 8.
When the two estimates disagree, the disagreement itself carries information. A 2025 meta-analysis found that discordance between creatinine-based and cystatin C-based eGFR is associated with worse clinical outcomes than concordant results 9. The sensible response to a gap between the two numbers is to bring it to a clinician, not to pick the friendlier figure.
Read a cystatin C eGFR next to serum creatinine first, because the comparison is the point. Cystatin C on its own tells you what one filtration marker is doing, while the pair tells you whether body composition, diet or a non-kidney influence is distorting the picture.
Beyond creatinine, a few results round out the view.
Trend matters more than any single draw. Two results separated by months, run on the same standardised assay, say far more than one number in isolation.
FitScript measures cystatin C within its Organ Health panel, alongside the creatinine, urea, electrolyte and liver markers that give a filtration estimate its context. Membership covers 150+ biomarkers tested per year, with results interpreted by Atlas, FitScript's AI health advisor, and any treatment decision made by a licensed provider after reviewing the member.
The practical advantage of testing both filtration markers in the same draw is simple. You get the creatinine estimate, the cystatin C estimate and the combined estimate from one blood sample, at the same point in time, which is the only fair way to compare them.
Bring your results to a clinician if an eGFR sits below 60 mL/min/1.73 m², if the creatinine and cystatin C estimates differ substantially, if a previously stable estimate has fallen, or if a urine albumin result is elevated regardless of what the eGFR shows. A result in the severely reduced bands is worth discussing with a clinician promptly rather than at the next routine visit.
Also flag anything that could be distorting the marker, including thyroid disease, a recent course of steroid medication, an active infection or a rapidly changing illness, since a licensed provider evaluates whether the estimate is trustworthy in your situation and whether repeat or formal measured GFR testing is appropriate. This page is educational and is not medical advice, and no calculator output should be used to diagnose a condition or to change treatment on your own.
If you want a deeper comparison of the two filtration markers, see why is cystatin c better than creatinine.
It is a second filtration marker. Combining it with creatinine gave the most accurate eGFR in the CKD-EPI studies.
CKD-EPI 2012 for cystatin C alone, and the race-free CKD-EPI 2021 creatinine and cystatin C equation when you add creatinine.
No. The cystatin C equation works alone; adding creatinine from the same blood draw gives the combined estimate.
It uses the CKD-EPI 2012 cystatin C equation when you enter cystatin C, age and sex, and the CKD-EPI 2021 combined creatinine and cystatin C equation when you also enter a serum creatinine. Both return an eGFR indexed to a body surface area of 1.73 m².
Many adult laboratory reference intervals fall roughly between 0.5 and 1.0 mg/L, but the interval varies by lab, assay, age and sex. Use the reference range printed on your own report rather than a general figure, since the calculator result depends on how the assay was calibrated.
It is usually more accurate in people whose muscle mass, diet or body composition distorts creatinine, and the combined creatinine and cystatin C equation generally performs better than either marker alone. Neither estimate measures filtration directly, so both are read against the clinical picture.
The two markers are produced and handled differently, so muscle mass, diet, thyroid status, inflammation or steroid exposure can move one without moving the other. A meaningful gap between the two estimates is worth discussing with a clinician, because the discordance itself is clinically informative.
No. The CKD-EPI 2012 cystatin C equation and the CKD-EPI 2021 combined equation do not include a race coefficient. The inputs are cystatin C, age, sex and, in combined mode, serum creatinine.
Cystatin C itself does not generally require fasting. If it is drawn as part of a wider panel that includes glucose, insulin or a lipid profile, the fasting instructions for those markers apply, so follow the guidance given with your lab requisition.
Heavy training, a high-meat meal and creatine-containing supplements can raise serum creatinine without any change in kidney filtration, which inflates or deflates a creatinine-based estimate. Cystatin C is far less affected by these, which is one reason the two markers are read side by side.
No. A chronic kidney disease assessment requires a kidney abnormality that persists for at least three months, usually interpreted alongside urine albumin. One calculator output is a snapshot, and a low or falling result should be repeated and reviewed with a clinician rather than self-interpreted.