Cystatin C with eGFR
A kidney filtration marker produced at a constant rate by all nucleated cells — and crucially unaffected by muscle mass, diet or creatine supplementation.
This is the kidney test lifters actually need. Standard creatinine-based eGFR routinely flags muscular people and creatine users as having 'reduced kidney function' when their kidneys are entirely healthy.
Check a Cystatin C with eGFR result against this range →
What Cystatin C with eGFR actually measures — the analyte, and the assay
The analyte is cystatin C: a 13.3 kDa, 120-amino-acid inhibitor of cysteine proteases, produced at a near-constant rate by all nucleated cells. That last clause is the whole design. Creatinine comes from muscle; cystatin C comes from every cell that has a nucleus, so its production is not a function of how much muscle you carry Avila 2025.
Its renal handling is unusual and it matters. Being small and positively charged at physiological pH, cystatin C is freely filtered at the glomerulus and then almost completely reabsorbed and catabolized by the proximal tubule. Essentially none reaches the urine intact, which means there is no such thing as a cystatin C clearance measurement — the serum concentration is the only available signal.
The measurement itself is a latex-enhanced immunoturbidimetric or nephelometric assay: antibody-coated particles agglutinate in proportion to analyte and the change in light scattering is read. Unlike insulin, it has an international certified reference material, and assays calibrated to it are comparable between laboratories. That is why an eGFR built on cystatin C is portable at all, and it is the technical precondition for everything below Delanaye 2023 Pottel 2023.
The number that reaches you is usually two numbers. A concentration in mg/L, and an eGFR calculated from it. The 2021 CKD-EPI work refitted both the cystatin C equation and the combined creatinine-cystatin C equation without a race coefficient, and validated them in 4,050 participants across 12 studies; the combined equation was more accurate than either single-marker equation and showed smaller differences between race groups Inker 2021. If your report gives a cystatin C eGFR without also giving the combined one, it is using the weaker of the two available answers.
Cystatin C with eGFR: what changes the blood, and what only changes the reading
What changes the cystatin C in your blood:
- Glomerular filtration rate. The intended signal, and the dominant term.
- Thyroid status. Hyperthyroidism raises cystatin C production and hypothyroidism lowers it, independently of the kidney. An untreated thyroid is the most under-recognized confounder on this test.
- Glucocorticoids. Corticosteroid exposure raises cystatin C production in a dose-related way — which matters to anyone using them for anything.
- Adiposity and inflammation. Fat mass and circulating inflammatory activity both raise it, so an obese person's cystatin C modestly overstates kidney impairment.
- Smoking, and some malignancies, both upward.
- Age, through the genuine decline in filtration that comes with it Pottel 2023.
What does NOT change it, which is the entire point of ordering it:
- Muscle mass. Cystatin C is not a muscle product Avila 2025. This is why the marker exists and why it is the right kidney test for a heavily muscled reader Tidmas 2022.
- Creatine supplementation. Creatine raises serum creatinine by non-enzymatic conversion and does nothing to cystatin C Longobardi 2023.
- Dietary protein or a cooked-meat meal.
- A hard training session.
- Ketones, bilirubin and the other Jaffe interferents, because this is not a picrate reaction Boss 2023.
What changes only the reading: which manufacturer's calibration was used, and whether it is traceable to the certified reference material; and gross lipemia or an extreme paraprotein, which interfere optically with a turbidimetric endpoint Delanaye 2023.
Reference interval or decision threshold — which kind of number Cystatin C with eGFR is
The concentration is a reference interval. The eGFR derived from it is a decision threshold. They are printed adjacent and they are different kinds of claim. A cystatin C of 0.95 mg/L is inside the middle 95% of a measured population; an eGFR of 62 mL/min/1.73 m² is a position inside an outcome-defined staging system that also requires the abnormality to persist three months Delanaye 2023.
The interval is only as good as the calibration behind it, and this is where cystatin C is on much firmer ground than most of the cohort: because a certified reference material exists, the interval travels between laboratories in a way an insulin interval does not Pottel 2023.
There is no validated 'optimal' cystatin C. A target of 'lower half of the range' is an inference from the continuity of risk, not a threshold anyone tested, and it should be read the same way as the optimal HbA1c — a reasonable goal, not a finding.
One caution about the equations themselves. Estimating equations are fitted to populations, and different consortia have fitted different ones — the CKD-EPI 2021 family and the European Kidney Function Consortium equation among them Inker 2021 Pottel 2023. Two laboratories can print different eGFRs from the same cystatin C, and neither is wrong.
How you would know your Cystatin C with eGFR was wrong — and when to redraw
Cystatin C reaches a new steady state within days of a change in filtration, so the analyte is not what sets the interval. What sets it is the question: three months is the interval that answers 'is this chronic', because that is the duration criterion inside the definition of chronic kidney disease Delanaye 2023. Annually is right for surveillance in someone with risk factors; immediately is right when a creatinine-based eGFR has flagged and the person is muscular Tidmas 2022.
Conditions that must match: the same laboratory and calibration; stable thyroid status; no new or changed corticosteroid; no acute inflammatory illness; and comparable body composition, since adiposity moves it.
What would have to change for the retest to mean something. The internal consistency check is the pair. Run creatinine and cystatin C together and compare the two eGFRs — the combined equation exists because they disagree in informative ways Inker 2021. A large gap has a differential with a clear direction:
- eGFR-creatinine much LOWER than eGFR-cystatin C: high muscle mass, creatine supplementation, a large meat meal, recent heavy training, or a Jaffe method with an interferent Avila 2025 Longobardi 2023 Boss 2023.
- eGFR-creatinine much HIGHER than eGFR-cystatin C: low muscle mass, which is the dangerous direction because creatinine flatters the kidney. In 145 patients with primary neuromuscular disease measured against iohexol clearance, all equations overestimated kidney function, by 22 to 60 mL/min/1.73 m², and the cystatin C estimates carried the smallest bias at 22 Aldenbratt 2022. Or, in the other population that reads this site, corticosteroid exposure, hyperthyroidism or obesity pushing cystatin C up for non-renal reasons.
How you would know the value was wrong. A cystatin C that disagrees with creatinine in someone with ordinary muscle, ordinary thyroid function and no steroid exposure should prompt a urine albumin-to-creatinine ratio and a repeat rather than a diagnosis.
What Cystatin C with eGFR cannot tell you
It is still an estimate, not a measurement. The reference standard is measured clearance of an exogenous marker, and estimating equations built on cystatin C are more accurate than creatinine-based ones in several groups without being accurate in the absolute sense Aldenbratt 2022 Delanaye 2023.
It says nothing about protein in the urine. Chronic kidney disease is defined by filtration or damage, and albuminuria is the damage half. A perfect cystatin C eGFR alongside significant albuminuria is still kidney disease, and this test cannot see it.
It cannot distinguish acute from chronic any better than creatinine can. One value is one moment Delanaye 2023.
Its own non-GFR determinants point the wrong way for the people most likely to buy it. Corticosteroid use, higher fat mass and systemic inflammation all raise cystatin C without any change in filtration — so in a heavy, inflamed, steroid-exposed reader it can under-read kidney function just as creatinine over-reads it. The marker escapes the muscle confound; it does not escape confounding.
It does not tell you why. A genuinely reduced filtration rate has a differential — hypertension, diabetes, NSAIDs, obstruction, glomerular disease — and this test contributes nothing to separating them.
The wrong inference readers actually draw is that a normal cystatin C clears the kidney entirely, so the flagged creatinine can be ignored forever. What it clears is the muscle explanation for that specific creatinine Longobardi 2023. It does not clear albuminuria, it does not clear blood pressure, and in anyone using androgens the relevant kidney risk is a glomerular one that neither filtration marker is designed to detect Tidmas 2022.
Sources read for these sections
- Inker LA, et al. New Creatinine- and Cystatin C-Based Equations to Estimate GFR without Race. New England Journal of Medicine 2021 · PMID 34554658
- Aldenbratt A, et al. Estimation of kidney function in patients with primary neuromuscular diseases: is serum cystatin C a better marker of kidney function than creatinine?. Journal of Nephrology 2022 · PMID 34351595
- Avila M, et al. The Metabolism of Creatinine and Its Usefulness to Evaluate Kidney Function and Body Composition in Clinical Practice. Biomolecules 2025 · PMID 39858438
- Delanaye P, et al. Diagnostic standard: assessing glomerular filtration rate. Nephrology Dialysis Transplantation 2023 · PMID 37950562
- Pottel H, et al. Exploring Renal Function Assessment: Creatinine, Cystatin C, and Estimated Glomerular Filtration Rate Focused on the European Kidney Function Consortium Equation. Annals of Laboratory Medicine 2023 · PMID 37909162
- Boss K, et al. Effect of serum creatinine difference between the Jaffe and the enzymatic method on kidney disease detection and staging. Clinical Kidney Journal 2023 · PMID 37915891
- Longobardi I, et al. Is It Time for a Requiem for Creatine Supplementation-Induced Kidney Failure? A Narrative Review. Nutrients 2023 · PMID 36986197
- Tidmas V, et al. Nutritional and Non-Nutritional Strategies in Bodybuilding: Impact on Kidney Function. International Journal of Environmental Research and Public Health 2022 · PMID 35409969
The plan of attack
In this order. Most people start at step four, which is why they change five things at once and learn nothing.
- Confirm the number is real
Which lab ran it. Cystatin C assays are calibrated against an international reference material, but not every lab and not every platform reports on the same scale. A value from one lab is not reliably comparable to a value from another, and the eGFR equation applied on top magnifies the difference. Trend cystatin C at the SAME lab. A jump between providers is a calibration difference until proven otherwise. - Read it with its partner
The single best money spent by any lifter who's been told their kidneys look off. Draw it alongside: Comprehensive Metabolic Panel (CMP), Urinalysis, Routine. - Work out which direction is yours
If it's high — Real reduction in kidney filtration — far more concerning than an isolated high creatinine, and worth prompt follow-up. - Fix it in this order
Nutrition. Adequate hydration; moderate sodium if hypertensive. High protein and creatine raise creatinine without harming kidneys — cystatin C sees through that noise.
Lifestyle. Blood pressure and glucose control are the two things that actually preserve kidney function long-term. Hydrate, avoid NSAID overuse.
Supplements. Avoid chronic high-dose NSAIDs. No supplement improves GFR directly; control the drivers.
Hormones. Manage hypertension and diabetes aggressively — they're the leading causes of CKD. ACE inhibitors/ARBs are renoprotective where indicated.
Compounds. If a routine panel ever flags your eGFR, get cystatin C before you panic or change anything. KDIGO explicitly recommends it when creatinine-based estimates may be unreliable — which describes most of this community.
Work down the list, not across it. Adding a compound on top of an unfixed diet is why generic protocols fail. - Retest
Annually, or to confirm any abnormal creatinine-based eGFR. Change one thing at a time, or the retest can't tell you which thing worked.
How to fix it
📚 KDIGO 2024 CKD Guideline. Inker LA et al., NEJM 2021 — race-free eGFR equations including cystatin C.
This page can tell you what could have made your Cystatin C with eGFR wrong. It cannot tell you whether it did.
Everything above is free and stays free — the assay, what changes the reading rather than the blood, the retest window and the sources. What no page can do is look at your draw: which laboratory ran it, at what hour, what you were taking that week, and what else was flagged beside it. Every one of those changes the answer, and none of them is on any page. Bringing a real result to people who know that list is what the members' area is for.
Bring your result — $10/mo →🩸 Test your Cystatin C with eGFR
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Order this test — 10% off → Browse all 103 markers →What Cystatin C with eGFR is usually tested alongside
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What people use Cystatin C with eGFR to decide
Nobody orders a test for its own sake. Cystatin C with eGFR is on the test list for these pathways — each one links to what the pathway claims, and what its test list is read for before you spend anything on it.
Cystatin-C reads kidney function independently of muscle mass, which matters if you're muscular — creatinine systematically overstates decline in that group and understates it in the frail.
These are the numbers with real causal mortality evidence. ApoB beats LDL-C, Lp(a) is genetic and worth measuring exactly once in your life, and cystatin-C catches kidney decline that creatinine misses. If you test nothing else on this page, test these.
What moves your Cystatin C with eGFR
1 compound and 5 supplements in the Vault have a documented effect on this marker, or are a reason to have measured it first:
Browse all 278 compounds & 371 supplements →
Would you feel it? Symptoms Cystatin C with eGFR helps explain
People search for how they feel, not for a marker. These are the complaints where this one is worth checking, and whether it is first-line or a follow-up.
Why your Cystatin C with eGFR might be wrong
Most abnormal results are interference, not disease. Check these before you change anything. Each says whether the number is wrong (repeat it), badly timed (redraw it), or real with a cause.
Creatine raises serum creatinine without touching the kidney, which is why lifters get told their kidneys are failing every day. Cystatin C is not affected by creatine at all.
If you take creatine, ask for cystatin C with eGFR rather than arguing about a creatinine-based estimate.
Cystatin C is the answer to creatinine being confounded by muscle mass and creatine — but it has its own confounders. Hyperthyroidism and corticosteroids raise it; hypothyroidism lowers it, in both cases without any change in kidney function.
If thyroid status is abnormal or steroids are on board, read cystatin C and creatinine together rather than trusting either alone.
Cystatin C assays are calibrated against an international reference material, but not every lab and not every platform reports on the same scale. A value from one lab is not reliably comparable to a value from another, and the eGFR equation applied on top magnifies the difference.
Trend cystatin C at the SAME lab. A jump between providers is a calibration difference until proven otherwise.
Both raise cystatin C independently of kidney function — steroids notably so.
Note steroid and thyroid status; this is cystatin C's main weakness.
Obesity, smoking and inflammation each raise cystatin C modestly without any reduction in kidney function, which is the main limitation of an otherwise better marker.
Interpret alongside creatinine — where the two disagree, the reason is usually one of these three.
What Cystatin C with eGFR means in combination
One marker tells you a little; combinations tell you the story. These are the named patterns this one takes part in.
Creatinine is a muscle breakdown product — more muscle and creatine supplementation raise it without any kidney dysfunction. Standard eGFR equations systematically misclassify lifters.
Get Cystatin C before panicking or changing anything. KDIGO specifically recommends it when creatinine-based estimates may be unreliable. If cystatin C is normal, your filtration is fine.
Cystatin C is not affected by muscle mass, so when it agrees with a low eGFR the filtration problem is genuine rather than an artefact of being muscular.
This is a clinician conversation, not a supplement one. Blood pressure and glucose control are the two things that change the trajectory. Review NSAID use, creatine, and protein intake honestly, and get urine albumin checked.
Creatinine is a breakdown product of muscle, and eGFR equations based on it quietly assume an average-sized person. Carry a lot of muscle and you generate more creatinine at a perfectly normal filtration rate, so the equation reports a kidney problem you do not have. Creatine supplementation and a hard training session in the day or two before the draw both push it further. Cystatin C is the way out. It is produced by all nucleated cells at a roughly steady rate and is largely independent of muscle mass — so a raised creatinine with a normal cystatin C is the signature of a big person, not a failing kidney. This is one of the most common false alarms in this population, and it regularly leads to compounds being stopped that did not need stopping.
Get a cystatin C eGFR before you conclude anything, and before you stop anything on kidney grounds. It is inexpensive and it answers the question directly. Standardize the draw: no hard training and no creatine for 48 hours, properly hydrated, and ideally fasted at the same time of day. Then compare like with like. A genuinely reduced eGFR confirmed on cystatin C is a different matter and belongs with a clinician — this pattern exists to stop the false alarm, not to explain away a real one. Urine albumin-to-creatinine ratio is the other half of the picture and is not affected by muscle.
Settles it: 🫘 Kidney Health — Read Properly — 6 markers, $83.25 with the discount applied.
What to test next
These put Cystatin C with eGFR in context — each with its own full breakdown.
Frequently asked questions
0.62–1.15 mg/L. Ranges vary by laboratory and assay — always compare to the range printed on your own report.
eGFR >90 mL/min/1.73m² with cystatin C in the lower half of range.
Real reduction in kidney filtration — far more concerning than an isolated high creatinine, and worth prompt follow-up.
Favorable.
You can order Cystatin C with eGFR directly through Marek Diagnostics without a doctor's visit — drawn at any Quest Diagnostics location in the US. Code CAMERON applies 10% off automatically.
Where this goes next
The pages here are the frameworks. The protocols — the dosing, the order to correct things in, the week-by-week schedule and what to retest — are inside Skool.