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.
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.
🩸 Test your Cystatin C with eGFR
Order directly through Marek Diagnostics — no doctor's visit needed, drawn at any Quest location in the US. Code CAMERON applies 10% off automatically.
Order this test — 10% off → Browse all 102 markers →What Cystatin C with eGFR is usually tested alongside
On its own, one marker is a data point. These panels include Cystatin C with eGFR plus the markers that make it interpretable — each names every test, why it is on the list, and loads the whole set into your cart in one click at 10% off.
includes this + 5 more markers — High blood pressure, diabetes, a family history of kidney disease, long-term NSAID use — or you lift heavily and your eGFR came back looking alarming.
includes this + 12 more markers — Healthy, asymptomatic, and want the most informative baseline you can get — the panel to run once a year and track for decades.
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 237 compounds & 350 supplements →
Would you feel it? Symptoms Cystatin C with eGFR helps explain
People rarely search for a marker — they search for how they feel. These are the complaints where this marker is worth checking, and whether it is a first-line test or a follow-up once the obvious causes are ruled out.
Why your Cystatin C with eGFR might be wrong
Most abnormal results are interference, not disease. These are the things that measurably move this specific marker — check them before you change anything. Each one says whether the number is wrong and should be repeated, badly timed and should be redrawn, or real with a cause worth explaining.
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
A single marker tells you a little. Combinations tell you the story — these are the named patterns this marker takes part in, and what each one points at.
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. Standardise 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.
What to test next
Markers rarely answer alone. These are the ones that 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.