Cystatin-C is a small protein produced at a steady rate by all cells in the body, filtered freely by the kidneys, and not influenced by muscle mass, diet, or sex. These properties make it a more accurate marker of kidney function than creatinine in many populations.
Cystatin-C is increasingly used when creatinine-based eGFR may mislead, such as in very muscular or very frail individuals, in older adults, and when early kidney dysfunction is suspected before creatinine rises. It detects early kidney dysfunction more sensitively than creatinine and is emerging as the preferred marker in longevity-focused kidney assessment. A Cystatin-C-based eGFR gives a second estimate that, combined with creatinine-based eGFR, improves accuracy.
Optimal Cystatin C: 0.6-0.9 mg/L.
Typically re-tested every 12 weeks.
Typical lab reference ranges run from about 0.5 to 1.1 mg/L, and FitScript's optimal range is 0.6 to 0.9 mg/L. Many labs also report an estimated filtration rate calculated from the cystatin C value, either on its own or combined with creatinine.
Cystatin C is a small protein, released at a fairly constant rate by virtually all nucleated cells in the body, that the kidneys remove from the blood, which makes its blood concentration a practical marker of how fast the kidneys filter.
The mechanism is simple and that is why the marker is useful. Cystatin C belongs to a family of proteins that inhibit cysteine proteases, enzymes involved in normal protein turnover. Because it is small and carries a positive charge at the pH of blood, it passes freely through the glomerulus, the filtering unit of the kidney. Once filtered, it is almost completely taken up and broken down by cells lining the proximal tubule, so very little returns to the circulation and almost none appears intact in urine. The level you see on a blood test is therefore set mostly by filtration rate rather than by secretion, diet or urinary losses.
That is the key difference from creatinine. Creatinine is a breakdown product of muscle, so a heavily muscled person can carry a higher creatinine with entirely normal filtration, and someone with very little muscle can sit inside the normal creatinine range while filtering poorly. Cystatin C sidesteps most of that problem. Research in people with spinal cord injury, a group where muscle mass is often substantially reduced, has examined cystatin C alongside imaging measures precisely because creatinine-based estimates can be misleading in that setting[1].
Related: Cystatin C eGFR Calculator (CKD-EPI) Explained
It matters because kidney filtration usually declines quietly, and cystatin C is one of the earlier and more muscle-independent ways to notice the drift. Your kidneys filter the entire blood volume many times a day, regulate blood pressure and electrolytes, and clear most medications. A meaningful amount of filtering capacity can be lost before anyone feels different.
In chronic kidney disease, research has looked at cystatin C among a panel of markers used to anticipate how kidney function will behave over time rather than simply describing where it sits today[2]. In acute settings, studies have evaluated it as an early signal of kidney injury, including work on patients hospitalised with COVID-19 where investigators assessed which early measurements tracked with later acute kidney injury[3].
For someone tracking long-term health rather than managing a known condition, the practical value is trend. A single value is a snapshot shaped by that day's hydration, illness and lab assay. A series of values over years is far more informative, because a slow upward drift inside the standard range can appear before any result is flagged. Kidney filtration also sets how your body handles many prescription medications and supplements, which is one reason clinicians want an accurate filtration estimate before adjusting anything.
None of this means a single cystatin C result predicts your future. It is one input, and it is most useful read next to creatinine, urine testing and blood pressure.
Most adult results fall between roughly 0.5 and 1.1 mg/L, and FitScript's optimal range is narrower.
| Measure | Range | What it represents |
|---|---|---|
| Standard lab reference range (adults) | 0.5 to 1.1 mg/L | Typical lab reference range, the spread seen across a general population |
| FitScript optimal range | 0.6 to 0.9 mg/L | FitScript's optimization target |
FitScript's optimal range is an optimization target, not a diagnostic cut-off. Reference ranges vary by lab, age and sex, and the ranges above are typical rather than universal. Always read your result against the range printed on your own report, because assays and calibration differ between laboratories.
Two practical notes. First, no age-stratified range is given here, because ranges in infants, children and older adults differ substantially and belong to the reporting lab rather than to a general article. Second, many reports convert the value into an estimated glomerular filtration rate, sometimes from cystatin C alone and sometimes from a combined creatinine and cystatin C equation. Those equations are estimates with their own margins of error, so the raw concentration and the estimate are best read together.
An elevated cystatin C suggests the kidneys are filtering more slowly than expected, or that something other than filtration is pushing the level up. It does not, by itself, identify a cause.
Many people with a modestly raised result notice nothing at all. When symptoms are reported, they tend to be non-specific: tiredness that does not resolve with sleep, puffiness around the ankles or eyes, needing to urinate at night, foamy urine, reduced appetite, itchy skin, or difficulty concentrating. These overlap with dozens of ordinary explanations, which is exactly why the lab value is checked rather than guessed.
Common contributors include reduced filtration in people with diabetes or long-standing high blood pressure, acute illness or dehydration, recent surgery, and exposure to medications that stress the kidneys. A systematic review in children and adolescents with type 1 diabetes examined cystatin C as a marker of early kidney injury in that group[4]. Research in children and adolescents with obesity has compared serum cystatin C with filtration-rate formulae and proteinuria to see which measure is most informative[5]. Levels have also been studied after major vascular surgery, where investigators assessed kidney function following open and endovascular aortic aneurysm repair[6], and during cisplatin chemotherapy, where cystatin C was evaluated as a way to assess filtration rate[7].
The picture is not one-sided. An earlier study in patients with diabetes concluded that cystatin C was not more sensitive than creatinine for detecting early kidney impairment[8], which is a useful reminder that no single marker outperforms the others in every population.
Observation, meaning, next step. The observation is a value above your lab's range or above the optimal band. The meaning may be slower filtration, or a transient effect from illness, dehydration or a medication. The next step is to bring it to a clinician, who will usually want a repeat test under stable conditions plus creatinine and a urine albumin measurement before drawing any conclusion. A clearly elevated result, or any result with swelling, a sharp drop in urine output or breathlessness, is worth discussing with a clinician promptly.
A low cystatin C usually reflects brisk filtration and is far less often a concern than a high one, though it is not automatically meaningless.
Low values rarely produce symptoms of their own. If symptoms are present alongside a low reading, they usually belong to whatever else is going on, for example the fatigue, cold intolerance and weight change that can accompany an underactive thyroid, or increased thirst and urination in early metabolic disease.
Common explanations include a genuinely high filtration rate in a healthy younger adult, hyperfiltration in pregnancy or in the early phase of some metabolic conditions, reduced thyroid hormone activity, low body mass, and simple assay or laboratory variation. Age matters a great deal at the extremes of life. Work on glomerular function in neonates has described how filtration measures behave in newborns, where adult ranges do not apply[9], and a systematic review and meta-analysis has compared kidney health outcomes in children born very prematurely with full-term peers[10].
Long term, a low-normal value in an otherwise healthy adult is generally reassuring. The situation that deserves more attention is sustained hyperfiltration, where the kidneys are working above their usual rate for a reason, because that pattern is associated over years with strain on the filtering units. That pattern is identified by a clinician using the whole picture, not by one low number.
Several everyday factors shift the result, which is why a surprising value is normally repeated before it is acted on.
Testing conditions. Fasting is usually not required for this marker, but follow whatever instruction your lab gives. Hydration status, an acute infection, a recent hard training session and the time of day can all nudge the number. Different laboratories use different assays and calibrators, so comparing a result from one lab directly against a result from another is less reliable than tracking within the same lab. Fluid strategy around procedures has itself been studied, including research comparing kidney function after percutaneous coronary intervention depending on the type of hydration used[11].
Medications. Immunosuppressants used in kidney conditions can affect filtration and the markers that estimate it, and cystatin C has appeared in research on immunosuppressive therapy for severe Henoch-Schonlein glomerulonephritis[12]. No dose or schedule is suggested here for anything, and you should never stop or change a prescribed medication on the basis of a lab value without speaking to the prescriber.
Habits and body composition. Smoking, higher body fat, chronic inflammation and very high protein intake tend to go with higher readings, although the size of the effect varies and causation is not established. Muscle mass influences cystatin C far less than it influences creatinine, which is the main reason both markers are often run together.
FitScript does not currently include cystatin C in a panel. The membership covers 150+ biomarkers tested per year, including the kidney markers most laboratories run as standard, and results are read by Atlas, FitScript's AI health advisor, which interprets each marker in the context of the rest of your biology rather than one line at a time.
If you already have a cystatin C result from another laboratory, you can upload your existing labs so Atlas reads it alongside your creatinine, filtration estimate, albumin and metabolic markers. That context is what turns a single number into something useful, because a cystatin C of 1.0 mg/L in someone with normal creatinine, normal urine albumin and stable blood pressure is a different conversation from the same value with other markers drifting.
Atlas writes your FitScript from what your results actually show, and tracking matters more than any one draw. Where a result suggests something that may call for medication or further investigation, a licensed provider evaluates whether any prescription option is appropriate after reviewing you as a person, not as a data point.
Kidney function is a system, and cystatin C is most informative when it is read with the markers below.
eGFR is the estimated filtration rate your lab calculates from creatinine, cystatin C, or both, and it is the number most clinicians anchor to. Creatinine is the long-standing filtration marker, influenced by muscle mass and recent protein intake, and the discrepancy between it and cystatin C is often the most interesting finding. BUN reflects urea clearance and is sensitive to hydration, protein intake and bleeding in the gut, so it adds context that filtration markers alone do not. Albumin speaks to liver synthesis, nutrition and inflammation, and urinary albumin is one of the earliest signs of filter damage. The BUN:Creatinine Ratio helps separate a hydration effect from a filtration effect. SDMA (Symmetric Dimethylarginine) is another filtration-linked molecule that, like cystatin C, is less dependent on muscle mass.
Treated as a single snapshot, cystatin C answers one question: how fast are the kidneys filtering right now, independent of how much muscle you carry. Treated as a trend, it answers a better one: is that capacity holding steady over years. It is most useful where creatinine is unreliable, and it may help most when it is paired with creatinine rather than used instead of it.
If your result sits outside your lab's range or outside the optimal band, the reasonable next move is a repeat test under stable conditions together with the companion markers above. This page is educational and is not medical advice. Discuss your results, and any treatment decisions that follow from them, with a clinician or licensed provider who can see your full history.
Calculators that use Cystatin C:
It measures the concentration of a small protein that body cells release at a steady rate and the kidneys clear from the blood. Because clearance depends almost entirely on filtration at the glomerulus, the blood level acts as a marker of filtration rate. Labs often convert the value into an estimated glomerular filtration rate, sometimes using cystatin C alone and sometimes combined with creatinine.
Typical adult lab reference ranges run from about 0.5 to 1.1 mg/L, and FitScript's optimal range is 0.6 to 0.9 mg/L, which is an optimization target rather than a diagnostic cut-off. Ranges vary by lab, age and sex, so compare your result with the range printed on your own report.
Neither replaces the other. Cystatin C is far less affected by muscle mass, so it tends to be more informative in people with unusually high or low muscle. Most clinicians get the clearest picture when both are read together.
It suggests filtration may be slower than expected, or that another factor is raising the level, such as acute illness, dehydration, thyroid status or inflammation. It does not identify a cause on its own. A repeat test under stable conditions plus creatinine and a urine albumin measurement is the usual next step, and a clearly elevated result is worth discussing with a clinician promptly.
A low-normal value in an otherwise healthy adult is usually reassuring and rarely causes symptoms. Low readings can reflect brisk filtration, pregnancy, reduced thyroid hormone activity, low body mass or assay variation. Sustained hyperfiltration is the pattern a clinician would look at more closely, and that assessment uses the whole picture rather than one number.
Fasting is usually not required for this marker, but follow the instruction your lab gives, since other tests drawn at the same time may require it. Hydration, acute illness and a recent hard training session can all shift the result slightly, so testing under your usual conditions makes trends easier to read.
Yes. Never stop or change a prescribed medication because of a lab value without speaking to the prescriber first.
FitScript does not currently include cystatin C in a panel. The membership covers 150+ biomarkers tested per year, including standard kidney markers. If you already have a cystatin C result from another laboratory, you can upload your existing labs so Atlas reads it alongside your creatinine, filtration estimate and albumin.
There is no single schedule that fits everyone. Recheck intervals depend on why the test was ordered, what the first result showed and what other kidney markers are doing. A clinician who can see your history is the right person to set the interval, and a surprising value is normally repeated before any conclusion is drawn from it.