Cystatin C Test

What is Cystatin C?

Cystatin C is a small protein that nearly all nucleated cells in the body produce at a fairly steady rate. Unlike creatinine, which is a waste product of muscle breakdown, cystatin C production is not tied to how much muscle you have. The kidneys filter cystatin C out of the blood through the glomeruli, the tiny filtering units inside each kidney; the renal tubules then reabsorb and break it down, and it is not secreted back into the blood.

Because the body makes cystatin C at a near-constant rate and the kidneys are its main route out, the level in the blood mainly reflects how well the kidneys are filtering, which is your glomerular filtration rate (GFR). Doctors rarely order cystatin C on its own; it is usually measured together with creatinine so the two can be combined into a more accurate estimated GFR (eGFR).

Cystatin C Normal Range

For most healthy adults, cystatin C falls roughly between 0.6 and 1.0 mg/L, though limits vary between laboratories and assays, and older adults may see a slightly higher upper limit. Some labs report results in µg/mL, which is numerically identical (0.6 mg/L = 0.6 µg/mL). As always, the range printed on your own report rules, because it reflects the exact assay your lab uses.

Clinicians usually interpret cystatin C through the estimated GFR it produces (eGFR from cystatin C, or eGFRcys), reported in mL/min/1.73 m². That estimate, not the raw cystatin C number, is what guides decisions, so ask your doctor to walk you through the eGFR if your report shows only the raw value.

Cystatin C vs Creatinine: When Is It Better?

Creatinine is the standard kidney marker because it is cheap, familiar, and included in nearly every kidney panel. But it is half a muscle number: creatinine is produced when muscle tissue breaks down, so anything that changes muscle mass or muscle breakdown changes creatinine without any change in kidney function. Cystatin C largely avoids this problem.

Cystatin C is not universally better — it is better in specific situations. The 2021 race-free CKD-EPI equations allow eGFR to be calculated from creatinine, from cystatin C, or from both together, and the combined creatinine–cystatin C equation is generally the most accurate of the three. The KDIGO 2024 chronic kidney disease guideline recommends confirming a creatinine-based eGFR with cystatin C (or a measured GFR) when the diagnosis of kidney disease hinges on the eGFR and the creatinine result may be unreliable.²

In the study that established the combined equation, the improvement was largest exactly where it matters most: among people whose creatinine-based eGFR sat in the borderline 45–59 range, adding cystatin C correctly reclassified about 17% as actually having a filtration rate of 60 or above — meaning they did not have the reduced kidney function the first test implied.²

Who Should Get a Cystatin C Test?

Cystatin C is a selective test, not part of routine health checkups. Your clinician is most likely to order it when a creatinine-based result may be misleading, for example:

• Very muscular or very slight build — bodybuilders can have high creatinine with healthy kidneys; frail or elderly people can have “normal” creatinine while their kidneys are actually struggling.
• Amputation or muscle-wasting conditions — less muscle means less creatinine production regardless of kidney function.
• Chronic liver disease — the liver makes less of the creatine that becomes creatinine, so creatinine-based eGFR can overstate kidney function.
• Borderline eGFR — when a chronic kidney disease diagnosis would change your care, confirming with cystatin C is the recommended next step.²
• Pregnancy — kidney filtration rises substantially during pregnancy and creatinine reference ranges shift; some clinicians use cystatin C as an additional marker, interpreted cautiously.

A single result is interpreted together with your history, urine tests (especially albumin), and repeat measurements. It is a doctor-ordered, doctor-interpreted test.

What Does High Cystatin C Mean?

A high cystatin C level corresponds to a lower estimated GFR — the kidneys appear to be filtering less blood than expected. This can reflect genuine kidney dysfunction, which is why the next steps usually mirror those for an abnormal creatinine result: the test is repeated to confirm it, urine is checked for albumin (protein), and the clinician reviews blood pressure, diabetes, and medication history.

Before concluding anything about the kidneys, two things are checked. First, whether a non-kidney factor explains the result (see below): thyroid disorders and corticosteroid use can shift cystatin C without any change in filtration. Second, whether the result fits the clinical picture — a lone mildly raised value in a well person deserves repeat testing before any label is applied.

What Can Affect Cystatin C Besides the Kidneys?

Cystatin C is more independent of muscle than creatinine, but it is not independent of everything. Factors reported to shift cystatin C without a change in kidney function include:

• Thyroid disorders — untreated hyperthyroidism tends to raise cystatin C; hypothyroidism tends to lower it. Share your thyroid status and medication with the interpreting doctor.
• Corticosteroids — systemic steroids such as prednisone can raise cystatin C independently of kidney function. A result drawn during a steroid course needs clinical interpretation.
• Inflammation, obesity, and smoking — have been reported to nudge cystatin C upward in population studies; the effect on any single result is uncertain and must be judged clinically.

These factors do not mean the test is unreliable — they mean the result is interpreted with your full history in view, which is exactly why doctors order it selectively rather than as a standalone screen.

How Is the Test Done?

Cystatin C is measured from a standard blood draw. Fasting is usually not required, but follow the instructions from the ordering lab. Labs increasingly report the eGFR calculated from cystatin C (eGFRcys), or the combined creatinine–cystatin C eGFR, alongside or instead of the raw value — that estimate is the number your clinician acts on.

Cystatin C test price in India

In India, a cystatin C test typically costs roughly ₹1,400–₹2,800, though the price varies by city and laboratory. NABL-accredited labs in most metros offer home sample collection, sometimes with an added phlebotomy charge, so confirm the all-in price before the sample is drawn.

In the United States, the test is typically in the region of $80–$150 when paid out of pocket at large reference labs, with the actual amount varying by lab and insurance coverage. Because cystatin C is a specialized, doctor-ordered test rather than a routine panel item, always confirm the price with the ordering lab before testing.

Frequently Asked Questions

Pharmacist’s practical notes

The clinical scenario where cystatin C earns its keep is the borderline case. Think of the frail 78-year-old whose creatinine sits comfortably “in range” while her creatinine-based eGFR reads 62, or the weightlifter whose creatinine flags high alongside a spotless urine albumin result. Cystatin C is the tiebreaker — it exposes the cases where creatinine is misleading in either direction. If your eGFR from creatinine sits in the borderline band and a kidney disease label would change your care, that is the moment to ask your clinician whether a cystatin C confirmation is appropriate.

Medication review matters before interpreting the result. Systemic corticosteroids such as prednisone can push cystatin C upward independently of kidney function, so a result drawn during a steroid course needs clinical interpretation — discuss the timing of testing with your doctor rather than changing any medication on your own. Thyroid status matters too: untreated hyperthyroidism tends to raise cystatin C and hypothyroidism to lower it, so bring your thyroid medication and recent thyroid function results when the report is reviewed.

Read the eGFR, not the raw number. Labs increasingly report eGFRcys or the combined creatinine–cystatin C eGFR rather than raw cystatin C, and that estimate is what your clinician acts on. Most modern assays are traceable to the same international reference material, but results can still drift between laboratories, so compare serial results from the same lab where possible.

Finally, the practical ordering reality: cystatin C costs noticeably more than creatinine and appears in neither routine Indian health checkups nor standard US panels, so it is ordered selectively — when the creatinine-based eGFR is uncertain, or when kidney disease staging would change management. It is a simple blood test with no special preparation; if you are paying out of pocket, confirm the all-in price, including any home-collection charge, before the sample is drawn.

References

1. Inker LA et al. New Creatinine- and Cystatin C–Based Equations to Estimate GFR without Race. N Engl J Med. 2021.
2. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease.
3. National Kidney Foundation. Cystatin C: what it is and when it is used to estimate kidney function.
4. Chronic kidney disease: diagnosis. NHS.