Skip to content
Blood Test Tracker

Cystatin C

A small protein that nearly every cell makes and the kidneys clear from the blood, used to check kidney filtration without depending on muscle mass.

AI-verified against cited sources. Awaiting expert reviewAlso called CysC, Cys C, CST3LOINC 33863-2

Check your result

Sex (optional)

Type your result to see where it falls.

Nothing you type here leaves your browser.

What it is

Cystatin C is a small protein made at a fairly steady rate by almost every cell in your body. The filters in your kidneys remove it from the blood, so when filtration slows, the level in your blood rises. Unlike creatinine, cystatin C does not depend much on how much muscle you have. That makes it useful when creatinine may be misleading. It is mostly used together with creatinine to work out an estimated GFR (eGFR).

Why it matters

Clinicians use cystatin C when a creatinine-based eGFR may not tell the whole story, for example when someone has unusually low or high muscle mass, or when a precise kidney-function number will guide a decision. KDIGO 2024 recommends using creatinine and cystatin C together when creatinine alone is less accurate, and says that if cystatin C is available, the combined estimate should be used to place kidney function into stages. It is not part of routine panels, so you will usually see it on longevity or specialty panels, or when your clinician adds it.

If it’s low

A low cystatin C usually means your kidneys are clearing it well, which is common in younger adults. An underactive thyroid can also lower it. Your clinician can put a low result in context with your other kidney tests.

Common causes

  • Healthy kidney filtration, especially in younger adults
  • An underactive thyroid
  • Differences between lab methods or labs

If it’s high

A higher cystatin C usually means your kidneys are clearing less of it from the blood. But it is not a perfect kidney marker. Extra body fat, smoking, inflammation and some medicines can raise it without a real drop in kidney filtration. One result is not enough to say anything about kidney disease, and your clinician can put it in context with your creatinine, eGFR and urine tests.

Common causes

  • Lower kidney filtration, including a temporary drop from dehydration or illness
  • Chronic kidney disease, which is more common with diabetes, high blood pressure and heart disease
  • Higher body fat
  • Smoking
  • Long-term inflammation, for example a raised CRP
  • An overactive thyroid
  • Steroid medicines or excess natural cortisol
  • Older age, since reference ranges are higher from age 50

What can help

  • Ask your clinician whether your creatinine and cystatin C results agree with each other
  • Ask whether inflammation, thyroid function, weight or any medicines could be affecting the result
  • Ask whether a repeat test or an eGFR that uses both creatinine and cystatin C would be useful
  • Not smoking, regular physical activity and keeping blood pressure and blood sugar in the range your clinician recommends support kidney health

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

The usual range depends on age. Choose an age to see the one that applies.

See Cystatin C by age and sex, with the U.S. survey numbers where they exist.

If this applies to you

Cystatin C means a little more for some people. Open the one that fits you.

Kidney disease or at risk

Creatinine is tied to muscle mass, while cystatin C is swayed by other things, such as steroid medicines and thyroid problems. KDIGO 2024 recommends adding it when a creatinine-based eGFR may be off and the number will guide a decision, such as confirming or staging CKD or choosing a medicine dose. The estimate then combines creatinine and cystatin C.

Blood tests for kidney health

Strength training

Cystatin C is a second kidney marker that doesn't depend on muscle. Kidney guidelines say an eGFR from cystatin C may suit people whose only reason to doubt their creatinine is extra muscle. The same guidelines note that steroids, including anabolic steroids, can affect cystatin C as well as creatinine.

Blood tests for strength training and bodybuilding

What can affect your result

  • Age: reference ranges are higher from age 50
  • Body fat: higher adiposity is linked to higher cystatin C
  • Smoking
  • Thyroid function: an overactive thyroid tends to raise cystatin C and an underactive thyroid tends to lower it
  • Steroid medicines and excess cortisol
  • Long-term inflammation, such as a raised CRP or a low albumin
  • The lab's method and calibration, since results are not identical across labs
  • Sample quality: hemolyzed or very jaundiced samples can be rejected
  • Cystatin C is affected by things other than kidney filtration. KDIGO lists higher body fat, smoking, over- and underactive thyroid, excess glucocorticoids (steroids) and long-term inflammation (higher CRP, lower albumin) as likely factors.
  • Mayo Clinic Laboratories rejects samples with gross hemolysis or gross icterus (jaundice); gross lipemia (fatty blood) is acceptable to them.
  • Assays differ between labs, even though most are calibrated to the international reference material ERM-DA471/IFCC. Compare results from the same lab and method where possible.

Reflects kidney filtration at the time of the draw rather than a long-term average. Lasting changes in kidney function are judged over at least 3 months.

When to talk to a clinician

Talk with a clinician if your cystatin C is above your lab's range, especially if your creatinine-based eGFR is also below 60 or the two do not agree. KDIGO lists an eGFR below 30 among the reasons to see a kidney specialist. Contact a clinician soon if you notice much less urine than usual, swelling in your legs or feet, nausea or vomiting, or severe tiredness. If your lab flagged the result as urgent, follow its instructions.

Questions you could ask

  • Would an eGFR that uses both creatinine and cystatin C be more reliable for me?
  • How often should my kidney function be checked, given my health conditions?
  • Does my cystatin C agree with my creatinine-based eGFR, and if not, what could explain the difference?
  • Could my weight, smoking history, thyroid, inflammation or any medicines be affecting my cystatin C?
  • Should we repeat the test or add a urine albumin test (uACR)?
  • Does this change how my eGFR should be read?

Questions people ask

Why would my clinician order cystatin C when I already had a creatinine test?

Creatinine depends on muscle mass and diet, so it can mislead when your muscle mass is unusually low or high. Cystatin C is affected by different things, so using both gives a more accurate estimate of kidney filtration than either one alone, according to KDIGO 2024.

Is the reference range the same at every lab?

No. Mayo Clinic Laboratories prints 0.63–1.03 mg/L for ages 18 to 49 and 0.67–1.21 mg/L for 50 and older, Labcorp prints 0.60–1.00 mg/L for ages 2 to 50, and ARUP prints 0.61–0.95 mg/L for all adults. Methods differ, so the range on your own report is the best guide.

Can my weight, smoking or inflammation change my cystatin C?

Yes. KDIGO lists higher body fat, smoking, thyroid disease, steroid exposure and long-term inflammation as things that can raise or shift cystatin C without a true change in kidney filtration. That is why clinicians usually read cystatin C together with creatinine and not on its own.

Do I need to fast or prepare for a cystatin C test?

Mayo Clinic Laboratories lists no special preparation for this test, such as fasting. Follow any instructions your own lab gives you.

What is the difference between cystatin C and eGFR?

Cystatin C is a blood measurement, reported in mg/L. eGFR is a calculated estimate in mL/min/1.73 m² that turns a marker (creatinine, cystatin C or both) plus your age and sex into a kidney-filtration number. A higher cystatin C goes with a lower eGFR.

Show the science and sources

Is a change real?

Cystatin C naturally varies about 4% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 12% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Critical appraisal and meta-analysis of biological variation estimates for kidney related analytes (Jonker N et al., EFLM WG-BV).

For clinicians and researchers

Cystatin C is a 13 kDa cysteine-protease inhibitor produced by all nucleated cells at a fairly constant rate, freely filtered at the glomerulus, then reabsorbed and catabolized by proximal tubule cells (Skidmore 2024), so little returns to the blood or reaches the urine. It is less influenced by muscle mass, physical activity and protein intake than creatinine (Skidmore 2024). Commercial assays are immunoassays calibrated to the certified reference material ERM-DA471/IFCC. KDIGO 2024 Table 11 asks labs for an assay CV below 2.0% and bias below 3.2%, to report cystatin C to the nearest 0.01 mg/L, and to measure creatinine on the same sample so eGFRcr-cys can be calculated. Non-GFR determinants of cystatin C are less well studied than those of creatinine but are thought to include higher adiposity, smoking, hypo- and hyperthyroidism, glucocorticoid excess and chronic inflammation (CRP, TNF, low albumin). A 2021 meta-analysis of 11 studies found cystatin C higher in hyperthyroidism and lower in hypothyroidism than in controls (Xin 2021). KDIGO Practice Point 1.2.2.7 notes that 25% to 30% of people have eGFRcr and eGFRcys values that differ by 15 mL/min/1.73 m² or more, or 20% or more, and that factors linked to a higher eGFRcr than eGFRcys include older age, female sex, higher BMI, weight loss and current smoking. In pooled cohort data, risk gradients were consistently stronger for most outcomes with eGFRcys than eGFRcr, and the J-shaped risk seen with eGFRcr above about 105 mL/min/1.73 m² was not seen with eGFRcr-cys. Mayo Clinic Laboratories reports 0.63–1.03 mg/L for ages 18 to 49 and 0.67–1.21 mg/L for 50 and older, Labcorp reports 0.60–1.00 mg/L for ages 2 to 50, and ARUP reports 0.61–0.95 mg/L for all adults, so intervals are assay-specific. For adults Mayo converts the result to an eGFR with the CKD-EPI cystatin C equation, and for children with the Schwartz cystatin C equation.

LOINC 33863-2 (primary). Specimen: serum-or-plasma.

Sources

  1. CSTCE - Cystatin C with Estimated Glomerular Filtration Rate (eGFR), Serum, Mayo Clinic Laboratories (via the Schneck Medical Center test catalog)
  2. Pediatric Testing Reference Ranges (includes Cystatin C, test 121251), Labcorp
  3. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease, Kidney International 105(4S):S117-S314 (2024)
  4. CKD-EPI Cystatin C Equation (2012), National Kidney Foundation
  5. Cystatin and Creatinine With eGFR (test 3018316), ARUP Laboratories
  6. Cystatin C as a Marker of Kidney Function in Children, Biomolecules (2024)
  7. Association Between Serum Cystatin C and Thyroid Diseases: A Systematic Review and Meta-Analysis, Frontiers in Endocrinology (2021)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

Calculators

Related markers