Cystatin C with Glomerular Filtration Rate, Estimated (eGFR)
The Cystatin C with eGFR test measures serum cystatin C and calculates estimated glomerular filtration rate independent of muscle mass, offering greater accuracy than creatinine-based eGFR in older adults, extremes of body composition, and chronic illness.
2
Biomarkers
Blood (serum)
Sample
2–3 business days
Turnaround
No
Fasting
What is the Cystatin C eGFR test?
The Cystatin C with eGFR test measures serum cystatin C and calculates an estimated glomerular filtration rate to assess kidney function with less bias from muscle mass and body composition than creatinine-based estimates. It is ordered when creatinine eGFR is discordant with clinical impression, when precision is needed for drug dosing or pre-operative risk stratification, and in populations where creatinine performs poorly — older adults, those with extremes of muscle mass or chronic illness, and individuals whose creatinine may be affected by diet or supplements. Cystatin C eGFR improves CKD detection and staging, reclassifying a substantial minority of patients whose creatinine eGFR falls near clinical thresholds. It is increasingly used in cardiovascular and oncology settings where accurate filtration estimates guide nephrotoxic therapy, and KDIGO guidelines now recommend combined creatinine–cystatin C equations as the confirmatory standard when initial eGFR is uncertain or treatment decisions hinge on precise staging.
What does the Cystatin C eGFR test measure?
Cystatin C is a low-molecular-weight cysteine protease inhibitor produced at a constant rate by all nucleated cells and freely filtered by the glomerulus, then catabolised by proximal tubular cells. Its production rate is independent of muscle mass, diet, and most inflammatory states, making its serum concentration a direct reflection of glomerular filtration. Because it is not secreted or reabsorbed by the tubule, cystatin C rises earlier than creatinine when filtration declines and offers particular advantages in populations where creatinine-based estimates are less reliable.
Cystatin C is used to estimate GFR with greater accuracy than creatinine alone, especially in older adults, those with extremes of muscle mass, and patients with chronic disease where creatinine may underestimate true kidney impairment. The eGFRcys equation improves CKD detection and staging, and combined creatinine–cystatin C equations offer the best overall performance. Practitioners order cystatin C when creatinine-based eGFR is discordant with clinical suspicion or when precision matters for therapeutic decisions.
The eGFRcys is calculated from serum cystatin C, age, and sex using the CKD-EPI 2012 cystatin C equation. It estimates the volume of plasma filtered by the glomeruli per unit time, standardised to body surface area, without the confounding influence of muscle mass that limits creatinine-based estimates.
eGFRcys is a more accurate filtration marker in patients with sarcopaenia, obesity, limb amputation, chronic illness, or extremes of age. It detects earlier declines in kidney function and reclassifies a meaningful proportion of patients compared with creatinine eGFR alone. Combined creatinine–cystatin C equations (eGFRcr-cys) are now recommended by KDIGO when confirmatory testing is needed.
Why is the Cystatin C eGFR test ordered?
- Discordance between creatinine eGFR and clinical suspicion of impaired kidney function, or when creatinine eGFR hovers near decision thresholds (e.g., 45 or 60 mL/min/1.73 m²)
- Older adults, in whom sarcopaenia and reduced creatinine production can mask declining filtration
- Patients with extremes of muscle mass — sarcopaenia, cachexia, limb amputation, neuromuscular disease, obesity, or high muscle mass from athletic training or anabolic use
- Pre-operative assessment or oncology dosing when precision is critical for nephrotoxic chemotherapy or contrast exposure
- Evaluation of potential living kidney donors, where accurate GFR is required before nephrectomy
- Monitoring of chronic kidney disease when creatinine trends are ambiguous or influenced by dietary protein, creatine supplementation, or medications that inhibit tubular secretion
- Diabetic or hypertensive patients with early CKD, where cystatin C may detect decline before creatinine rises
- Conditions associated with altered creatinine metabolism, such as cirrhosis, severe malnutrition, or chronic corticosteroid use
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
2–3 business days
Collection notes
Phlebotomy at Quest draw site; no special preparation. Avoid gross haemolysis or severe lipaemia, which may interfere with the immunoassay. Note current thyroid status and corticosteroid use in the clinical history.
Specimen requirements
Minimum 0.5 mL serum (gel separator acceptable). Cystatin C is stable refrigerated for up to one week and frozen for longer storage. Specify the performing laboratory for serial comparability, as assays and reference intervals differ.
What can affect Cystatin C eGFR results?
Hyperthyroidism increases and hypothyroidism decreases cystatin C production independent of filtration, so uncontrolled thyroid disease can distort the result; thyroid function should be stable or interpreted alongside.
Glucocorticoids increase cystatin C transcription, raising serum levels independent of kidney function; this effect is dose- and duration-dependent.
Unlike CRP-driven effects on creatinine, cystatin C appears relatively stable in inflammation, though severe critical illness and sepsis have been reported to alter production; interpret with caution in the acutely unwell.
Adipose tissue may modestly increase cystatin C production, but the effect is small compared with creatinine's muscle-mass dependency; eGFRcys remains more accurate than eGFRcr in obesity.
Active smoking has been associated with slightly higher cystatin C, independent of kidney function; the clinical significance is modest but may contribute to minor overestimation of impairment.
Immunoassays for cystatin C can be affected by gross haemolysis or severe lipaemia; the laboratory will flag interference and request a recollection if necessary.
What do Cystatin C eGFR results mean?
Elevated
Elevated cystatin C (>1.10–1.30 mg/L, assay-dependent) and reduced eGFRcys (<60 mL/min/1.73 m²) indicate impaired glomerular filtration. When eGFRcys is lower than creatinine eGFR, true kidney function is likely overestimated by creatinine alone — common in sarcopaenia, older age, or chronic illness. When both are low, CKD is confirmed; when cystatin C is elevated but creatinine is normal, consider early decline masked by low muscle mass, or non-renal factors (thyroid disease, high-dose steroids). Stage by KDIGO criteria and integrate albuminuria for risk stratification.
Normal
Cystatin C 0.50–1.10 mg/L and eGFRcys ≥60 mL/min/1.73 m² suggest preserved glomerular filtration. When creatinine eGFR is also normal, significant CKD is unlikely; when creatinine eGFR is reduced but cystatin C eGFR is normal, consider creatinine confounders (low muscle mass, vegetarian diet, creatine supplements, tubular secretion inhibitors). Combined creatinine–cystatin C equations resolve most discordance.
Reduced
Low cystatin C (<0.50 mg/L) is uncommon and not clinically actionable; it may reflect assay variation or exceptionally high filtration in young, healthy individuals. It does not exclude kidney disease, which is assessed by eGFR and albuminuria.
Note: eGFRcys is more accurate than eGFRcr in most populations, but it is not a direct measurement — measured GFR (inulin, iohexol, iothalamate clearance) remains the gold standard when absolute precision is required. Cystatin C should be interpreted with stable thyroid function and awareness of corticosteroid dose; KDIGO recommends confirmatory testing with combined eGFRcr-cys when initial eGFR is near clinical thresholds. Acute kidney injury is still monitored with creatinine, as cystatin C kinetics are slower.
Frequently asked questions
Cystatin C is produced at a constant rate by all nucleated cells and is independent of muscle mass, diet, and most inflammatory states, so its serum concentration directly reflects glomerular filtration. Creatinine is influenced by muscle mass, dietary protein, and tubular secretion, which can mask declining kidney function in older adults, sarcopaenia, and chronic illness.
How to access the Cystatin C eGFR test
The Cystatin C eGFR reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int. (2024)
- Inker LA, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. N Engl J Med. (2021)
- Shlipak MG, et al. Cystatin C versus creatinine in determining risk based on kidney function. N Engl J Med. (2013)
- Stevens LA, et al. Estimating GFR using serum cystatin C alone and in combination with serum creatinine: a pooled analysis of 3,418 individuals with CKD. Am J Kidney Dis. (2008)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.