What this test measures
Cystatin C is a 13-kDa protein produced at a constant rate by all nucleated cells, freely filtered at the glomerulus, and almost completely reabsorbed and catabolised by proximal tubule cells (so it does not normally appear in urine). Unlike creatinine, cystatin C is independent of muscle mass, diet, and age, making it a more accurate GFR marker in patients where creatinine is misleading — elderly, sarcopenic, amputees, paediatric, very obese, vegetarian, anorexic.
Why it matters
KDIGO 2024 explicitly endorses cystatin C as a confirmatory test for CKD diagnosis when creatinine-based eGFR is borderline (45–59 mL/min/1.73 m²). In India, where vegetarian diets and sarcopenia in the elderly are common, creatinine consistently overestimates GFR. The combined eGFR formula (CKD-EPI 2021 using both creatinine and cystatin C) is the most accurate non-invasive GFR estimate and is now the gold standard for borderline CKD diagnosis and for chemotherapy / drug dose adjustment.
How to prepare
No fasting required. Random sample acceptable. Cystatin C is unaffected by recent meals, exercise, or muscle mass. Disclose corticosteroid use (steroids modestly raise cystatin C independent of GFR) and active hyperthyroidism (raises) / hypothyroidism (lowers).
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Serum Cystatin C (mg/L)[1][2] | Adults 0.6 – 1.0 (assay-dependent; values slightly higher in elderly) | Low cystatin C is unusual and not clinically meaningful. Hypothyroidism and large-dose steroids lower it slightly. | Raised cystatin C indicates reduced GFR. Calculate eGFR-cystatin-C; ≥ 60 mL/min/1.73 m² rules out CKD when creatinine-based eGFR is borderline. < 60 confirms CKD. Hyperthyroidism, large steroid doses, and active inflammation raise cystatin C independent of GFR. |
When to add cystatin C to a creatinine eGFR
| Scenario | Why | Action |
|---|---|---|
| Borderline eGFR-Cr 45–59 mL/min | Confirm CKD before labelling | Order cystatin C; combined formula is the answer |
| Elderly / frail / sarcopenic | Creatinine overestimates GFR (low muscle) | Use cystatin C-based eGFR |
| Amputee, bedridden | Creatinine unreliable | Cystatin C is the standard |
| Pre-chemotherapy dosing | Carboplatin / methotrexate need accurate GFR | Combined eGFR-Cr-Cys is preferred |
| Pregnancy | Creatinine drops 30%; cystatin C also drops | Pregnancy-specific reference ranges apply |
Frequently asked questions
Why not just use creatinine?
Creatinine is generated by muscle and influenced by diet, age, ethnicity, and muscle mass. In sarcopenic or vegetarian patients it underestimates the true serum creatinine for a given GFR, making CKD harder to diagnose. Cystatin C avoids all of those variables.
Is cystatin C affected by anything else?
Yes — hyperthyroidism raises it, hypothyroidism lowers it, large-dose corticosteroids raise it modestly, and active inflammation can raise it. Disclose these so the result is interpreted correctly.
Should everyone get cystatin C?
No. KDIGO recommends it specifically for confirming CKD when creatinine-eGFR is borderline (45–59), or in patients where creatinine is known to be unreliable, or when accurate GFR is needed (e.g., chemotherapy dosing).
Do I need to fast?
No. Cystatin C is unaffected by recent meals or exercise.
How does it compare with iohexol GFR?
Combined creatinine + cystatin C eGFR has bias and precision similar to measured iohexol GFR — close enough to use for nearly all clinical decisions, including chemotherapy dose adjustment.
Related Toxicology / Trace Elements tests
Tests commonly ordered alongside CYSTATIN C, or that help interpret an unexpected result.
Sources & references
- KDIGO 2024 CKD Guideline · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Cystatin C · accessed 2026-05-30T00:00:00.000Z
- NKDEP — Cystatin C eGFR · accessed 2026-05-30T00:00:00.000Z
- Mayo Clinic Labs — Cystatin C · accessed 2026-05-30T00:00:00.000Z
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