Cystatin C Unit Converter
Cystatin C Unit Converter
Convert cystatin C between mg/L, µg/mL and nmol/L, with reference ranges and when to prefer it over creatinine.
Cystatin C converter
Mass ⇄ molarCystatin C 1.3 mg/L
Formula and conversion factor
mg/L = µg/mL (numerically identical)
- 74.95
- derived from the cystatin C molecular weight, 13,343 Da
- standardisation
- results should be traceable to the ERM-DA471/IFCC reference material; pre-standardisation values are not comparable
Worked example
Cystatin C 1.3 mg/L
1.3 × 74.95 = 97.4 nmol/L
1.3 mg/L = 1.3 µg/mL
What moves cystatin C independently of GFR
| Direction | Cause |
|---|---|
| Raises | Corticosteroids, hyperthyroidism, obesity, smoking, systemic inflammation |
| Lowers | Hypothyroidism, ciclosporin |
| No effect | Muscle mass, diet, tubular secretion — the reason it complements creatinine |
What cystatin C measures
Cystatin C is a 13 kDa cysteine protease inhibitor produced at a near-constant rate by all nucleated cells. It is freely filtered at the glomerulus and then almost entirely reabsorbed and catabolised by proximal tubular cells rather than returned to the circulation, so plasma concentration depends on filtration rate and very little else.
Crucially, it is independent of muscle mass, dietary protein and tubular secretion — the three factors that distort creatinine. That makes it the more reliable marker in cirrhosis, cachexia, amputation, paraplegia, very high muscle bulk, and in patients taking trimethoprim, dolutegravir or another drug that blocks creatinine secretion.
It has its own confounders. Corticosteroids raise it, thyroid dysfunction moves it in both directions, and obesity, smoking and systemic inflammation all raise it independently of GFR. Assay standardisation was historically poor and is now resolved through the ERM-DA471/IFCC certified reference material, but results predating standardisation cannot be compared with current ones.
In practice it is used as a confirmatory rather than a screening test, largely on cost. KDIGO recommends it to confirm a creatinine-based eGFR when the result will drive a consequential decision, and the combined creatinine-cystatin C equation is more accurate than either marker alone.
Frequently asked questions
How do I convert cystatin C to nmol/L?
Multiply the mg/L value by 74.95, from its molecular weight of 13,343 Da. In practice cystatin C is almost always reported in mg/L.
Is mg/L the same as µg/mL for cystatin C?
Yes, exactly — they are numerically identical, so no conversion is needed between them.
What is a normal cystatin C?
Roughly 0.6 to 1.0 mg/L in adults, though intervals vary between assays. Unlike creatinine it does not need separate reference ranges by sex.
When is cystatin C preferred over creatinine?
When muscle mass is atypical — cirrhosis, cachexia, amputation, paraplegia or high muscle bulk — or when a drug is blocking creatinine secretion. KDIGO also recommends it to confirm a creatinine eGFR before a consequential decision.
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References
- Inker LA et al. Estimating glomerular filtration rate from serum creatinine and cystatin C. N Engl J Med. 2012;367(1):20–29.
- Grubb A et al. First certified reference material for cystatin C in human serum ERM-DA471/IFCC. Clin Chem Lab Med. 2010;48(11):1619–21.