What are the estimating equations that use cystatin C?
The serum level of cystatin C can be used to estimate GFR (CKD-EPI 2012 Cystatin C equation):
where SCysC is serum cystatin C
min indicates the minimum of SCysC/0.8 or 1
max indicates the maximum of SCysC/0.8 or 1
Cystatin C is less influenced than creatinine by race, so GFR estimates based on cystatin C alone (eGFRcys) include only age and sex, and not race. This allows GFR to be estimated without specification of race. This can be quite helpful in mixed-race populations. However, eGFRcys is not more accurate than creatinine-based estimating equations (eGFRcr); rather, it is the combination of the two markers (eGFRcr-cys) that results in the most accurate eGFR.
The CKD-EPI 2012 creatinine-cystatin and cystatin C equations were developed using cystatin C measured using assays traceable to the international reference standard in a large database of subjects with diverse characteristics. KDIGO recommends using the CKD-EPI cystatin 2012 and creatinine-cystatin 2012 estimating equations:
where SCr is serum creatinine
SCysC is serum cystatin C
x is 0.7 for females and 0.9 for males
α is −0.248 for females and −0.207 for males
min(SCr/k,1) indicates the minimum of SCr/k or 1
max (SCr/k,1) indicates the maximum of SCr/k or 1
min(SCysC/0.8,1) indicates the minimum of SCysC/0.8 or 1
max(SCysC/0.8,1) indicates the maximum of SCysC/0.8 or 1
All eGFR equations are available online at http://ckdepi.org/equations/gfr-calculator/ .
