Cystatin C–Based Equation to Estimate GFR without the Inclusion of Race and Sex
Résumé
BACKGROUND
The accuracy of estimation of kidney function with the use of routine metabolic
tests, such as measurement of the serum creatinine level, has been controversial.
The European Kidney Function Consortium (EKFC) developed a creatinine-based
equation (EKFC eGFRcr) to estimate the glomerular filtration rate (GFR) with a
rescaled serum creatinine level (i.e., the serum creatinine level is divided by the
median serum creatinine level among healthy persons to control for variation related to differences in age, sex, or race). Whether a cystatin C–based EKFC equation would increase the accuracy of estimated GFR is unknown.
METHODS
We used data from patients in Sweden to estimate the rescaling factor for the
cystatin C level in adults. We then replaced rescaled serum creatinine in the EKFC
eGFRcr equation with rescaled cystatin C, and we validated the resulting EKFC
eGFRcys equation in cohorts of White patients and Black patients in Europe, the
United States, and Africa, according to measured GFR, levels of serum creatinine
and cystatin C, age, and sex.
RESULTS
On the basis of data from 227,643 patients in Sweden, the rescaling factor for
cystatin C was estimated at 0.83 for men and women younger than 50 years of age
and 0.83+0.005×(age–50) for those 50 years of age or older. The EKFC eGFRcys
equation was unbiased, had accuracy that was similar to that of the EKFC eGFRcr
equation in both White patients and Black patients (11,231 patients from Europe,
1093 from the United States, and 508 from Africa), and was more accurate than
the Chronic Kidney Disease Epidemiology Collaboration eGFRcys equation recommended by Kidney Disease: Improving Global Outcomes. The arithmetic mean of
EKFC eGFRcr and EKFC eGFRcys further improved the accuracy of estimated GFR
over estimates from either biomarker equation alone.
CONCLUSIONS
The EKFC eGFRcys equation had the same mathematical form as the EKFC eGFRcr
equation, but it had a scaling factor for cystatin C that did not differ according to
race or sex. In cohorts from Europe, the United States, and Africa, this equation
improved the accuracy of GFR assessment over that of commonly used equations.
(Funded by the Swedish Research Council.)