eGFR using CKD-EPI (Creatinine-Cystatin C) Equation (2021)

The CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation was developed in an effort to create a more precise formula to estimate glomerular filtrate rate (GFR) from serum creatinine and serum cystatin, especially at when actual GFR is >60 mL/min per 1.73m².

The estimated glomerular filtration rate (GFR) is a quick way to assess how well the kidneys filter toxins from the blood, balance fluids, control the production of red blood cells, and more. Estimated glomerular filtration rate (eGFR) is one of the primary diagnostic methods for detecting and managing kidney diseases. To find the eGFR, healthcare professionals use a blood test and a special calculator.

Researchers pooled data from multiple studies to develop and validate new equation. They randomly divided 10 studies which included 8254 participants, into separate data sets for development and internal validation. 16 additional studies, which included 3896 participants, were used for external validation.

The CKD-EPI equation performed better than the MDRD (Modification of Diet in Renal Disease Study) equation, especially at higher GFR, with less bias and greater accuracy. When looking at NHANES (National Health and Nutrition Examination Survey) data, the median estimated GFR was 94.5 mL/min per 1.73 m² vs. 85.0 mL/min per 1.73 m², and the prevalence of chronic kidney disease was 11.5% versus 13.1%.

The eGFR CKD-EPI 2009 equation includes age, sex, race and/or body weight to approximate directly measured kidney function. However, race is a social, not a biological, construct. After rigorous scientific study, discussion, and patient input, the Task Force recommended using the new eGFR 2021 CKD EPI Creatinine Equation. Although not yet widely used, the serum cystatin C level is recommended for confirmatory testing of eGFR. The current equation for eGFR that uses serum cystatin C (eGFR_cys) does not include race and has similar accuracy to eGFR_cr. The current equation that uses both markers (eGFR_cr-cys) is more accurate than either eGFR_cr or eGFR_cys.

This calculation, originally published by Dr. Inker in the New England Journal of Medicine, uses creatinine and cystatin C to estimate kidney function without using race. It has been endorsed by the United States Pathology and Laboratory Society Leadership and The Organ Procurement Transplantation Network (OPTN) who unanimously agreed to require transplant hospitals to use the new race-neutral calculation.

"One of the points that we made over and over again in the task force discussion is that race is a social construct. It's not a biological construct," Glenda said. "By removing it from the equation, we have acknowledged the fact that we want to look at biological factors that might impact kidney disease."

Equation: eGFR using the CKD-EPI Creatinine-Cystatin C Equation (2021).
eGFR_cr-cys = 135 x min(S_cr/к, 1)^α x max(S_cr/к, 1)^-0.544 x min(S_cys/0.8, 1)^-0.323 x max(S_cys/0.8, 1)^-0.778 x 0.9961^Age x 0.963 [if female]

where:

S_cr = standardized serum creatinine in mg/dL
к = 0.7 (females) or 0.9 (males)
α = -0.219 (female) or -0.144 (male)
min(S_cr/к, 1) is the minimum of S_cr/к or 1.0
max(S_cr/к, 1) is the maximum of S_cr/к or 1.0
S_cys = standardized serum cystatin C in mg/L
Age (years)