Glomerular Filtration Rate Equations

Glomerular filtration rate (GFR) equations use laboratory measurements and demographic variables to calculate an estimated GFR (eGFR). While no single eGFR equation offers an overwhelming advantage in estimating kidney function for all patients or clinical situations, the recommended adult and pediatric equations below are the preferred equations. Laboratories are encouraged to use the recommended equations to standardize practice.

Regardless of the equation used to calculate eGFR, the eGFR value is an estimate and is not a precise measure of kidney function. Imprecision is inherent in all estimating equations. Despite various factors that can affect the performance of eGFR equations, estimating equations are still the preferred method for assessing kidney function. Laboratories can help health care professionals assess and track patients’ kidney function by following best practices for estimating and reporting eGFR.

Using urine albumin and the urine albumin-to-creatinine ratio in individuals with or at risk for kidney disease is also recommended for a more complete assessment of kidney disease.

Adult eGFR Equations

NIDDK supports the National Kidney Foundation–American Society of Nephrology Task Force’s recommendation to calculate eGFR using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations that do not include a race coefficient.¹

• 2021 CKD-EPI creatinine equation (2021 CKD-EPI eGFRcr)
• 2021 CKD-EPI creatinine-cystatin C equation (2021 CKD-EPI eGFRcr-cys)
• 2012 CKD-EPI cystatin C equation (2012 CKD-EPI eGFRcys)

Using both creatinine and cystatin C to estimate GFR is preferred and is more accurate than using serum creatinine alone. Using the combined creatinine-cystatin C equation is particularly important when the eGFR is close to a critical decision value, such as in certain cases of drug dosing or kidney transplant evaluation. Using the combined creatinine-cystatin C equation may also be particularly important when assessing racial disparities in risk of kidney failure or death. Use of the 2021 CKD-EPI eGFRcr equation appears to attenuate these disparities compared to the 2021 CKD-EPI eGFRcr-cys, the 2012 CKD-EPI eGFRcys, and the 2009 race-based CKD-EPI eGFR equations.²

Limitations of the 2021 CKD-EPI eGFRcr Equation

Although combined use of creatinine and cystatin C is preferred, cystatin C is not currently routinely assessed in clinical contexts. Therefore, the 2021 CKD-EPI eGFRcr equation is likely to become the most widely used equation in the near future. Thus, it is particularly important to acknowledge its limitations. Estimated GFR calculated using the 2021 CKD-EPI eGFRcr equation may be less precise compared to the 2021 CKD-EPI eGFRcr-cys, the 2012 CKD-EPI eGFRcys, and the 2009 race-based CKD-EPI equations.^1,2,3,4 On average, the 2021 CKD-EPI eGFRcr equation appears to overestimate GFR (higher eGFR) in non-Black individuals by approximately 4%, while underestimating kidney function (lower eGFR) in Black individuals by approximately -4%.

Decisions on how to best account for biases in various eGFR calculations when clinical decisions are being made, such as with drug dosing and listing for kidney transplant, can be individualized by health care teams based on conditions specific to each patient.

Pediatric eGFR Equations

The following pediatric eGFR equations are available for use in children, adolescents, and young adults

• Chronic Kidney Disease in Children under (age) 25 (CKiD U25) equations⁵
• Neonatal eGFR equation⁶
• European Kidney Function Consortium (EKFC) equations^7,8
• 2009 Creatinine-based CKiD “bedside” equation⁹

Best Practices for Reporting eGFR

Reporting eGFR when serum creatinine, cystatin C, or both markers are measured is a simple and effective way laboratories can give health care professionals the information they need to detect, monitor, and treat CKD in patients.

Given that the eGFR value is an estimate and not a precise measure of kidney function, health care teams should use eGFR trends, when available, rather than a single point estimate. To best understand the change in a person’s kidney function over time, health care teams compare eGFR values calculated using the same equation. If eGFR values calculated using different equations are compared, some of the change in eGFR could be a consequence of using different equations rather than a change in kidney function.

NIDDK suggests laboratories

• Report eGFR whenever serum creatinine is measured, even if the information system cannot identify if eGFR reporting is appropriate for the patient. This allows the health care team to assess the suitability of a result for the patient’s condition.
• Use the recommended equations to calculate eGFR. Using these equations will standardize eGFR reporting in all laboratories.
• If both serum creatinine and serum cystatin C are measured, use the combined creatinine-cystatin C equation. This equation provides greater accuracy than the serum creatine-only equation. Also report the eGFRcr and eGFRcys because if important non-GFR determinants for one or the other biomarker are present, the combined equation may not be the best estimate.
• Clearly identify the equation used when reporting eGFR.
• If your system has changed equations, indicate the date of the change. This allows the health care teams to more accurately assess trends in eGFR over time.
• Report all eGFR values as an exact number, rounded to a whole number (e.g., 62 mL/min/1.73 m²). The increased accuracy of the CKD-EPI equations allows for more precise reporting of eGFR values than would have previously been reported as ≥ 60 mL/min/1.73 m².
• Round serum creatinine and cystatin C values as follows, to reduce rounding errors using the estimating equations.
  • Creatinine in mg/dL to two decimal places (e.g., 0.95 mg/dL)
  • Creatinine in µmol/L to the nearest whole number (e.g., 84 µmol/L).
  • Cystatin C in mg/L to two decimal places (e.g., 0.85 mg/L)

References