Estimating kidney function by calculating the estimated glomerular filtration rate (eGFR) is important, and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine-cystatin C equation (CKD-EPIcr-cys) is one of the most accurate ways of estimating eGFR. However, after reviewing thousands of laboratory tests from institutions nationwide, I've seen only ONE lab automatically calculate the eGFR using CKD-EPIcr-cys.
Instead, labs typically calculate eGFR using either the CKD-EPI creatinine equation (CKD-EPIcr) based on serum creatinine or the CKD-EPI cystatin C equation (CKD-EPIcys) based on serum cystatin C. But if you want the more accurate CKD-EPIcr-cys? You're on your own, and you must calculate CKD-EPIcr-cys using a website like the National Kidney Foundation.
A more practical and quicker method to estimate the eGFR of CKD-EPIcr-cys is to average the eGFR values obtained from CKD-EPIcr and CKD-EPIcys. This method is almost as accurate and significantly faster.
Consider an 81-year-old man with a serum creatinine level of 1.6 mg/dL and a serum cystatin C level of 2.2 mg/L.
Using the equations from the NKF website:
- Using CKD-EPIcr: eGFR = 43 mL/min/1.73 m²
- Using CKD-EPIcys: eGFR = 25 mL/min/1.73 m²
The estimated eGFR by averaging these values is (43 + 25) / 2 = 34 mL/min/1.73 m².
For comparison, the calculated eGFR using CKD-EPIcr-cys is approximately 33 mL/min/1.73 m², showing a difference of only 1 mL/min/1.73 m².
In this example, the difference in eGFR was only 1. Using the average eGFR of CKD-EPIcr and CKD-EPIcys -- both printed on most lab tests -- is usually an excellent way to estimate the eGFR of CKD-EPIcr-cys.
You can also create a macro to automate the averaging process, reporting the estimated eGFR in patient charts as "eGFR = X (avg. of CKD-EPIcr & CKD-EPIcys)."