Difference Between GFR and eGFR

Glomerular Filtration Rate (GFR) is a test used to measure the level of kidney function. Basically, it measures how much blood passes through the glomeruli each minute. Estimated Glomerular Filtration Rate (eGFR) is the calculated value based on different GFR definitions. This is the key difference between GFR and eGFR; further differences will be summarized in this article.

What is Glomerular Filtration Rate (GFR)?

GFR is assumed to be the best available index of kidney function in health and disease. It is referred to as the volume of the fluid filtered from the renal glomerular capillaries into the Bowman’s capsule per unit time. Filtration rate depends on the blood pressure difference created by vasoconstriction of input versus vasoconstriction of output. GFR can be measured by clearance techniques using both endogenous (creatinine, urea) or exogenous (inulin, iothalamate) filtration markers. In clinical practice, GFR is often measured based upon serum creatinine concentrations.

What is Estimated Glomerular Filtration Rate (eGFR)?

eGFR (Estimated Glomerular Filtration Rate) is a calculated value based on filtration marker (basically, serum creatinine) concentration to assess the kidney function. Estimated GFR can vary with age even in a healthy population. Age-based average eGFR for healthy population is listed below

eGFR is basically performed to diagnose CKD (Chronic Kidney Disease) and currently it is classified into five stages based on eGFR as recommended by professional guidelines.

Equations for Calculating eGFR

In the past, a 24-hour creatinine clearance has been considered as the sensitive method of measuring kidney function. But due to practical limitations of collecting timed urine samples and failure to collect the entire specimen, National Kidney Foundation Disease Outcomes Quality Initiative (K-DOQI) recommends the use of eGFR calculated from prediction equation based on plasma/serum creatinine.

This offered an easy and practical approach for calculating eGFR taking into consideration factors like patient’s age, sex, weight and ethnicity (depending on the type of equation). The commonly used equations are Modification of Diet in Renal Disease [(MDRD) (1999)] and Chronic Kidney Disease Epidemiology Collaboration [(CKD-EPI) (2009)]

For estimating GFR in patients under age 18 years, the Bedside Schwartz equation can be used.

MDRD Equation

MDRD eGFR = 186×[Plasma Creatinine (μmol/L)×0.0011312]−1.154 ×[age (years)]−0.203 ×[0.742 if female]×[1.212 if black]

Units – mL/min/1.73m²

This equation was validated in patients with diabetic kidney disease, renal transplant recipients and African Americans with non-diabetic kidney disease. But it has not been validated in children age under 18 years, pregnant women and patients over 70 years of age.

CKD-EPI Equation

White or other

Female with Creatinine≤0.7mg/dL; use eGFR=144×(Cr/0.7)^−0.329×(0.993)Age

Female with Creatinine>0.7mg/dL; use eGFR=144×(Cr/0.7)^−1.209×(0.993)Age

Male with Creatinine≤0.9mg/dL; use eGFR=141×(Cr/0.9)^−0.411×(0.993)Age

Male with Creatinine>0.9mg/dL; use eGFR=141×(Cr/0.9)^−1.209×(0.993)Age

Black

Female with Creatinine≤0.7mg/dL; use eGFR=166×(Cr/0.7)^−0.329×(0.993)Age

Female with Creatinine>0.7mg/dL; use eGFR=166×(Cr/0.7)^−1.209×(0.993)Age

Male with Creatinine≤0.9mg/dL; use eGFR=163×(Cr/0.9)^−0.411×(0.993)Age

Male with Creatinine>0.9mg/dL; use eGFR=163×(Cr/0.9)^−1.209×(0.993)Age

Units – mL/min/1.73m²

CKD-EPI equation minimizes the over diagnosis of CKD with MDRD equation. This includes log serum creatinine model with gender, race and age on natural scale.

What is the difference between GFR and eGFR?

Definition

GFR: GFR is the rate of blood passed through the kidney

eGFR: eGFR is a result which can be obtained through GFR.

Use

GFR: GFR acts as one of the best ways of measuring kidney function.

eGFR: eGFR provide a value for that.

This value is totally based on equations validated on different conditions. Therefore significant errors are possible in people with extreme body weight, pregnant women, and  children. In addition, most equations validate US white and Black patients and may not compatible with other ethnic groups.

References

National Kidney Foundation [Online]. Available: https://www.kidney.org/atoz/content/gfr [Accessed June 23 2016].

National Institute of Daibetes and Diagestive Kidney Diseases [Online]. Available: http://www.niddk.nih.gov/health-information/health-communication-programs/nkdep/lab-evaluation/gfr/estimating/Pages/estimating.aspx [Accessed June 23 2016].

American Kidney Fund [Online]. Available: http://www.kidneyfund.org/prevention/tests-for-kidney-health/?referrer=https://www.google.lk/?referrer=http://www.kidneyfund.org/prevention/tests-for-kidney-health/ [Accessed June 23 2016].

FLORKOWSKI, C. M. & CHEW-HARRIS, J. S. 2011. Methods of Estimating GFR – Different Equations Including CKD-EPI. Clin Biochem Rev, 32, 75-9.

MULA-ABED, W. A., AL RASADI, K. & AL-RIYAMI, D. 2012. Estimated Glomerular Filtration Rate (eGFR): A Serum Creatinine-Based Test for the Detection of Chronic Kidney Disease and its Impact on Clinical Practice. Oman Med J, 27, 108-13.

XIE, D., JOFFE, M. M., BRUNELLI, S. M., BECK, G., CHERTOW, G. M., FINK, J. C., GREENE, T., HSU, C. Y., KUSEK, J. W., LANDIS, R., LASH, J., LEVEY, A. S., O’CONNER, A., OJO, A., RAHMAN, M., TOWNSEND, R. R., WANG, H. & FELDMAN, H. I. 2008. A comparison of change in measured and estimated glomerular filtration rate in patients with nondiabetic kidney disease. Clin J Am Soc Nephrol, 3, 1332-8.

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“Physiology of Nephron.” y Madhero88 – Own work ( References here) (CC BY 3.0) via Commons Wikimedia