CKD-EPI Equations for Glomerular Filtration Rate (GFR)

Introduction
CKD-EPI Equations are the standard used to measure kidney function in patients with stable, chronic kidney disease (not acute). These equations use creatinine or a combination of creatinine and cystatin C to measure creatinine clearance as a proxy to estimate glomerular filtration rate(rate). 

The 2021 CKD-EPI creatinine-cystatin C version is the recommended standard and the 2021 CKD-EPI creatinine version is currently recommended by the ASN and NKF for GFR reporting in the United States. However, cystatin C is not available in all laboratories. Yet, the creatinine-based version is adequate for many clinical applications even though the combined creatinine and cystatin C can add accuracy. However, specific populations, such as diabetics, pregnant women, and individuals with unusual body mass (obese, amputees, and severe malnutrition), are at risk for less accurate creatinine-based estimates of kidney function. The 2021 CKD-EPI creatinine-cystatin may offer improved accuracy in these special populations with the addition of cystain C, and is recommended as a confirmatory test in patients with extremes of body composition, to include body fat or muscle mass. CKD-EPI is not indicated for patients on dialysis.

Some of the CKD-EPI equations include adjustments based on race. However, adjustments base on race appear to be controversial and the pros/cons of adjustments ought to be part of a shared decision-making approach between the clinician and the patient. For more information on the topic of eGFR equations and race, NephJC (FitzGerald, 2019) provides an excellent primer on the subject.

The CKD-EPI (creatinine) was developed by Levey et al (2009). 

• Uses the new gold standard: Clearance of iothalamate

• Uses linear regression to estimate the logarithm of measured GFR from standardized creatinine levels, sex, race, and age.

2021 CKD-EPI Creatinine

• = 142 x (Scr/A)^B x 0.9938^age x (1.012 if female)

• A & B as follows:

2021 CKD-EPI Creatinine-Cystatin C

• = 135 x (Scr/A)^B x (Scys/C)^D x 0.9961^age x (0.963 if female)

• A, B, C & D as follows:

 2009 CKD-EPI Creatinine*

• = A x (Scr/B)^C x 0.993^Age x (1.159 if black)

• A, B & C as follows:

2012 CKD-EPI Cystatin C 

• = 133 x (scys/0.8)^A x 0.996^age x B

• A& B as follows:

2012 CKD-EPI Creatinine-Cystatin C*

• = A x (Scr/B)^C x (Scys/0.8)^D x 0.995^age x (1.08 if Black)

• A, B, C, & D as follows:

*Race may or may not provide better estimates of GFR. Using the adjustment coefficient for black patients is controversial (Inker et al, 2021) and is an optional component.

Abbreviations

• Scr = serum creatinine (mg/dL)

• Scys = serum cystatin C (mg/L)

• GFR = glomerular filtration rate (ml/min/1.73 m²)

Chronic Kidney Disease (CKD) Staging

Source: KIDGO 2012 Clinical Practice Guidelines

Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI)