Glomerular Filtration Rate (CKD-EPI 2021)

What eGFR by CKD-EPI 2021 is and when it is used

The glomerular filtration rate (GFR) is the volume of plasma filtered by the glomeruli per minute and is the best overall index of kidney function. Measuring GFR directly requires exogenous markers, such as iohexol or iothalamate, which does not fit routine practice. For that reason, estimated GFR (eGFR), calculated from serum creatinine, is used.

The CKD-EPI 2021 equation was published by Inker and colleagues in the New England Journal of Medicine. The group refit the 2009 equation without the race variable, on the understanding that race is a social rather than a biological construct. The development data came from 10 studies, with 8,254 participants; the validation, from 12 studies, with 4,050 participants.

The KDIGO 2024 guideline recommends estimating GFR with a validated equation, rather than using creatinine alone, and states that the use of race in the calculation should be avoided. The National Kidney Foundation recommends CKD-EPI 2021 for adults. To compare with the earlier equation, see the MDRD GFR calculator.

How to calculate GFR in the calculator: inputs and units

The calculator asks for three inputs:

1. Serum creatinine, in mg/dL, greater than zero.
2. Age, in years, from 18 to 120, because the equation was developed for adults. The tool uses the whole number part: 59.8 years is entered as 59.
3. Sex: female (default option) or male.

The result appears in mL/min/1.73 m², as a whole number, as KDIGO 2024 recommends for reports, followed by the corresponding G category.

Before entering values, check:

• creatinine unit: values in µmol/L convert to mg/dL by dividing by 88.4, a factor from the KDIGO 2024 conversion table;
• laboratory method: the NKF states that the equation was developed for IDMS-standardized creatinine, and KDIGO 2024 recommends an enzymatic method whenever possible;
• stability: the equation assumes stable creatinine; in acute kidney injury, the result does not reflect current filtration.

Icteric samples may have creatinine altered by the method; the topic is covered on the page on creatinine and bilirubin.

CKD-EPI 2021 formula and symbol definitions

The calculator applies the 2021 CKD-EPI creatinine equation, as published by Inker and colleagues and reproduced by the National Kidney Foundation.

eGFR = 142 × min(S_Cr/κ; 1)^α × max(S_Cr/κ; 1)^−1.200 × 0.9938^age × 1.012 [if female]

• eGFR: estimated glomerular filtration rate, in mL/min/1.73 m².
• S_Cr: standardized serum creatinine, in mg/dL.
• κ: 0.7 for women and 0.9 for men.
• α: −0.241 for women and −0.302 for men.
• min(x; 1): the smaller of x and 1; max(x; 1): the larger.
• age: in years.

In practice, only one of the two creatinine terms changes the result. If creatinine is below κ, the exponent α applies; if it is above, the exponent −1.200 applies, and the other term equals 1.

The calculator applies no race factor and includes the 1.012 factor for women, as in the 2021 equation. It reproduces the NKF verification cases, such as an 18-year-old man with 0.90 mg/dL giving 127.

Worked example: creatinine of 1.2 mg/dL at age 60

Hypothetical case: serum creatinine of 1.2 mg/dL, age 60. The calculation changes according to sex.

Man (κ = 0.9; α = −0.302)

1. S_Cr/κ = 1.2 ÷ 0.9 = 1.3333. Because it is greater than 1, the term with α equals 1.
2. 1.3333^−1.200 = 0.7081.
3. 0.9938⁶⁰ = 0.6886.
4. 142 × 0.7081 × 0.6886 = 69.2, which on the report becomes 69 mL/min/1.73 m².

Woman (κ = 0.7; α = −0.241)

1. S_Cr/κ = 1.2 ÷ 0.7 = 1.7143, also greater than 1.
2. 1.7143^−1.200 = 0.5237.
3. 142 × 0.5237 × 0.6886 × 1.012 = 51.8, which on the report becomes 52 mL/min/1.73 m².

The calculator returns 69 and 52 for these data, with categories G2 and G3a. The example shows that the same creatinine corresponds to different filtration rates depending on sex, because average creatinine production differs. For that reason, creatinine alone is not enough to assess kidney function.

How to understand the result: KDIGO GFR categories

The calculator classifies eGFR using the G categories of the KDIGO 2024 guideline, applied to the whole number shown in the result.

The category alone does not diagnose chronic kidney disease. KDIGO 2024 defines the disease as an abnormality of kidney structure or function present for at least three months and classifies risk by combining the G category with albuminuria (categories A1 to A3). An eGFR between 60 and 89 without another marker of damage, for example, does not constitute chronic kidney disease.

Albuminuria can be assessed with the urine albumin-to-creatinine ratio. The final interpretation is up to the healthcare professional.

Common errors, units and verification checklist

The most frequent eGFR errors come from creatinine, not from the equation.

• Creatinine in µmol/L in the mg/dL field: a value of 106 µmol/L corresponds to 1.2 mg/dL; entered without conversion, it produces an eGFR close to zero.
• Using a race factor: the 2021 equation has no such factor. Results from older calculators that multiply the value by a race factor are not comparable.
• Applying the sex adjustment twice: the equation already contains κ, α and the 1.012; do not multiply creatinine by sex factors beforehand.
• Calculating during acute kidney injury: creatinine has not yet stabilized and eGFR overestimates function.
• Comparing reports from different methods: Jaffe and enzymatic may disagree, especially with icterus.

Checklist:

1. Creatinine in mg/dL, from a standardized method?
2. Adult patient, with stable creatinine?
3. Correct age and sex?
4. Result compared as a whole number, as on the report?
5. Was the category read together with albuminuria?

Limitations of CKD-EPI 2021 and comparison with other estimates

CKD-EPI 2021 is an estimate, not a measurement. In the validation study by Inker and colleagues, 85% or more of the estimates fell within 30% of measured GFR, which leaves a relevant individual margin of error.

The creatinine-based estimate is less reliable when creatinine production departs from the average:

• very low or very high muscle mass, amputations, malnutrition;
• diets very rich or very poor in meat, creatine supplements;
• frail people or those with severe acute illness;
• children, for whom KDIGO 2024 indicates equations validated in pediatric populations.

When the estimate seems inconsistent with the clinical picture, measured creatinine clearance is an alternative, with its own limitations.

Frequently asked questions about eGFR by CKD-EPI 2021

References

1. Inker LA, Eneanya ND, Coresh J, Tighiouart H, Wang D, Sang Y, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. N Engl J Med. 2021;385(19):1737-49. View
2. National Kidney Foundation. CKD-EPI Creatinine Equation (2021). View
3. Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int. 2024;105(4S):S117-S314. View
4. Levey AS, Stevens LA, Schmid CH, Zhang YL, Castro AF 3rd, Feldman HI, et al. A new equation to estimate glomerular filtration rate. Ann Intern Med. 2009;150(9):604-12. View