eGFR Calculator (CKD-EPI 2021)

About the eGFR Calculator (CKD-EPI 2021)

eGFR (estimated glomerular filtration rate) is the standard estimate of how well the kidneys filter blood, in mL/min per 1.73 m² of body surface. Labs work it out from your blood creatinine, age and sex. This calculator uses the race-free CKD-EPI 2021 equation that the US National Kidney Foundation and American Society of Nephrology recommend, adds the 2021 creatinine–cystatin C equation (and the 2012 cystatin C equation) when you have a cystatin C result, and for children uses the CKiD U25 2021 equations, with the older bedside Schwartz equation alongside.

With a urine albumin–creatinine ratio (ACR) it places you on the KDIGO 2024 risk heat map (G1–G5 by A1–A3) and explains in plain words what the category means — and when an eGFR is likely to be wrong. Creatinine can be entered in mg/dL or µmol/L. Nothing leaves your browser.

The equations

• CKD-EPI 2021 creatinine (adults): eGFR = 142 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^−1.200 × 0.9938^age × 1.012 (if female), with κ = 0.7 (female) or 0.9 (male) and α = −0.241 (female) or −0.302 (male). Scr is serum creatinine in mg/dL (µmol/L ÷ 88.4).
• CKD-EPI 2021 creatinine–cystatin C: 135 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^−0.544 × min(Scys/0.8, 1)^−0.323 × max(Scys/0.8, 1)^−0.778 × 0.9961^age × 0.963 (if female), with α = −0.219 (female) or −0.144 (male).
• CKD-EPI 2012 cystatin C: 133 × min(Scys/0.8, 1)^−0.499 × max(Scys/0.8, 1)^−1.328 × 0.996^age × 0.932 (if female).
• CKiD U25 2021 (children, used for the category under 18): eGFR = K × height (m) ÷ creatinine (mg/dL). K for boys is 39.0 × 1.008^(age − 12) up to 12 and 39.0 × 1.045^(age − 12) from 12 to 18; for girls 36.1 × 1.008^(age − 12) and 36.1 × 1.023^(age − 12) (50.8 and 41.4 from 18). With cystatin C, eGFR = K ÷ cystatin C (mg/L), K for boys 87.2 × 1.011^(age − 15) below 15 and 87.2 × 0.960^(age − 15) from 15; for girls 79.9 × 1.004^(age − 12) below 12 and 79.9 × 0.974^(age − 12) from 12 — and the category uses the average of the two results, which the authors found unbiased and more accurate. KDIGO 2024 judges that many clinicians would choose the CKiD U25 creatinine equation for children.
• Bedside Schwartz (children, for comparison): 0.413 × height (cm) ÷ creatinine (mg/dL), from children aged 1–16 with CKD; many labs still report it.

None of the 2021 equations uses race. They are valid only with standardised (IDMS-traceable) creatinine and standardised cystatin C. KDIGO 2024 recommends estimating the GFR category from creatinine and cystatin C together when cystatin C is available, which is why that result is shown first when you enter it.

KDIGO 2024 categories and the heat map

• GFR categories (mL/min/1.73 m²): G1 ≥ 90 normal or high · G2 60–89 mildly decreased · G3a 45–59 mildly to moderately decreased · G3b 30–44 moderately to severely decreased · G4 15–29 severely decreased · G5 < 15 kidney failure.
• Albuminuria categories (urine ACR): A1 < 30 mg/g (< 3 mg/mmol) normal to mildly increased · A2 30–300 mg/g (3–30 mg/mmol) moderately increased · A3 > 300 mg/g (> 30 mg/mmol) severely increased.
• Heat map: green = low risk (if there are no other markers of kidney disease, no CKD) for G1–G2 with A1; yellow = moderately increased risk (G1–G2 A2, G3a A1); orange = high risk (G1–G2 A3, G3a A2, G3b A1); red = very high risk (G3a A3, G3b A2–A3 and all of G4–G5).

CKD means abnormalities of kidney structure or function present for at least 3 months. KDIGO says not to assume chronicity from one abnormal eGFR or ACR, because it may reflect a recent acute kidney injury; an ACR of 30 mg/g or more on a random sample should be confirmed with a first-morning sample. In people with CKD, a change in eGFR of more than 20% on a later test exceeds the expected variability and warrants evaluation.

When an eGFR can be wrong

Creatinine comes from muscle and diet as well as reflecting filtration, so the estimate is less accurate when:

• Kidney function is changing quickly — acute kidney injury, or any time creatinine is not stable (the equations assume a steady state).
• You are pregnant — the CKD-EPI equations have not been validated in pregnancy.
• Muscle mass or body size is unusual — very muscular or very frail people, severe malnutrition or obesity, muscle disease, amputation, paraplegia or quadriplegia.
• Diet — vegetarian or very low-protein diets, high-protein diets or creatine supplements, a large meat meal before the test.
• Medicines that block creatinine secretion in the kidney or interfere with the lab method.
• Age under 18 — use a children’s equation (this calculator switches to CKiD U25 2021 and shows bedside Schwartz too).
• Transgender and gender-diverse people, especially on gender-affirming hormones, which can change both creatinine and cystatin C: KDIGO advises a shared decision about how to estimate GFR.

In several of these situations cystatin C (or a measured GFR) gives a better answer — ask your doctor.

Sources

• Inker LA, Eneanya ND, Coresh J, et al. New creatinine- and cystatin C–based equations to estimate GFR without race. N Engl J Med 385(19):1737-49 · National Kidney Foundation: CKD-EPI creatinine equation (2021), creatinine–cystatin C equation (2021) and cystatin C equation (2012)
• Delgado C, Baweja M, Crews DC, et al. A unifying approach for GFR estimation: recommendations of the NKF-ASN Task Force on reassessing the inclusion of race in diagnosing kidney disease. Am J Kidney Dis 79(2):268-88
• KDIGO CKD Work Group. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int 105(4S):S117-S314 (nomenclature and heat map p. S126; Recommendation 1.1.2.1; Practice Points 1.1.3.2, 1.2.4.3, 1.3.1.2, 2.1.3; Table 8, conditions that make creatinine-based eGFR less accurate; Table 14, validated equations), with its executive summary, Kidney Int 105:684-701
• Pierce CB, Muñoz A, Ng DK, et al. Age- and sex-dependent clinical equations to estimate glomerular filtration rates in children and young adults with chronic kidney disease. Kidney Int 99(4):948-56 (Table 2: CKiD U25 K values)
• Schwartz GJ, Muñoz A, Schneider MF, et al. New equations to estimate GFR in children with CKD. J Am Soc Nephrol 20(3):629-37

Limitations

• An estimate: KDIGO 2024 notes that at best about 90% of eGFR results fall within 30% of the measured GFR, so a single number is never exact.
• It cannot tell which unit your report uses. Creatinine of 9–20 fits both mg/dL and µmol/L, so the calculator warns you and keeps the number as typed when you switch the unit; the ACR is never converted on a switch — choose the unit printed on your report.
• For adults 18 and over with the CKD-EPI equations; children 1–17 with CKiD U25 2021 (and bedside Schwartz for comparison). Both children’s equations were developed in children with CKD; CKiD U25 was not tested in children without CKD and is less certain in the very young and at very low GFR. Not for infants, dialysis or kidney transplant care.
• Creatinine 0.1–20 mg/dL (9–1,768 µmol/L), cystatin C 0.2–10 mg/L, height 45–220 cm.
• It does not calculate drug doses. Doses for medicines cleared by the kidneys are decided by your prescriber.

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