eGFR Calculator (CKD-EPI 2021) — Estimated Glomerular Filtration Rate

About

The estimated glomerular filtration rate (eGFR) is a calculated measure of kidney function derived from serum creatinine, age, and sex. The CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation was first introduced in 2009 by Inker, Levey and colleagues as an improvement over the MDRD Study equation, offering superior accuracy across the full spectrum of kidney function, particularly at eGFR values ≥60 mL/min/1.73m². The equation was developed and validated in a diverse population of over 8,000 individuals from 10 studies, encompassing both healthy volunteers and patients with chronic kidney disease. In 2021, a landmark revision removed the race coefficient following evidence that its inclusion perpetuated health disparities and delayed CKD diagnosis and nephrology referral in Black patients. The updated equation uses serum creatinine, age, and sex, with an optional cystatin C-based version for improved accuracy in specific clinical scenarios. eGFR is central to CKD screening, diagnosis, staging, and prognostication according to KDIGO guidelines. It guides medication dosing, determines nephrology referral timing, stratifies cardiovascular and renal risk, and monitors disease progression. The equation has been extensively validated against measured GFR using gold-standard filtration markers such as iothalamate and iohexol clearance. eGFR also plays a role in preoperative risk assessment, living kidney donor evaluation, and epidemiological studies of kidney disease prevalence. Despite its utility, eGFR has important limitations — it assumes steady-state renal function, is influenced by non-renal factors such as muscle mass, dietary protein intake, and drugs affecting tubular creatinine secretion (e.g., cimetidine, trimethoprim), and demonstrates reduced accuracy at the extremes of age, body size, and in certain ethnic subgroups not well-represented in the original validation cohorts. The ongoing Nephrotic Syndrome Study Network and Chronic Kidney Disease in Children studies continue to refine these equations for special populations.

Formula

eGFR = 142 × min(SCr/κ, 1)^α × max(SCr/κ, 1)^-1.2 × 0.9938^Age × (1.012 if female)

The CKD-EPI 2021 equation calculates eGFR in mL/min/1.73m² using four input variables: serum creatinine (SCr), age, sex, and optionally cystatin C. The equation uses two sex-specific constants: κ (kappa), which represents the population median SCr value — 0.7 mg/dL for females and 0.9 mg/dL for males; and α (alpha), the sex-specific coefficient for the interaction between SCr and age — -0.241 for females and -0.302 for males. The equation computes two terms: a "min" term that handles SCr values at or below the population median, and a "max" term for values above the median. Specifically, the min term is min(SCr/κ, 1)^α, and the max term is max(SCr/κ, 1)^-1.209. When SCr is low relative to the sex-specific κ, the min term predominates; when SCr is high, the max term drives the result downward. The age term is 0.9938^Age, reflecting the natural decline in GFR with aging. For females, a multiplication factor of 1.012 is applied, accounting for the lower average muscle mass and therefore lower creatinine production relative to males. All terms are multiplied by 142 (the intercept or scaling factor) to yield the final eGFR. For example, a female with SCr 0.7 mg/dL (equal to κ) yields min term = 1^α = 1 and max term = 1^-1.209 = 1, so only age and sex factor influence the result. As SCr rises above κ, the max term progressively reduces the eGFR. The result is interpreted in the context of KDIGO CKD stages G1 through G5, where G1 is eGFR ≥90 and G5 is eGFR <15 mL/min/1.73m². Note that eGFR values above 60 mL/min/1.73m² are reported as "≥60" in many clinical laboratories because precision decreases at higher GFR levels.

Score Interpretation

eGFR calculated by CKD-EPI is the cornerstone of CKD diagnosis and staging according to KDIGO 2024 guidelines, which recommend using the CKD-EPI 2021 equation without race for all adults. The KDIGO classification system combines eGFR categories (G1-G5) with albuminuria categories (A1-A3) to stratify risk of CKD progression, cardiovascular events, and mortality. eGFR is essential for early detection of CKD in at-risk populations including patients with diabetes, hypertension, cardiovascular disease, and family history of kidney disease. Annual eGFR screening in these groups enables timely intervention with renoprotective therapies such as ACE inhibitors, ARBs, SGLT2 inhibitors, and finerenone. eGFR thresholds guide medication safety — for example, metformin is contraindicated when eGFR falls below 30 mL/min/1.73m², SGLT2 inhibitors require dose adjustment or discontinuation below 30, and many antibiotics, anticoagulants, and chemotherapeutic agents require renally adjusted dosing based on eGFR ranges. eGFR also determines nephrology referral timing: KDIGO recommends referral when eGFR is <30 mL/min/1.73m² (G4-G5), when there is rapid decline (>5 mL/min/1.73m² per year), or when eGFR is <45 with significant albuminuria. Beyond clinical care, eGFR is a critical endpoint in clinical trials of kidney disease and is used to define disease progression (sustained decline of ≥40% in eGFR from baseline). The UK National Institute for Health and Care Excellence, the American Diabetes Association, and the European Renal Association all recommend CKD-EPI as the preferred equation. Despite its central role, eGFR should not be used in isolation — it must be interpreted alongside albuminuria, urine sediment, renal imaging, and clinical history for comprehensive kidney assessment.

Reference Ranges