GFR Calculator (CKD-EPI & MDRD)

GFR is estimated, not measured directly at the bedside, because the gold-standard methods — inulin clearance or nuclear medicine studies using iohexol or ⁵¹Cr-EDTA — are impractical for routine care. The CKD-EPI 2021 creatinine equation, now the standard reported by most U.S. and European labs, uses a two-slope (spline) relationship between serum creatinine and filtration rate, with separate coefficients above and below a sex-specific creatinine threshold (0.7 mg/dL for women, 0.9 mg/dL for men). The 2021 refit removed the previous race coefficient after evidence showed it did not reflect biological difference and could delay specialist referral and transplant listing for Black patients. Cockcroft-Gault, though older and less accurate for eGFR staging, remains embedded in drug-dosing tables (renally cleared antibiotics, DOACs, chemotherapy) because the original pharmacokinetic studies that set those dose adjustments were validated against Cockcroft-Gault creatinine clearance, not CKD-EPI. This calculator reports both for that reason: CKD-EPI to stage chronic kidney disease, Cockcroft-Gault to cross-check a drug label's dosing threshold. eGFR is only reliable at a steady-state creatinine — in acute kidney injury, when creatinine is actively rising or falling, none of these formulas produce a meaningful number, and clinical judgment or urine output must guide dosing instead.

Creatinine clearance as a filtration surrogate traces to Homer Smith's renal physiology work in the 1930s-40s, which established creatinine's near-ideal properties as an endogenous filtration marker: it is produced at a fairly constant rate from muscle metabolism and is filtered by the glomerulus with only modest tubular secretion. Donald Cockcroft and Henry Gault, two Canadian nephrologists, published their simple weight-and-age-based clearance estimate in Nephron in 1976, derived from 249 male inpatients and intended specifically to guide drug dosing at the bedside — not as a population screening tool. The Modification of Diet in Renal Disease (MDRD) study of the 1990s produced the first widely used eGFR equation from creatinine alone, but it lost accuracy at near-normal GFR values above roughly 60. The CKD-EPI collaboration, pooling data from over 8,000 patients across 10 studies, published a more accurate equation in 2009 that became the basis for KDIGO's global CKD staging guidelines. Following the 2020 National Kidney Foundation-ASN task force review of race-based diagnostics, CKD-EPI released a race-free refit in 2021, and U.S. laboratories transitioned to it as the default reporting equation over the following two years.