Beyond Numbers: CKD-EPI Versus MDRD in Primary Care—Differences in Chronic Kidney Disease Stage Classification in 117,055 Patients

<b>Background/Objectives</b>: Chronic kidney disease (CKD) is a global public health concern, posing significant diagnostic and management challenges in primary care. Estimated glomerular filtration rate (eGFR) is central to CKD staging, yet different estimating equations may yield substantially different stage classifications when applied to the same population. This study aims to compare the eGFR-based CKD stage classification and stage distribution obtained using the Chronic Kidney Disease: Epidemiology Collaboration (CKD-EPI) and Modification of Diet in Renal Disease (MDRD) equations in a large primary care cohort, and to explore the implications of these classification differences for routine use in primary healthcare (PHC). <b>Methods</b>: A cross-sectional analysis was conducted using standardized electronic health records from 117,055 PHC patients in the Matosinhos Health Unit, Portugal, spanning 22 years (2000-2022). CKD staging followed KDIGO guidelines and focused on stages G3-G5, based on the most recent available serum creatinine value. CKD-EPI and MDRD equations were compared overall and across age strata, BMI categories, albuminuria categories (when available), and major comorbidity subgroups, including heart failure, diabetes, and hypertension. <b>Results</b>: Using CKD-EPI, a higher proportion of individuals were classified as CKD stages G3-G5 (9042; 7.73%) compared with MDRD (7686; 6.57%). Classification differences were most pronounced in advanced stages (relative increase with CKD-EPI: G3b +29.4%, G4 +23.6% and G5 +34.4%). Among individuals aged ≥80 years, equation-related classification differences were particularly marked in advanced stages (G5). Similarly, CKD-EPI was associated with higher CKD stage classification rates in high-risk subgroups, including patients with heart failure. <b>Conclusions</b>: Compared with MDRD, CKD-EPI yields a higher proportion of individuals classified into CKD stages, particularly advanced stages and among older adults and high-risk comorbidity subgroups. These findings highlight the substantial impact of equation choice on CKD stage classification in primary care and support the use of CKD-EPI for standardized eGFR reporting, while emphasizing that observed differences reflect classification rather than confirmed CKD diagnosis.