Appropriate interpretation of kidney function is essential for clinical and therapeutic management of heart failure (HF). We evaluated the prognostic accuracy of 6 glomerular filtration rate estimation (eGFR) formulas in HF patients with reduced ejection fraction (HFrEF) and their impact on the Metabolic Exercise test data combined with Cardiac and Kidney Indexes (MECKI) score prognostic accuracy. We retrospectively analyzed 6933 patients enrolled in the MECKI score database. GFR was estimated using Modification of Diet in Renal Disease (MDRD); MRDR modified (MDRDm); Cockcroft-Gault (CG), CG modified (CGm); Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI), and the European Kidney Function Consortium (EKFC). Survival was assessed as the composite of cardiovascular death, left ventricular assist device implantation and urgent heart transplantation at 2 years. Each GFR estimation demonstrated similar but moderate prognostic capacity, with the area under the curve (AUC) for predicting survival ranging from 0.6271 (EKFC) to 0.635 (MDRD). For cardiovascular death, the AUC values ranged from 0.668 to 0.677. The prevalence of severe CKD, defined as eGFR <30 mL/min/1.73 m², ranged from 3.2% (MDRD) to 4.5% (EKFC). When included in MECKI score, the 6 formulas showed a MECKI AUC for prognosis ranging from 0.7841 to 0.7883, with the EKFC and CKD-EPI showing the best performance. GFR estimations play a role in HFrEF prognosis without difference among the 6 most frequently used formulas. Furthermore, using eGFR calculated from the 6 different formulas in MECKI score did not significantly alter its strong prognostic power, highlighting MECKI reliability in risk stratification.