Association Between the Modified Furosemide Response Index and Prognosis in Patients with Chronic Kidney Disease: A Retrospective Cohort Study Based on the MIMIC-IV Database

Background: The modified furosemide response index (mFRI), derived from a standardized furosemide stress test, is a biomarker of diuretic responsiveness and AKI progression. Its prognostic value in chronic kidney disease (CKD) remains unclear. This study examined the association between mFRI and clinical outcomes in critically ill patients with CKD. Methods: We conducted a retrospective cohort study using the MIMIC‑IV database. Critically ill CKD patients receiving furosemide were included. Restricted cubic splines were used to examine the non‑linear relationship between mFRI and mortality. Patients were grouped by mFRI quartiles. Multivariable Cox and logistic regression assessed outcome associations, with subgroup analyses for robustness. Results: Among 3,311 patients, a significant L‑shaped non‑linear association was observed between mFRI and 28‑day mortality (P < 0.001). Mortality risk declined sharply as mFRI increased to 0.143, then plateaued. Kaplan‑Meier analysis showed progressively lower mortality from the lowest (Q1) to the highest (Q4) mFRI quartile. Higher mFRI quartiles were associated with lower all‑cause mortality and reduced use of renal replacement therapy. Conclusion: mFRI has a non‑linear, L‑shaped association with prognosis in critically ill patients with CKD. Because it is simple, low‑cost, and repeatable, mFRI could be a practical bedside tool for risk stratification in this population.