The prognostic value of a laboratory-based frailty index (FI-Lab) in carbapenem-resistant Acinetobacter baumannii infections: a multicentre cohort study

Background Carbapenem-resistant Acinetobacter baumannii (CRAB) infections remain a major cause of mortality despite advances in antimicrobial therapy. Frailty, reflecting cumulative physiological decline, may strongly influence outcomes beyond age or comorbidity. This study evaluated whether a 30-item Frailty Index–Laboratory (FI-Lab), calculated within 48 h of admission, independently predicts early (7-day) and short-term (30-day) mortality among hospitalized patients with CRAB infections. Methods We conducted a multicentre retrospective cohort study including adults with microbiologically confirmed CRAB infection admitted to four Infectious Diseases Units in Apulia, Italy (Bari, Foggia, Taranto–Moscati, Bisceglie) between January 2020 and August 2025. The FI-Lab, derived from 30 biochemical parameters, quantified the proportion of abnormal laboratory values (range 0–1). Cox regression models assessed associations between FI-Lab and mortality, adjusting for age, comorbidity, SOFA score, septic shock, infection site, therapy type, and other covariates. Results Among 199 patients (mean age 70 ± 15 years; 38.2% male), the median FI-Lab was 0.56 (IQR 0.45–0.68). Seven-day, 30-day, and in-hospital mortality were 12.6%, 19.1%, and 20.6%, respectively. Each + 0.10 increase in FI-Lab was associated with higher 7-day (HR 1.52, 95% CI 1.22–1.89), 30-day (HR 1.68, 95% CI 1.36–2.08), and in-hospital (HR 1.63, 95% CI 1.32–2.01) mortality (all p < 0.01). Patients with respiratory infections had significantly higher 30-day mortality than those with bloodstream infections (HR 1.62, 95% CI 1.05–2.51, p = 0.029). No mortality difference was observed between cefiderocol- and colistin-based regimens. FI-Lab demonstrated excellent discrimination (AUC = 0.89 for 30-day mortality). Conclusions The FI-Lab independently predicted early and short-term mortality in CRAB infections, outperforming traditional indices and highlighting the prognostic weight of biological frailty. The strong impact of respiratory infection and the absence of therapy-related differences suggest that host vulnerability may outweigh antimicrobial choice in determining outcomes. FI-Lab, easily derived from routine tests, could be integrated into clinical algorithms for early risk stratification in multidrug-resistant infection.