Timing of Antibiotics in ICU Pneumonia: An Observational Association Between Early Treatment and Higher Mortality

Background: Early administration of antibiotics is commonly recommended for pneumonia in intensive care unit (ICU) patients. However, the clinical benefit of very early empirical treatment remains uncertain and may reflect differences in illness severity, baseline risk, or care pathways, particularly in non-septic or hemodynamically stable ICU populations. Methods: We performed a retrospective cohort study using the Medical Information Mart for Intensive Care IV (v2.2) database to evaluate the observational association between antibiotic timing and in-hospital mortality among adult ICU patients with pneumonia. Patients were categorized as receiving early (<3 h) or delayed (≥3 h) antibiotic therapy after ICU admission. A multivariable logistic regression model adjusted only for age and sex. Given the absence of detailed severity-of-illness measures, no causal inference was intended, and all analyses were considered hypothesis-generating. Additional analyses exploring antibiotic class, dosing frequency, and combination therapy were conducted in an exploratory manner, given substantial variation in sample sizes and a high risk of confounding by indication, misclassification, immortal-time, and survivorship bias. Results: Among 7569 ICU patients with pneumonia, 56.5% received antibiotics within three hours of ICU admission. Early antibiotic initiation was associated with higher in-hospital mortality than delayed therapy (26.1% vs. 21.5%; OR 1.30, 95% CI 1.16-1.44; p < 0.001). Because validated severity-of-illness measures were unavailable, residual confounding and confounding by indication are likely and may largely explain this association. A potential signal of increased mortality was observed in patients receiving ≥3 doses of levofloxacin (OR 4.39, 95% CI 1.13-17.02); however, this subgroup was small and the finding is highly susceptible to survivorship and indication bias. Mortality appeared lower in patients receiving two or three antibiotics compared with monotherapy, but marked group imbalances, lack of restriction or stratification, and clinical selection effects limit interpretability. Regimens involving ≥4 agents were rare and primarily associated with prolonged ICU length of stay rather than a clear mortality difference. Conclusions: In this large retrospective ICU cohort, very early antibiotic administration for pneumonia was observationally associated with higher in-hospital mortality. Causality cannot be inferred, and early treatment likely represents a marker of higher baseline risk or clinical urgency rather than a harmful exposure. These findings challenge the assumption that earlier antibiotic initiation is uniformly beneficial and underscore the importance of individualized, stewardship-aligned, and context-dependent decision-making regarding antimicrobial timing and intensity in critically ill patients.