# Evaluation of factors affecting Pneumonia Severity Index and antibiotic resistance status in culture-proven bacterial pneumonia

## TL;DR

This study shows that comorbidities, antibiotic resistance, and immune markers are linked to higher severity scores in pneumonia patients.

## Contribution

The study identifies new associations between MDR pathogens, immune biomarkers, and Pneumonia Severity Index scores.

## Key findings

- Higher PSI scores are associated with comorbidities, MDR pathogens, and elevated immune-inflammation markers.
- MDR pathogens like Acinetobacter baumannii and Klebsiella spp. are linked to significantly higher PSI scores.
- Elevated NLR, PLR, MLR, and SII values correlate with increased pneumonia severity.

## Abstract

The Pneumonia Severity Index (PSI) is a widely used to assess disease severity and guide hospitalization decisions in patients with pneumonia. However, PSI does not incorporate microbiological characteristics, antimicrobial resistance patterns, or immune–inflammatory biomarkers. This study aimed to evaluate the impact of comorbidities, etiological agents, antimicrobial resistance, and immune–inflammatory markers on PSI scores in patients with culture-proven bacterial pneumonia.

This retrospective study included patients with culture-proven bacterial pneumonia treated at a tertiary care center between January 2019 and April 2023. Demographic characteristics, comorbidities, intensive care unit (ICU) admission, etiological agents, antimicrobial resistance patterns, and multidrug resistance (MDR) status were analyzed. Laboratory parameters, including neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), monocyte-to-lymphocyte ratio (MLR), systemic immune–inflammation index (SII), C-reactive protein, and albumin levels were evaluated in relation to PSI scores.

Most patients were aged ≥65 years and had at least one comorbidity. Gram-negative bacteria predominated, with Pseudomonas aeruginosa being the most frequently isolated pathogen. Multidrug resistance was most commonly observed in Acinetobacter baumannii, Klebsiella spp., and Escherichia coli. Higher PSI scores were significantly associated with a greater comorbidity burden, presence of MDR pathogens, elevated NLR, PLR, MLR, and SII values, and lower albumin levels (p < 0.05). Patients with MDR isolated had significantly higher mean PSI scores than those without MDR.

In culture-proven bacterial pneumonia, comorbidity burden, antimicrobial resistance–particularly multidrug resistance–and elevated immune–inflammatory biomarkers are associated with higher PSI scores. These findings highlight the importance of considering community-onset healthcare-associated pneumonia and integrating local antimicrobial resistance surveillance and laboratory biomarkers into clinical risk stratification. Further multicenter prospective studies are warranted to validate these associations.