Methods

The methods used to create this database have been described previously. ^{Reference Ilges, Ritchie and Krekel4} This retrospective cohort comprised patients was admitted to Barnes Jewish Hospital, a large academic center in St. Louis, Missouri, between January 16, 2016, and December 31, 2019. The study was approved by the Washington University institutional review board. Adult patients with ICD-9 or ICD-10 codes and who met the following criteria for HAP or VAP were included: (1) index date of infection onset ≥48 hours after hospital admission for HAP or ≥48 hours after initiation of mechanical ventilation for VAP, (2) chest radiograph completed within ±24 hours of index pneumonia date, (3) active orders for either anti–methicillin-resistant Staphylococcus aureus (MRSA) or anti–Pseudomonas aeruginosa antibiotics, and (4) at least 1 sign of infection, including a white blood cell count ≥11 or ≤4×10⁹ cells/L or temperature ≥38 or ≤36°C. The index date of pneumonia diagnosis was defined as day 0. Pharmacy-verified intravenous and oral antibiotic orders of interest were collected for each day of admission.

Antibiotic spectrum scores modified based on the Barnes-Jewish Hospital antibiogram and the addition of antibiotics not listed in the original description reported by Gerber et al ^{Reference Gerber, Hersh, Kronman, Newland, Ross and Metjian7} were calculated on day 0 and for each day thereafter. ^{Reference Ilges, Ritchie and Krekel4,Reference Gerber, Hersh, Kronman, Newland, Ross and Metjian7} Microbiologic data, including respiratory cultures, respiratory viral polymerase chain reaction tests, MRSA nasal culture swabs, and blood cultures, were collected from days –1 to 3 of index pneumonia diagnosis. Respiratory cultures were classified as sputum-like specimens, which included sputum and tracheal aspirates, and bronchoscopy specimens, which included bronchoalveolar lavage, washings, and brushings. Respiratory samples identified as positive for a likely pathogen excluded those with only yeast, fungal structural elements, and/or clinically insignificant flora. Among the blood cultures identified as positive for a likely pathogen, we excluded those with only coagulase-negative staphylococci. Culture negative was defined as no growth from any of the cultures described above.

Study participants were categorized in 4 groups based on the pathogen associated with their pneumonia: Staphylococcus aureus (n = 134), Pseudomonas aeruginosa (n = 59), Enterobacterales spp (n = 87), and bacteria-culture negative (n = 1,404). Due to small sample sizes, study participants with pathogens not of interest were excluded from analysis. These pathogens included Stenotrophomonas, Burkholderia, and Acinetobacter as well as S. pneumoniae, H. influenzae, and S. agalactiae.

The spectrum scores from each pathogen group were compared separately on days 0, 3, 7, 10, and 14 using Kruskal-Wallace tests and subsequent Dunn post-hoc tests if warranted. The initial spectrum scores were calculated on the day study participants had a new active order for either anti-MRSA or anti–Pseudomonas aeruginosa antibiotics and at meeting the study criteria for HAP and VAP described above. The composite outcome of all-cause mortality and hospital readmission within 30 days of pneumonia diagnosis was also calculated similar to our prior study. ^{Reference Ilges, Ritchie and Krekel4} All analyses were performed using R version 4.1.3 and R Studio version 2022.02.3 (R Foundation for Statistical Computing, Vienna, Austria).