Blood Cultures Versus Respiratory Cultures: 2 Different Views of Pneumonia

Abstract

Background: Choice of empiric therapy for pneumonia depends on risk for antimicrobial resistance. Models to predict resistance are derived from blood and respiratory culture results. We compared these results to understand if organisms and resistance patterns differed by site. We also compared characteristics and outcomes of patients with positive cultures by site.

Methods: We studied adult patients discharged from 177 US hospitals from July 2010 through June 2015, with principal diagnoses of pneumonia, or principal diagnoses of respiratory failure, acute respiratory distress syndrome, respiratory arrest, or sepsis with a secondary diagnosis of pneumonia, and who had blood or respiratory cultures performed. Demographics, treatment, microbiologic results, and outcomes were examined.

Results: Among 138 561 hospitalizations of patients with pneumonia who had blood or respiratory cultures obtained at admission, 12 888 (9.3%) yielded positive cultures: 6438 respiratory cultures, 5992 blood cultures, and 458 both respiratory and blood cultures. Forty-two percent had isolates resistant to first-line therapy for community-acquired pneumonia. Isolates from respiratory samples were more often resistant than were isolates from blood (54.2% vs 26.6%; P < .001). Patients with both culture sites positive had higher case-fatality, longer lengths of stay, and higher costs than patients who had only blood or respiratory cultures positive. Among respiratory cultures, the most common pathogens were Staphylococcus aureus (34%) and Pseudomonas aeruginosa (17%), whereas blood cultures most commonly grew Streptococcus pneumoniae (33%), followed by S. aureus (22%).

Conclusions: Patients with positive respiratory tract cultures are clinically different from those with positive blood cultures, and resistance patterns differ by source. Models of antibiotic resistance should account for culture source.

Keywords: antibiotic resistance; pneumonia; respiratory cultures.

Conflict of interest statement

Potential conflicts of interest.

P. K. L. has received a midcareer award in patient-oriented research (K24) from the National Heart, Lung, and Blood Institute. M. D. Z. has received research funding from Astellas, The Medicines Company, Tetraphase, Lungpacer, Merck, Melinta, Pfizer, and Spero; serves as a consultant for Nabriva, Melinta, Spero, Arsanis, Shinogi, Tetraphase, Pfizer, and Paratek; and has received stocks/stock options from Johnson & Johnson. T. H. has received personal fees from the Cerner Corporation. A. D. has received institutional research support from the Clorox Company and consulting fees from Ferring Pharmaceuticals. All other authors report no potential conflicts of interest. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.

© The Author(s) 2019. Published by Oxford University Press for the Infectious Diseases Society of America. All rights reserved. For permissions, e-mail: journals.permissions@oup.com.

Figures

Figure 1.

Flowchart indicating derivation of the study sample. Abbreviations: CLABSI, central line–associated bloodstream infection; CT, computed tomography; LOS, length of stay; POA, present on admission.

Figure 2.

Rates of bacterial resistance to Infectious Diseases Society of America guideline–recommended antibiotics for community-acquired pneumonia, stratified by culture source and Gram stain result. Standard errors of all depicted proportions are ≤1.1%. Abbreviations: CAP, community-acquired pneumonia.