SCAP was defined as meeting either one major criterion or at least three minor criteria of the Infectious Diseases Society of America/American Thoracic Society criteria (11 (link)). Immunosuppression was defined based on consensus, determined by meeting one of the following criteria (15 (link)): primary immune deficiency disease; active malignancy; receiving cancer chemotherapy; HIV infection with CD4 T-lymphocyte count < 200 cells/μL or percentage < 14%; solid organ transplantation; hematopoietic stem cell transplantation; receiving corticosteroid therapy with a prednisone dose of 20 mg or equivalent daily for ≥14 days or a cumulative dose >700 mg; receiving biologic immune modulators; or receiving disease-modifying anti-rheumatic or other immunosuppressive drugs.
Microbiological tests were performed within 48 h of ICU admission in all patients, using bronchoalveolar lavage fluid (BALF) or endotracheal aspirates. Microbiological tests included bacterial and fungal smear and culture, acid-fast stain, and real-time polymerase chain reaction for cytomegalovirus (CMV), Pneumocystis jirovecii (PJ), influenza virus, respiratory syncytial virus, adenovirus, Legionella pneumophila, Chlamydia pneumoniae, and Mycoplasma pneumoniae. Metagenomic next-generation sequencing (mNGS) was performed, if necessary, at the clinician's discretion (more details see Additional file 1).
Pathogens were identified by clinicians based on microbiological tests, clinical manifestations, and chest radiology findings. Atypical pathogens included Legionella, Mycoplasma, and Chlamydia (16 (link)). Polymicrobial infection was defined as having more than one type of pathogen diagnosed by clinicians within 48 h of ICU admission.
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