Severe Community-acquired pneumonia (SCAP) is an emergence infection disease of lung parenchyma that acquired outside of a hospital setting. SCAP is a leading global infectious cause of intensive care unit (ICU) admission (approximately 20%-30%), and the primary reason of mortality and morbidity in immunocompromised patients. There is a global increase of patients with distinct immunocompromised conditions due to the advance of cancer treatment, increasing biologics, and immunosuppressants for autoimmune diseases and growing organ transplant recipients, and it has been estimated that patients with immunocompromised conditions account for approximately 35% of all intensive care unit (ICU) admissions. Immunocompromised patients, who always at risk of mixed and unusual pathogens infection, have more factors to complicate with sepsis, respiratory failure, acute respiratory distress syndrome, and the mortality rate can be up to 50%. Moreover, the outcomes in immunocompromised patients with SCAP not only related to disease severity but also related to delays initiation of receiving appropriate therapy. 2019American Thoracic Society (ATS)/Infectious Diseases Society of America (IDSA) community-acquired pneumonia (CAP) guideline recommends that administering appropriate antimicrobials as soon as possible is the most effective measure to improve clinical prognosis and reduce mortality rate for SCAP patients. Therefore, timely identification of pathogenic microorganisms is particularly crucial for antimicrobial treatment in immunocompromised patients with SCAP.

Conventional microbiology diagnostic methods, such as standard microbiologic cultures, microscopy, polymerase chain reaction (PCR), respiratory virus multiplex PCR, as well as pathogen-specific antigens and antibody assays, are associated with relevant limitations: (1) long culture cycle and low positive rate; (2) usually only one pathogen can be detected at a time; (3) inability to detect fastidious or difficult culture organisms; (4) pathogen antibody-based testing may be unreliable in immunocompromised patients who are unable to mount antibody responses. Conventional diagnostic methods make big challenge for pathogens diagnosis of SCAP among immunocompromised patients due to above limitations and the complicated causative microorganisms. However, conventional antimicrobial therapy based on the results of conventional microbiology diagnostic techniques, which may delay timely accurate antimicrobial therapy at the initial stage, and the mortality of immunocompromised patients with SCAP may be increased. Metagenomic next-generation sequencing (mNGS), which can quickly (usually within 24h) detect a wide array of bacteria, viruses and fungi in an unbiased manner at the same time by analyzing cell-free nucleic acid (DNA) fragments of pathogens using appropriate lower respiratory tract (LRT) specimen, is increasingly used in severe infectious disease, especially among immunocompromised patients. This study speculates that mNGS (using LRT specimen) can guide early and accurate antimicrobial treatment for immunocompromised patients with SCAP. This multi-center, opening, randomized, controlled trail will enroll SCAP patients with immunocompromised conditions to determine whether mNGS-guided antimicrobial treatment improve the clinical prognosis and increase the clinical cure rate. The purpose of this study is to characterize the effect of mNGS-guided antimicrobial treatment for SCAP versus conventional treatment among immunocompromised patients. It is postulated that mNGS-guided antimicrobial treatment for immunocompromised patients with SCAP will improve clinical outcomes among these patients.