High-throughput sequencing technology/Metagenomic next generation sequencing (mNGS) has extensive applications in fields such as whole-genome sequencing, transcriptome, gene expression regulation, and epigenetics. Because mNGS has the characteristics of large sequencing throughput, short time and high sensitivity, it can detect thousands of pathogens including bacteria, viruses, true bacteria and parasites, and is widely used in infectious diseases. In 2020, the "Expert Consensus on the Clinical Application of Metagenomic Second-Generation Sequencing Technology in Detecting Infectious Pathogens in China" proposed that for immunodeficient patients, mNGS can significantly increase the detection rate of pathogens and can be used as a first-line detection method. However, at present, there is no unified standard for the interpretation of mNGS results in the environment with bacteria in the respiratory tract, and there are not many studies on the efficacy of mNGS applied in the detection of bacteria and fungi. This study explored the clinical application value of mNGS in the pathogen detection of pneumonia in immunosuppressed hosts.

Infection is the most common complication in immunosuppressed hosts, and lung infection is one of the leading causes of hospitalization and death in immunosuppressed hosts, and accurate pathogenetic diagnosis of pneumonia in immunosuppressed hosts remains challenging at present. Bronchoalveolar lavage is the standard method for sampling microorganisms in the lower respiratory tract, and with the progressive application of high-throughput sequencing (NGS) technology for pathogenetic testing and its success, we have clinically observed that bronchoalveolar lavage combined with NGS improves the rate of pathogenic diagnosis of pneumonia in the immunosuppressed host and benefits the patient. In this project, we plan to prospectively enroll patients with immunosuppressed host pneumonia and perform both bronchoalveolar lavage NGS and clinical conventional sputum culture, blood culture, TB-DNA test, fungal GM test, viral PCR test and other pathogenicity tests, and based on the above results, we will explore whether bronchoalveolar lavage in combination with NGS can increase the rate of pathogenic diagnosis of immunosuppressed host pneumonia and evaluate the effectiveness of the clinical value of NGS in serving as a tool for pneumonia detection and control and in optimizing antibiotic treatment strategies for pneumonia in immunosuppressed hosts.

This study compared the positive rate and accuracy rate of pathogen detection in immunosuppressive host pneumonia by mNGS and CMT, and explored the clinical value of bronchoalveolar lavage combined with high-throughput sequencing technology in the precise diagnosis of immunosuppressive host pneumonia. To increase the etiological diagnosis rate of pneumonia in immunosuppressed hosts, shorten the diagnosis time and improve its prognosis.

Inclusion criteria:

1. Voluntary signing of informed consent;
2. Age 14-75 years, No gender limitation；
3. Immunosuppressed host；
4. Symptoms，signs or imaging findings of pneumonia，and ineffective empirical anti-infection treatment；
5. Specimens collected for NGS testing.

Exclusion criteria：

1. No paired conventional microbiological tests (smear，culture，etc.)；
2. Incomplete case records；
3. Inability to complete bronchoalveolar lavage (BAL)

Patients who meet all of the following criteria are eligible to participate in the study:

Sign the informed consent form voluntarily; Age 18-75 years old, no gender limitation; Immunosuppressive host; Symptoms, signs, or imaging of pneumonia show signs of pneumonia, and empiric anti-infection treatment is ineffective.