A Prospective Observational Cohort Study Protocol for Disease Severity Stratification and Prognosis Assessment in Patients With Pneumonia

• Inclusion and Exclusion Criteria for Pneumonia Patients 1.1 Inclusion Criteria 1) Age ≥ 18 years old; 2) Meeting the diagnostic criteria for pneumonia (acute onset + new infiltrates on imaging); 3) Time from onset to admission ≤ 72 hours; 4) Signed written informed consent. 1.1.1 Inclusion Criteria for Severe Pneumonia Patients A diagnosis of "severe pneumonia" can be made if one of the main criteria or ≥ 3 of the secondary criteria are met.

Main Criteria:

① Requires mechanical ventilation with tracheal intubation

② Requires vasopressor drugs after aggressive fluid resuscitation for septic shock

Secondary Criteria:

• Respiratory rate ≥ 30 breaths/min

• PaO₂/FiO₂ ≤ 250 mm Hg

• Multiple lobe infiltrates

• Confusion and/or disorientation

⑤ Blood urea nitrogen ≥ 20 mg/dL

⑥ Leukopenia (WBC < 4×10⁹/L)

⑦ Thrombocytopenia (PLT < 100×10⁹/L)

⑧ Decreased body temperature (central body temperature < 36 ℃)

⑨ Hypotension requiring fluid resuscitation 1.2 Exclusion Criteria

• End-stage diseases (such as expected survival < 48 hours);
• Long-term immunosuppressive therapy (prednisone > 20mg/d for more than 3 weeks, or neutropenia after chemotherapy);
• Pregnant or lactating women. 2. Inclusion and Exclusion Criteria for Patients with Suspected Infection but Negative Pathogen Testing 2.1 Inclusion Criteria

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• Age ≥ 18 years old;
• Having infection-related symptoms or signs (such as fever, cough, sputum production, elevated white blood cells, etc.), clinically suspected of pneumonia;
• All pathogen tests are negative within 3 days after admission;
• Time from onset to admission ≤ 72 hours;
• Signed written informed consent. 2.2 Exclusion Criteria

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• End-stage diseases (such as expected survival < 48 hours);
• Long-term immunosuppressive therapy;
• Pregnant or lactating women;