What are the infectious and non‑infectious differential diagnoses for community‑acquired pneumonia and how can each be ruled in or ruled out?

Differential Diagnosis for Community-Acquired Pneumonia

The diagnosis of CAP requires clinical features (cough, fever, sputum production, pleuritic chest pain) plus demonstrable infiltrate on chest radiograph or other imaging, with systematic exclusion of infectious and non-infectious mimics through targeted diagnostic testing. 1

Infectious Differential Diagnoses

Bacterial Pathogens

• Streptococcus pneumoniae remains the most common bacterial cause, identified in approximately 15% of patients with confirmed etiology and 5% of all CAP cases. 2, 3
• Haemophilus species are frequent causes alongside S. pneumoniae in community-acquired disease. 1
• Atypical bacteria including Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophila account for 8-16% of hospitalizations, characterized by slow progression, malaise, and low-grade fever. 1
• Pseudomonas aeruginosa and Staphylococcus aureus occur with increased frequency in HIV-infected persons and those with chronic lung disease, representing community-acquired pathogens in this population. 1
• Mycobacterium tuberculosis should be considered in CAP failing standard therapy, particularly in immigrant populations and those with chronic lung disease. 1

Rule-in approach: Obtain pretreatment blood cultures and expectorated sputum for Gram stain and culture in hospitalized patients with ICU admission, failure of outpatient therapy, cavitary infiltrates, leukopenia, active alcohol abuse, chronic severe liver disease, severe obstructive lung disease, asplenia, or recent travel. 1 Urinary antigen tests for Legionella pneumophila and Streptococcus pneumoniae should be performed in severe CAP requiring ICU admission. 1

Viral Pathogens

• Respiratory viruses are detected in up to 40% of patients with identified CAP etiology, with human rhinovirus (9% of patients) and influenza virus (6%) being most common. 3
• Respiratory syncytial virus is the usual culprit in infants, toddlers, and preschool-aged children. 1
• COVID-19 and influenza must be tested in all patients when these viruses are common in the community, as their diagnosis affects treatment and infection prevention strategies. 2

Rule-in approach: All patients with CAP should be tested for COVID-19 and influenza when prevalent in the community. 2 Viral transport devices should be used for respiratory specimens, with immediate transport on ice. 1

Fungal and Parasitic Pathogens

• Endemic fungi (Histoplasma, Coccidioides, Blastomyces) should be suspected based on recent travel or endemic exposure history. 1
• Pneumocystis jirovecii must be considered in immunocompromised patients, particularly those with HIV infection or on immunosuppressive therapy. 1
• Cryptosporidiosis, microsporidiosis, and Strongyloides stercoralis can cause pulmonary disease in immunocompromised hosts. 1

Rule-in approach: History of recent travel or endemic exposure should be routinely sought to identify specific potential etiologies. 1 BAL fluid cultures can identify Pneumocystis jirovecii pneumonia in patients initially diagnosed with drug-related pneumonitis. 1

Non-Infectious Differential Diagnoses

Drug-Related Pneumonitis

• Molecular targeting agents and immune checkpoint inhibitors can cause pneumonitis with patterns including organizing pneumonia, nonspecific interstitial pneumonia, diffuse alveolar damage, hypersensitivity pneumonia, and pulmonary eosinophilia. 1
• Erlotinib, osimertinib, and everolimus are specific agents associated with drug-related pulmonary toxicity. 1

Rule-out approach: Review medication history for molecular targeting agents or immune checkpoint inhibitors. 1 Lung biopsy may be indicated when clinical and radiologic picture does not clearly point to a specific pattern or when differential diagnosis raises consideration of markedly different therapeutic strategies. 1 BAL fluid analysis can help exclude infectious organisms. 1

Cardiac Pulmonary Edema

• Acute decompensated heart failure can mimic pneumonia but typically presents with bilateral interstitial or alveolar patterns, Kerley B lines, cephalization, and absence of purulent sputum. 4
• Cardiorenal syndrome may complicate pneumonia in patients with underlying heart failure, where renal hypoperfusion is secondary to cardiac decompensation. 5

Rule-out approach: Obtain natriuretic peptides (BNP or NT-proBNP); values >500 pg/mL for BNP or >2000 pg/mL for NT-proBNP suggest acute decompensated heart failure rather than pneumonia. 4 Assess for jugular venous distension, S3 gallop, displaced apex beat, peripheral edema, and hepatomegaly. 4

Pulmonary Embolism with Infarction

• Pulmonary embolism should be considered in patients at risk who fail to respond to standard CAP therapy. 1

Rule-out approach: Lung scanning, spiral CT scanning, and/or pulmonary angiography should be considered if the patient is at risk for pulmonary embolism with infarction. 1

Atelectasis

• Subsegmental atelectasis typically presents without fever or with only low-grade temperature elevation, without purulent sputum production, with volume loss signs on imaging including crowded pulmonary vessels, displacement of interlobar fissures, diaphragm elevation, mediastinal shift, and compensatory hyperexpansion. 6

Rule-out approach: Pneumonia requires new infiltrate plus at least two of three criteria (fever >38°C, leukocytosis or leukopenia, purulent secretions), whereas atelectasis shows volume loss signs but lacks infectious symptoms. 6 Never diagnose "atelectatic pneumonia" based on radiographic atelectasis alone. 6

Malignancy

• Diffuse malignant infiltration can mimic or coexist with interstitial lung disease and should be ruled out in lung biopsies. 1
• Obstructing endobronchial lesion may delay resolution and should be considered in nonresponding patients, particularly smokers over age 55 with focal infiltrates. 1

Rule-out approach: Bronchoscopy should be considered in patients below age 55 who have multilobar disease and are nonsmokers, or in patients with persistent radiographic and clinical abnormalities. 1 CT may reveal unsuspected lung nodules or cavitation within a lung infiltrate. 1

Diagnostic Algorithm for Rule-In/Rule-Out

Initial Evaluation (All Patients)

1. Obtain chest radiograph to establish diagnosis and differentiate CAP from acute bronchitis. 1 CT scans are more sensitive but should be reserved for when chest radiograph is negative despite toxic appearance. 1
2. Screen with pulse oximetry to detect unsuspected hypoxemia. 1
3. Test for COVID-19 and influenza when these viruses are common in the community. 2

Outpatients Without Comorbidities

• Routine diagnostic tests to identify etiology are optional. 1
• Treat empirically with amoxicillin, doxycycline, or macrolide (only in areas where pneumococcal resistance to macrolides is <25%). 7

Hospitalized Patients With Specific Indications (Table 5 Criteria)

Obtain the following when indicated: 1

• Blood cultures (two sets pretreatment) for: ICU admission, failure of outpatient therapy, cavitary infiltrates, leukopenia, active alcohol abuse, chronic severe liver disease, asplenia
• Sputum Gram stain and culture (good-quality specimen only) for: ICU admission, failure of outpatient therapy, cavitary infiltrates, active alcohol abuse, severe obstructive/structural lung disease
• Legionella urinary antigen test for: ICU admission, failure of outpatient therapy, active alcohol abuse, recent travel
• Pneumococcal urinary antigen test for: ICU admission, active alcohol abuse

Severe CAP Requiring ICU Admission

• Minimum testing includes: blood cultures, urinary antigen tests for Legionella and S. pneumoniae, and endotracheal aspirate if intubated. 1
• Consider broader differential including opportunistic infections, drug-induced pneumonitis, cardiac pulmonary edema, and pulmonary emboli in immunocompromised patients. 6

Nonresponding Patients

If no clinical response within 72 hours or deterioration after initial improvement: 1

1. Collect lower respiratory tract secretions for culture to identify drug-resistant and unusual pathogens, including tuberculosis
2. Consider bronchoscopy in patients <55 years old with multilobar disease who are nonsmokers (diagnostic yield 41%)
3. Obtain CT to reveal unsuspected pleural fluid, multiple lung nodules, or cavitation
4. Send serologic tests for Legionella, Mycoplasma pneumoniae, viral agents, endemic fungi
5. Consider open lung biopsy if extensive evaluation is non-diagnostic and patient is seriously ill

Critical Pitfalls to Avoid

• Never assume all pulmonary infiltrates with fever are infectious—systematically exclude non-infectious causes. 6
• Never delay antibiotics if pneumonia cannot be excluded in clinically unstable patients, even while awaiting diagnostic results. 6, 4
• Never diagnose pneumonia without radiographic confirmation—clinical presentation alone lacks specificity to differentiate pneumonia from other acute respiratory diseases. 8
• Portable chest radiographs have only 27-35% specificity for pneumonia—obtain CT when clinical suspicion is high but radiograph is equivocal. 6
• Antibiotics can decrease the utility of bronchoscopy, but if organisms are recovered by this method while on antibiotics, they are often resistant to current therapy. 1