What diagnostic tests are recommended for an adult presenting with acute respiratory illness, specifically community‑acquired pneumonia, acute bronchitis, and aspiration pneumonia?

Diagnostic Testing for Acute Respiratory Illness: Community-Acquired Pneumonia, Acute Bronchitis, and Aspiration Pneumonia

For adults with suspected community-acquired pneumonia, obtain a chest radiograph (or lung ultrasound if unavailable) to confirm diagnosis, measure C-reactive protein (CRP) in outpatients to strengthen diagnostic accuracy, and reserve microbiological testing for hospitalized patients with severe disease or those requiring ICU admission.

Outpatient Setting: Acute Cough and Suspected Pneumonia

Clinical Assessment First

Focus on key clinical features that distinguish pneumonia from acute bronchitis 1:

• Suggestive of pneumonia: Cough, dyspnea, pleural pain, fever ≥38°C, tachypnea, new localizing chest signs (crackles, diminished breath sounds), absence of runny nose
• Against pneumonia: Normal vital signs, normal lung examination, presence of upper respiratory symptoms

Imaging Strategy

• Order chest radiography when abnormal vital signs are present or clinical suspicion is moderate-to-high 1
• Chest X-ray improves diagnostic accuracy and confirms radiographic pneumonia before initiating antibiotics
• If imaging cannot be obtained and pneumonia is clinically suspected, empiric antibiotics per local guidelines are appropriate 1

Biomarker Testing

C-reactive protein (CRP) is recommended for outpatients with suspected pneumonia 1:

• CRP ≥30 mg/L plus suggestive symptoms/signs increases likelihood of pneumonia
• CRP <10 mg/L or 10-50 mg/L without dyspnea and daily fever makes pneumonia less likely
• Adding CRP to clinical features strengthens both diagnosis and exclusion of pneumonia

Procalcitonin is NOT recommended for routine outpatient use 1. The evidence shows procalcitonin cannot reliably distinguish bacterial from viral pneumonia in outpatients, and sensitivity ranges from only 38-91% 2.

Microbiological Testing

Do NOT perform routine microbiological testing in outpatients 1. Reserve microbiological testing only when results would change therapy.

Inpatient Setting: Hospitalized CAP

Imaging

Chest radiography is required to confirm CAP diagnosis before initiating treatment 2.

Microbiological Testing Strategy

The approach differs based on severity 2:

Nonsevere inpatients:

• No routine cultures needed unless specific risk factors present
• If MRSA risk factors exist: obtain cultures and nasal PCR to allow de-escalation
• If Pseudomonas risk factors exist: obtain cultures but withhold empiric coverage unless PCR positive

Severe inpatients (ICU admission):

• Always obtain blood and respiratory cultures before antibiotics 2
• Obtain nasal PCR for MRSA
• Cultures allow de-escalation or confirmation of targeted therapy
• This is critical for mortality reduction in severe disease

Procalcitonin in Hospitalized Patients

Do NOT use procalcitonin to withhold antibiotics in patients with clinically suspected and radiographically confirmed CAP 2. The 2019 ATS/IDSA guidelines strongly recommend initiating empiric antibiotics regardless of initial procalcitonin level, as procalcitonin cannot reliably exclude bacterial infection (sensitivity 38-91%) and withholding antibiotics risks mortality.

Acute Bronchitis

Do NOT routinely use antibiotics when vital signs and lung examination are normal with no radiographic evidence of pneumonia 1. Acute bronchitis is predominantly viral and does not benefit from antibiotics.

If CRP is measured:

• CRP <10 mg/L or 10-50 mg/L without dyspnea/daily fever suggests acute bronchitis rather than pneumonia 1

Aspiration Pneumonia

The provided guidelines do not specifically address aspiration pneumonia separately. Apply the same diagnostic approach as CAP, but consider:

• History of witnessed aspiration, dysphagia, or altered consciousness
• Dependent lung segments on imaging (posterior segments of upper lobes, superior segments of lower lobes)
• For hospitalized patients, obtain cultures to guide therapy given different microbiology (anaerobes, gram-negatives)

Influenza Testing

For suspected influenza with acute cough, initiate antiviral treatment within 48 hours of symptom onset per CDC guidance 1. Early antiviral treatment may decrease antibiotic use, hospitalization, and improve outcomes.

Common Pitfalls to Avoid

1. Do not withhold antibiotics in radiographically confirmed CAP based on procalcitonin levels—this increases mortality risk 2
2. Do not order routine microbiological tests in outpatients—they rarely change management 1
3. Do not prescribe antibiotics for acute bronchitis with normal vital signs and examination 1
4. Do not skip chest imaging in hospitalized patients—radiographic confirmation is essential before CAP diagnosis 2