What is the diagnosis for a young male with fever, cough, and expectoration, and a normal computed tomography (CT) of the thorax?

Diagnosis: Community-Acquired Pneumonia or Atypical Infection

In a young male with fever, cough, and expectoration but a normal CT thorax, the most likely diagnosis is an early-stage respiratory tract infection, most commonly community-acquired pneumonia (bacterial or atypical) or acute bronchitis, where imaging may lag behind clinical symptoms.

Key Diagnostic Considerations

Why CT Can Be Normal Despite Active Infection

• Ultra-early stage infections may present with clinical symptoms (fever, cough, expectoration) before radiographic abnormalities become apparent, as documented in viral pneumonia cases where patients can be symptomatic with negative imaging initially 1

• The absence of CT findings does not exclude infection, particularly in:

  • Atypical pneumonia (Mycoplasma, Chlamydia)
  • Early bacterial pneumonia
  • Acute bronchitis
  • Viral respiratory infections

Critical Differential Diagnoses to Consider

Atypical Pneumonia (Most Likely):

• Mycoplasma pneumoniae commonly presents with fever, cough, and expectoration but may show minimal or no radiographic findings early in the disease course 2
• Young adults are particularly susceptible to atypical pathogens
• Symptoms often precede imaging findings by several days

Tuberculosis (Must Rule Out):

• In endemic areas, tuberculosis should be considered in any patient with cough lasting more than 3 weeks, regardless of radiographic findings 3
• Immediate action required: Obtain three sputum samples for acid-fast bacilli (AFB) smear and culture on different days to maximize diagnostic sensitivity 3
• Evening fever and productive cough are classic TB symptoms, even with normal initial imaging 3

Nocardiosis:

• Can mimic tuberculosis with fever, cough, and expectoration 4
• Consider if immunocompromised or if sputum is repeatedly negative for AFB 4

Recommended Diagnostic Algorithm

Immediate Laboratory Evaluation

1. Complete blood count with differential:

   • Look for lymphopenia (suggests viral or atypical infection) 1
   • Normal or mildly elevated white blood cell count is common in atypical pneumonia

2. Inflammatory markers:

   • C-reactive protein (CRP) and procalcitonin levels help differentiate bacterial from viral/atypical causes 1
   • Elevated CRP with low procalcitonin suggests atypical or viral infection

3. Sputum studies (CRITICAL):

   • Gram stain and culture
   • Three AFB smears and cultures on different days (mandatory to exclude TB) 3
   • Mycoplasma pneumoniae-specific IgM antibody testing 2

4. Additional serologies:

   • Atypical pathogen panel (Mycoplasma, Chlamydia, Legionella)
   • Consider HIV testing if risk factors present 4

Imaging Considerations

• Chest X-ray should be obtained as initial imaging, as it may show abnormalities not visible on CT or may become positive with disease progression 1
• Repeat CT in 3-7 days if symptoms persist or worsen, as imaging findings typically evolve during the rapid progression stage of infections 1
• Normal CT does not rule out significant respiratory infection, particularly in the first 1-3 days of symptoms 1

Common Pitfalls to Avoid

⚠️ Do not dismiss the diagnosis based solely on normal CT imaging—clinical symptoms may precede radiographic changes by several days 1

⚠️ Do not delay tuberculosis workup in endemic areas or with risk factors; obtain sputum samples immediately even with normal imaging 3

⚠️ Do not rely on chest radiography alone when clinical suspicion is high, as it has poor sensitivity (49% false negative rate in confirmed respiratory infections) 1

⚠️ Do not assume viral infection without excluding bacterial and atypical causes through appropriate microbiological testing

Treatment Approach Pending Results

• Empiric antibiotic therapy covering atypical pathogens (macrolide or respiratory fluoroquinolone) is reasonable while awaiting culture results in a symptomatic young patient
• Symptomatic management with antipyretics and hydration
• Close clinical follow-up within 48-72 hours to reassess response to therapy
• Repeat imaging if no clinical improvement within 3-5 days or if symptoms worsen 1