What tests are used to differentiate upper respiratory tract infections?

Diagnostic Tests for Differentiating Upper Respiratory Tract Infections

C-reactive protein (CRP) testing is the most valuable first-line test for differentiating between viral and bacterial upper respiratory tract infections, with levels <20 mg/L making bacterial infection highly unlikely and >100 mg/L strongly suggesting bacterial etiology. 1

Clinical Assessment for Differentiation

Signs and Symptoms to Evaluate

• Presence of focal chest signs
• Dyspnea
• Tachypnea (elevated respiratory rate)
• Pulse rate >100 beats/minute
• Fever lasting >4 days 1

Key Diagnostic Tests

First-Line Testing

1. C-reactive protein (CRP)

   • <20 mg/L: Viral infection likely

   • 100 mg/L: Bacterial infection likely

   • 20-100 mg/L: Indeterminate, consider additional testing 1

2. Nucleic acid amplification tests (NAATs)

   • Recommended as first-line for respiratory virus detection
   • Multiplex PCR panels can detect multiple pathogens simultaneously
   • Higher sensitivity than antigen detection or culture methods 1, 2

3. Chest radiography

   • Indicated when pneumonia is suspected based on clinical findings
   • Essential for confirming lower vs. upper respiratory tract involvement
   • Should be performed if CRP results are indeterminate and symptoms persist 1

Second-Line Testing

1. Microbiological studies

   • Gram staining and culture of expectorated sputum
   • Particularly valuable in patients with severe illness or treatment failure 1

2. Specific pathogen testing

   • For influenza A/B, RSV, and parainfluenza viruses as priority pathogens
   • Specimens should be taken from the site of clinical involvement 1
   • Pooled swabs for upper respiratory tract infections
   • BAL for lower respiratory tract infections when indicated

Diagnostic Algorithm

1. Initial assessment: Evaluate for signs of pneumonia (focal chest signs, dyspnea, tachypnea, pulse >100, fever >4 days)

2. CRP testing:

   • If <20 mg/L: Likely viral infection, supportive care
   • If >100 mg/L: Likely bacterial infection, consider antibiotics
   • If 20-100 mg/L: Proceed to additional testing

3. Respiratory virus testing:

   • Multiplex PCR for influenza A/B, RSV, and parainfluenza as priority pathogens
   • Consider testing for other viruses based on local epidemiology or if first-line testing is negative

4. Chest radiography:

   • If pneumonia is suspected based on clinical findings
   • If CRP is indeterminate and symptoms persist
   • To differentiate between upper and lower respiratory tract involvement

Special Considerations

Specimen Collection

• Upper respiratory tract: Pooled nasopharyngeal and throat swabs preferred
• Lower respiratory tract: BAL preferred over tracheal aspirates 1

Testing Technology Comparison

• NAATs (PCR): Highest sensitivity, can detect multiple pathogens, results in <24 hours
• Direct antigen detection: Good specificity, rapid (<4 hours), but lower sensitivity
• Virus isolation by cell culture: High specificity but slower (2-5 days) and less sensitive than NAATs 1

Common Pitfalls

1. Overreliance on clinical signs alone: Physical examination to detect rales or bronchial breath sounds is neither sensitive nor specific for detecting pneumonia 1

2. Inappropriate antibiotic use: Up to 56% of patients with upper respiratory infections receive antibiotics, though only a small percentage actually require them 3

3. Specimen quality issues: Poor specimen collection technique can lead to false-negative results, particularly with nasopharyngeal swabs

4. Timing of testing: Viral shedding may decrease after the first few days of symptoms, potentially leading to false-negative results if testing is delayed

By following this evidence-based approach to diagnostic testing, clinicians can more accurately differentiate between viral and bacterial upper respiratory tract infections, leading to more appropriate treatment decisions and improved antibiotic stewardship.