How to diagnose and differentiate various respiratory tract infections?

Diagnosing and Differentiating Respiratory Tract Infections

The diagnosis of respiratory tract infections requires a systematic approach focusing on specific clinical signs, symptoms, and targeted laboratory tests to differentiate between viral, bacterial, and other etiologies, with particular attention to distinguishing pneumonia from other respiratory infections due to its higher morbidity and mortality.

Upper Respiratory Tract Infections (URTIs)

Acute Rhinosinusitis

• Diagnostic criteria for viral rhinosinusitis 1:

  • Purulent nasal drainage (anterior, posterior, or both)
  • Nasal obstruction/congestion/blockage
  • Facial pain/pressure/fullness
  • Symptoms present for <10 days without worsening

• Diagnostic criteria for acute bacterial rhinosinusitis 1:

  • Symptoms persisting ≥10 days without improvement OR
  • Worsening symptoms after initial improvement ("double worsening") OR
  • Severe symptoms with high fever, unilateral facial/tooth pain, or orbital complications

• Laboratory assessment:

  • Radiographic imaging is NOT recommended for uncomplicated cases 1
  • CRP testing may help differentiate: CRP <20 mg/L makes bacterial infection unlikely 2

Pharyngitis/Tonsillitis

• Key diagnostic features:

  • Sore throat, fever, cervical lymphadenopathy
  • Absence of cough (suggests bacterial rather than viral etiology)
  • Presence of tonsillar exudates

• Laboratory assessment:

  • Rapid streptococcal antigen detection test for Group A Streptococcus
  • Throat culture when rapid test is negative but clinical suspicion is high

Lower Respiratory Tract Infections (LRTIs)

Differentiating Pneumonia from Acute Bronchitis

• Key clinical indicators for pneumonia 1:

  • New focal chest signs
  • Dyspnea/tachypnea
  • Fever >4 days
  • Pulse rate >100

• Laboratory assessment for pneumonia:

  • CRP testing: CRP >100 mg/L makes pneumonia likely 1, 2
  • CRP <20 mg/L with symptoms >24h makes pneumonia highly unlikely 1, 2
  • Chest X-ray should be performed when pneumonia is suspected based on clinical signs 1

• Acute bronchitis diagnostic features:

  • Cough (predominant symptom)
  • Absence of focal chest signs
  • Normal vital signs or mild abnormalities
  • Self-limiting course (typically 1-3 weeks)

Exacerbation of Chronic Bronchitis/COPD

• Diagnostic criteria 1:
  • Increased dyspnea
  • Increased sputum volume
  • Increased sputum purulence
  • History of COPD/chronic bronchitis
  • Consider in patients with persistent cough and at least two of: wheezing, previous consultations for wheezing/cough, dyspnea, prolonged expiration, smoking history, or allergic symptoms

Laboratory Parameters to Differentiate Bacterial vs. Viral Infections

• Suggestive of bacterial infection 2:

  • WBC >20,000/mL
  • Left shift (>1500 bands/mm³ or >16% band neutrophils)
  • Neutrophil percentage >90%
  • CRP >100 mg/L
  • PCT >0.5 ng/mL

• Suggestive of viral infection 2:

  • Normal WBC count
  • CRP <20 mg/L (with symptoms >24h)
  • PCT <0.1 ng/mL

Diagnostic Algorithm

1. Initial assessment:

   • Evaluate location of symptoms (upper vs. lower respiratory tract)
   • Assess vital signs (fever, respiratory rate, heart rate)
   • Check for focal chest signs
   • Evaluate duration and progression of symptoms

2. For suspected URTI:

   • If symptoms <10 days without worsening: likely viral rhinosinusitis
   • If symptoms ≥10 days OR worsening after improvement: suspect bacterial sinusitis
   • For pharyngitis: evaluate for bacterial features (exudates, cervical lymphadenopathy, absence of cough)

3. For suspected LRTI:

   • Check for pneumonia indicators (focal chest signs, dyspnea, tachypnea, fever >4 days)
   • If pneumonia suspected: obtain CRP and chest X-ray
   • If no pneumonia indicators: likely acute bronchitis (viral)
   • For patients with COPD: assess for exacerbation features

4. Laboratory testing:

   • CRP testing helps differentiate bacterial from viral infections
   • Microbiological tests generally not recommended in primary care 1
   • Consider PCT in hospital settings to guide antibiotic therapy

Common Pitfalls and Caveats

• Overdiagnosis of bacterial infections: Most URTIs and many LRTIs are viral in origin 3
• Misdiagnosis of pneumonia: Chest X-ray is the gold standard but not feasible for all patients with respiratory symptoms 1
• Failure to consider non-infectious causes: Conditions like heart failure, pulmonary embolism, and ARDS can mimic respiratory infections 1
• Inappropriate antibiotic use: Antibiotics are not indicated for viral infections or uncomplicated acute bronchitis
• Inadequate assessment of severity: Failure to identify high-risk patients who may need hospitalization or more aggressive management 1
• Relying solely on clinical signs: The sensitivity and specificity of clinical signs alone are low for establishing classification of LRTIs 1

By following this systematic approach, clinicians can more accurately diagnose and differentiate between various respiratory tract infections, leading to appropriate management decisions and judicious use of antibiotics.