What tests would you order to diagnose a patient with suspected respiratory issues?

Diagnostic Tests for Suspected Respiratory Issues

For a patient with suspected respiratory issues, the essential diagnostic tests should include spirometry with bronchodilator response, chest imaging, and specialized tests based on clinical presentation such as methacholine challenge testing for suspected asthma. 1

Initial Diagnostic Testing

Pulmonary Function Testing

• Spirometry: Fundamental first-line test that measures:

  • Forced Expiratory Volume in 1 second (FEV1)
  • Forced Vital Capacity (FVC)
  • FEV1/FVC ratio
  • Mid-expiratory flow (FEF25-75%)
  • Peak Expiratory Flow (PEF) 1, 2

• Bronchodilator Response Test:

  • Administer short-acting β2 agonist (e.g., salbutamol 400 μg)
  • Measure changes in lung capacity after 15 minutes
  • Positive response: Increase of ≥12% and ≥200 mL in FEV1 or FVC 1, 2

Imaging Studies

• Chest X-ray: Essential for all patients with atypical symptoms 1
• High-Resolution CT (HRCT): When more detailed evaluation is needed, especially for suspected interstitial lung disease 1

Specialized Testing Based on Clinical Suspicion

For Suspected Asthma

• Peak Flow Monitoring:

  • Record daily for 2-3 weeks
  • Calculate diurnal variability: (highest - lowest)/highest × 100
  • Diagnostic when variability >20% with minimum change of 60 L/min 1

• Methacholine Challenge Test (MCT):

  • Indicated when asthma diagnosis is uncertain with normal spirometry
  • Contraindicated if FEV1 <60% predicted or <1.0 L
  • Positive result: FEV1 decrease ≥20% from baseline 1, 3, 4
  • Highly specific but moderately sensitive for asthma diagnosis 4

For Suspected Interstitial Lung Disease

• Diffusion Capacity (DLCO):

  • Measures gas exchange across alveolar-capillary membrane
  • Reduced in interstitial lung disease and emphysema 1, 5

• Static Lung Volumes:

  • Total Lung Capacity (TLC)
  • Residual Volume (RV)
  • Helps distinguish restrictive from obstructive patterns 5

For Suspected Hypoxemia

• Arterial Blood Gas Analysis:

  • Measures PaO2, PaCO2, pH, and bicarbonate
  • Essential for evaluating respiratory failure 5

• Pulse Oximetry:

  • Non-invasive measurement of oxygen saturation
  • Should be performed at each clinic visit 1

Additional Tests for Specific Scenarios

For Exercise Intolerance

• 6-Minute Walk Test:
  • Assesses functional capacity
  • Measures distance walked, oxygen saturation, and dyspnea 5

For Suspected Sleep-Disordered Breathing

• Overnight Oximetry or Polysomnography:
  • Particularly in patients with obesity, snoring, or daytime somnolence 1

For Suspected Airway Hyperresponsiveness

• Bronchial Provocation Testing:
  • Exercise challenge
  • Eucapnic voluntary ventilation
  • Hypertonic saline or mannitol challenge 1

Testing Algorithm

1. Start with basic measurements:

   • Spirometry with bronchodilator response
   • Chest X-ray
   • Pulse oximetry

2. If obstructive pattern detected (FEV1/FVC <70%):

   • If reversible (>12% and >200 mL improvement with bronchodilator): Consider asthma
   • If not reversible: Consider COPD, bronchiectasis

3. If restrictive pattern detected (reduced FVC with normal/high FEV1/FVC):

   • Order HRCT
   • Measure DLCO and static lung volumes

4. If normal spirometry but persistent symptoms:

   • Consider methacholine challenge test
   • Measure exhaled nitric oxide if available
   • Consider exercise testing

Important Considerations

• Ensure proper technique and quality control for all pulmonary function tests

• Discontinue medications that may affect test results:

  • Oral and intranasal corticosteroids: at least seven days before testing
  • Antileukotrienes: at least seven days
  • Antihistamines: three days
  • Decongestants and cromones: one day 1

• Interpretation should account for patient's age, sex, height, and ethnicity

• A negative methacholine challenge does not completely rule out asthma, especially in cases of exercise-induced asthma 3

By following this systematic approach to diagnostic testing, you can efficiently identify the underlying respiratory condition and develop an appropriate treatment plan based on objective findings.