How is work-induced asthma diagnosed and treated?

Diagnosis of Work-Induced Asthma

Serial peak flow measurements are the best validated method for diagnosing work-related asthma and should be the first confirmatory test when a patient presents with suspected occupational asthma. 1

Diagnostic Approach to Work-Related Asthma

Step 1: Initial Assessment

• Occupational history: Ask all adults with new, recurrent, or worsening asthma symptoms about:

  • Their job and specific materials they work with
  • Whether symptoms improve when away from work (weekends/holidays)
  • Introduction of new materials before symptom onset
  • Any accidental high-level exposures or spills 1

• Screening question: "Does asthma improve away from work?" has high sensitivity but requires objective confirmation 2

Step 2: Confirm Asthma Diagnosis

• Spirometry with bronchodilator reversibility testing
• If normal spirometry or no significant bronchodilator response, perform methacholine/histamine challenge within 24 hours of work exposure 1
• Note: Normal lung function tests do not exclude occupational asthma when the patient is away from exposure 1

Step 3: Establish Work Relationship

Serial Peak Flow Monitoring (Gold Standard)

• Minimum criteria:
  • Measurements at least 4 times daily in triplicate
  • Record for ≥3 weeks of usual work exposure AND ≥10 days away from work
  • Keep treatment constant throughout monitoring period 1
• This method has high sensitivity and specificity for diagnosing work-related asthma 1

Non-Specific Bronchial Hyperresponsiveness (NSBHR)

• Perform methacholine challenge:
  • Within 24 hours of work exposure
  • After 2-3 weeks away from work
  • A threefold improvement in provocative concentration when away from work supports diagnosis 1
• Note: Changes in NSBHR alone have only moderate sensitivity and specificity 1

Pre/Post-Shift Testing

• Pre- to post-shift changes in lung function are not recommended for validation or exclusion of work-related asthma due to low sensitivity 1

Step 4: Identify Specific Causative Agent

Immunological Testing

• For high molecular weight agents (proteins, natural rubber latex):

  • Skin prick tests and specific IgE measurements have high sensitivity
  • Positive results support diagnosis but are not specific (sensitization can occur without disease) 1

• For low molecular weight agents (chemicals, isocyanates):

  • Immunological tests have lower sensitivity and specificity
  • Not useful for irritant-induced asthma 1

Specific Inhalation Challenge (SIC)

• Closest to a gold standard test for many agents causing occupational asthma
• Should be performed in specialized centers with expertise
• Indicated when diagnosis remains equivocal or causative agent is unknown
• Note: A negative test does not exclude occupational asthma if other evidence is strong 1

Supervised Workplace Challenge

• Alternative when SIC is not available or equivocal
• Technician supervises hourly spirometry during separate days of work exposure and non-exposure 1

Step 5: Additional Supporting Tests

Sputum Eosinophils

• Increase of >1% after SIC or workplace exposure may support diagnosis when FEV1 has fallen <20%
• However, presence/absence of increased sputum eosinophils alone is not useful for diagnosis 1, 3

Exhaled Nitric Oxide

• Normal values cannot exclude occupational asthma 1

Common Pitfalls and Caveats

1. Underdiagnosis: Work-related asthma is frequently underdiagnosed; consider in all cases of new-onset or worsening asthma 1, 2, 4

2. Timing of testing: Bronchial hyperresponsiveness testing should be performed within 24 hours of exposure, as results may normalize with longer periods away from work 1

3. False positives: History alone has numerous false positives; objective confirmation is essential 2

4. Diagnostic certainty: The degree of diagnostic proof required depends on consequences for the worker (job loss, compensation) 1

5. Negative SIC: A negative specific inhalation challenge does not exclude occupational asthma if other evidence is strong 1

6. Early diagnosis importance: Early diagnosis and removal from exposure improves prognosis 1, 5

7. Differential diagnosis: Always exclude other airway diseases such as COPD and vocal cord dysfunction 6

8. Exposure assessment: Consider not only substances used by the patient but also those used by coworkers that may become airborne 1

By following this structured diagnostic approach, clinicians can accurately identify work-related asthma, leading to appropriate management and improved outcomes for affected workers.