Occupational Asthma: Finding the Cause
Making a diagnosis of occupational asthma
Preconditions for making a diagnosis of occupational asthma are the typical diagnostic findings of asthma, along with an association of asthma symptoms with exposure at work (Aasen et al., 2013). Occupational asthma should be suspected in all those who have respiratory symptoms which improve on days away from work or on holiday (a few workers have no symptoms at all, and are recognized from their FEV1 decline only). A detailed occupational history should be taken which will enable the physician to determine the degree of pre-test probability of occupational asthma. In its evidence review and guidelines, the British Occupational Health Research Foundation (BOHRF) details work that it considers to be high-risk, which should create a high index of suspicion; this includes spray painting, health and dental care, chemical processing, baking and pastry making, food processing, welding, soldering, metalworking, woodworking, plastics and rubber manufacture, laboratory animal work, textiles, farming and hairdressing (Nicholson et al., 2010).
A diagnosis of occupational asthma should not be made on history alone and objective evidence should be gathered and triangulated:
Objective evidence of exposure to known sensitizers (or airway irritants in IIA);
Demonstration of an association between exposure and airflow limitation at work. Validation is best done with serial PEF measurements. Significant changes in non-specific reactivity at and away from work can be used, but are less sensitive and less specific than Oasys analysis of serial PEF measurements;
Demonstration of a specific allergen reaction to the occupational agent.
Further details about the principles of diagnosis and the validity of diagnostic tests can be found in the following publications:
- European Respiratory Society (ERS) guidelines: Baur et al., 2012; Aasen et al., 2013
- British Thoracic Society guidelines: Fishwick et al., 2012
- British Occupational Health Research Foundation (BOHRF) guidelines: Nicholson et al., 2010
In UK hospitals the availability of resources necessary for full investigation of a patient with occupational asthma varies and few centres have the ability to perform specilized tests such as specific inhalation challenge (SIC) to occuptional agents. Therefore if a diagnosis of occupational asthma is suspected, the British Thoracic Society (BTS) guidelines recommend early referral to a specialist centre (Fishwick et al., 2012).
Establishing a cause
Once occupational asthma has been validated the following can identify the cause:
1. Finding specific IgE to a well validated occupational allergen. Amylase in bakers, rat or mouse urinary antigens in laboratory animal workers or latex in a health-care worker would be good examples. Other agents can produce non-specific results, for instance wheat in a baker which may cross-react with grass pollen.
2. Exposure to a well recognised cause of occupational asthma which is known to sensitise others in the workplace. For instance a paint sprayer using isocyanate hardeners without exposure to other agents can reasonably be assumed to have isocyanate asthma.
3. Clues can come from serial PEF records when there are days with no workplace deterioration. Workers should be asked what was different on these workdays. PEF records can also monitor a worker moving to other areas of a workplace. Care is needed with interpretation as supposed relocation away from exposure may still result in enough exposure to cause asthma by walking through areas of exposure during clocking-on, going to meals or the toilet, or contact with workers from these areas with antigen on their clothes or hair.
4. Specific challenge testing is the best method for validating the cause, particularly for agents where methodology of exposure is well established. False negatives and some false positives occur
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