The diagnosis of extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis is difficult. Delays of years or even decades till the diagnosis is made are not uncommon; frequent misdiagnoses include allergic asthma, COPD, recurrent flue and other infections. The working group ALLERGIC DISORDERS from the EOMSociety recommends current diagnostics schema:
http://www.occup-med.com/content/10/1/15/abstract
Successful management of allergic diseases necessitates identifying their specific causes (i.e. identify the causative agent(s) and the route of contact to allergen: airborne, or skin contact) to avoid further exposure. Identification of sensitization by a sensitive and validated measurement of specific-IgE is an important step in the diagnosis.
See: Practical guideline “Immunological methods for diagnosis and monitoring of exposure-related type I allergic disorders caused by industrial sensitizing agents, IMExAllergy,”:
See also: the systematic review "Performance of specific immunoglobulin E tests for diagnosing occupational asthma: a systematic review and meta-analysis":