A physician will conduct a physical examination and compile a patient medical history and list of symptoms in attempting to diagnose hypersensitivity pneumonitis.

Hypersensitivity pneumonitis can be easily mistaken for a case of viral pneumonia. However, hypersensitivity pneumonitis is suspected if the patient repeatedly displays symptoms after exposure to the same environment. Crackles (rales) from the lungs are often heard when a stethoscope is used to examine the chest of a patient with hypersensitivity pneumonitis.

Tests used to diagnose the disease include:

• Chest x-rays. Small, diffuse nodules may appear on an x-ray of a patient with symptoms of acute hypersensitivity pneumonitis. These will disappear once symptoms subside. Evidence of chronic hypersensitivity pneumonitis may be evident on x-rays in the form of scarred lung tissue.
  
  
• Pulmonary function tests. During a bout of hypersensitivity pneumonitis, test results are likely to show a marked decline in forced vital capacity, total lung capacity and preserved airflow.
  
  
• Complete blood count. This is a screening test often used to diagnose various disorders. The presence of hypersensitivity pneumonitis is likely to be indicated by an increase in immunoglobulins and white blood cells.
  
  
• Hypersensitivity pneumonitis antibody panels and skin testing. These can be done to identify the specific offending molds, bacteria and particles.
  
  
• Aspergillus precipitins test. A laboratory test that detects antibodies in the blood resulting from exposure to the fungus Aspergillus.
  
  
• High-resolution CT scan of the chest. A test that allows for multiple x-rays to be taken from different angles around the patient.
  
  
• Bronchoscopy with washings or transtracheal biopsy. This is a procedure in which a bronchoscope (a flexible fiber optic instrument used to view the airways) is inserted through the nose or mouth and lung-tissue specimens are collected.

In some cases, a challenge test may be performed to pinpoint the responsible allergen. A small amount of the suspected allergen will be introduced to the patient, and any reaction will be studied. Challenge tests can produce potentially dangerous allergic reactions and should only be performed in a proper healthcare setting under the guidance of a physician.