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The most effective method to diagnose suspected occupational asthma is spirometry before and after rechallenge with workplace exposures. This method is crucial because it directly assesses the functional status of the lungs and evaluates the presence of airflow obstruction, which is a hallmark of asthma. The process typically involves measuring lung function with spirometry before exposure to potential allergens or irritants in the workplace and then re-evaluating lung function after exposure. A significant decrease in lung function following workplace exposure supports the diagnosis of occupational asthma.

This method is particularly valuable because it helps distinguish occupational asthma from other respiratory conditions and signifies a direct relationship between symptoms and work-related exposures. It also emphasizes the importance of a thorough occupational history and environmental assessment in the diagnostic process, reinforcing the connection between the patient's symptoms and specific workplace triggers.

Although other testing methods such as serum eosinophilia can indicate inflammation, and skin prick testing can evaluate allergic sensitivities, they do not provide the same direct evidence of the reversible airway obstruction that characterizes asthma. Furthermore, chest X-rays, while useful for ruling out other conditions, do not specifically assess lung function or diagnose asthma. Therefore, the rechallenge approach using spirometry effectively confirms the diagnosis of occupational asthma.