Differential Diagnosis for Chronic Cough

Single Most Likely Diagnosis

• Asthma: The improvement in FEF 25-75 by 13% in response to a bronchodilator suggests reversible airway obstruction, which is a hallmark of asthma. Despite normal spirometry and lung volumes, asthma can present with a normal pulmonary function test (PFT) if the disease is mild or if the patient is not experiencing an acute exacerbation at the time of testing.

Other Likely Diagnoses

• Chronic Bronchitis: Exposure to second-hand smoke increases the risk of chronic bronchitis, which can cause a chronic cough. Normal PFTs do not rule out chronic bronchitis, as the disease can be present even with normal lung function tests.
• Gastroesophageal Reflux Disease (GERD): GERD is a common cause of chronic cough, and the symptoms can be exacerbated by lying down or eating certain foods. Normal PFTs and lung volumes do not rule out GERD as a cause of chronic cough.
• Allergic Rhinitis: Allergic rhinitis can cause postnasal drip, leading to a chronic cough. The normal PFTs and lung volumes are consistent with this diagnosis.

Do Not Miss Diagnoses

• Lung Cancer: Although less likely, lung cancer can present with a chronic cough, and exposure to second-hand smoke increases the risk. It is crucial to consider this diagnosis to avoid missing a potentially life-threatening condition.
• Tuberculosis (TB): TB can cause a chronic cough, and normal PFTs do not rule out this diagnosis. It is essential to consider TB, especially if the patient has been exposed to someone with TB or has risk factors for TB.
• Sarcoidosis: Sarcoidosis can cause a chronic cough, and normal PFTs and lung volumes do not rule out this diagnosis. Although less common, sarcoidosis can be a cause of chronic cough, and missing this diagnosis could lead to delayed treatment.

Rare Diagnoses

• Cystic Fibrosis: Although rare in adults, cystic fibrosis can cause a chronic cough, and normal PFTs do not rule out this diagnosis. However, the lack of other symptoms, such as recurrent infections or malabsorption, makes this diagnosis less likely.
• Eosinophilic Granulomatosis with Polyangiitis (Churg-Strauss Syndrome): This rare condition can cause a chronic cough, and normal PFTs do not rule out this diagnosis. However, the presence of other symptoms, such as asthma, eosinophilia, and vasculitis, would increase the likelihood of this diagnosis.