ICD-10 Code: J45.909

Asthma is a chronic inflammatory disorder of the airways affecting approximately 25 million Americans (8% of population). It is characterized by bronchial hyperresponsiveness, reversible airflow limitation, and airway inflammation. Pathophysiology involves mast cell degranulation, eosinophil infiltration, increased mucus production, bronchial smooth muscle hypertrophy, and airway remodeling. Clinical presentation includes recurrent episodes of wheezing, shortness of breath, chest tightness, and coughing (often worse at night or early morning). Triggers include allergens (dust mites, pollen, pet dander), respiratory infections, exercise, cold air, tobacco smoke, air pollution, and stress. Between exacerbations, patients may be asymptomatic or have mild symptoms. Code J45.909 is used when: (1) provider documents "asthma" without specifying severity classification, (2) patient is currently stable (not experiencing exacerbation), and (3) no status asthmaticus is present. This is the most commonly used asthma code in practice, as detailed severity classification is often not documented at every visit. Diagnosis confirmed by spirometry showing reversible airflow obstruction (FEV1 improves ≥12% and ≥200 mL after bronchodilator).