Asthma is a chronic lung disorder that is characterized by airway inflammation, reversible airway obstruction, and episodic wheezing, coughing, or dyspnea. Asthma is diagnosed by symptoms and physical findings (wheezing, cough, dyspnea, decreased exercise tolerance, chest hyperinflation, atopy), and lung function testing. While lung function tests are essential for the diagnosis and monitoring of asthma, they focus on abnormal airway physiology and airflow obstruction, without evaluating the degree of airway inflammation. More direct assessment is possible using bronchial biopsy, bronchoalveolar lavage (BAL), and sputum analysis after induction using hypertonic saline. However, these procedures are invasive and uncomfortable. Due to these limitations, there has been much interest in biomarkers that can aid in diagnosis and monitoring, and define specific disease subgroups with different long-term prognoses or responses to therapy.