leading to alveolar destruction resulting in lung hyperinflation and air trapping. Diagnosis There are no widely accepted diagnostic criteria for ACO, however the diagnosis requires clinical features of both asthma and COPD. One diagnostic criteria, based on expert consensus, first described in 2016, requires the presence of 3 major and at least 1 minor criteria for the diagnosis of ACO. The major criteria are: a persistent airflow limitation (a ratio of forced expiratory volume in 1 second divided by forced vital capacity (FEV1/FVC) of less than 0.7 or below the lower limit of normal), a significant exposure history to tobacco smoke (defined as a greater than 10 pack year history)(or significant exposure to other indoor or outdoor air pollution) and a documented history of asthma or a significant improvement in FEV1 (of greater than 400mL) to an inhaled bronchodilator. The minor criteria include a history of atopy or allergic rhinitis, a more limited response to an inhaled bronchodilator (greater than 200 mL improvement in the FEV1 or a 12% improvement from baseline), and peripheral blood eosinophils greater than 300 cells/μL. Spirometry (documenting obstruction) is required for the diagnosis of ACO. In those with asthma, some features often seen in COPD that may aid in the diagnosis of ACO include emphysema seen on imaging or a decreased diffusion capacity (DLCO)(indicating significant lung tissue damage). In those with COPD, other features often seen in asthma that may aid in the diagnosis of ACO include an increase in the fraction of exhaled nitric oxide (FENO)(a marker that is specific to the degree of airway inflammation in those with asthma) or increased levels of IgE (either total IgE or specific to inhaled antigens). Treatment Treatment of ACO is based on expert opinion as there are no universally accepted clinical guidelines. Treatment is usually based on whether clinical features of asthma or COPD predominate. Inhaled corticosteroids are the primary treatment in those with ACOS. Inhaled corticosteroids (ICS) should be continued in those with asthma who develop decreased airway responsiveness to bronchodilators consistent with ACO. Therapy can be escalated to include a long acting beta-agonist (LABA) and inhaled steroid combination (ICS-LABA) or by adding on a long-acting anti-muscarinic inhaler (LAMA), known as triple therapy, in those with more severe or resistant disease. Monoclonal antibodies targeting type 2 inflammation (which is predominant in asthma) have been used to treat severe asthma, and may also be used in