Full guidance on the condition of Asthma-COPD Overlap Syndrome (ACOS) may be accessed via either the Global Initiative for COPD or the Global Initiative for Asthma websites. The summarised text below aims to give a brief introduction to the condition of ACOS but it is recommended that clinicians refer to the full guidance for more detailed information.

Some patients who present with chronic respiratory symptoms, particularly older patients, have diagnoses and/or features of both asthma and COPD, and are found to have airflow limitation that is not completely reversible after bronchodilation.

Some have an unequivocal history of asthma which has been poorly controlled and which has resulted in fixed airflow obstruction, but in these cases the diagnosis remains asthma and patients should be managed according to asthma guidelines.

However, a small group of patients have risk factors for and clinical features of both diseases and the concept of an 'overlap' syndrome has been proposed. These patients experience frequent exacerbations, have worse quality of life, a more rapid decline in lung function and higher mortality, and consume a disproportionate amount of healthcare resources than either asthma or COPD alone.

Best estimates suggest the overlap syndrome occurs in about 10% of people with airways disease and in the overwhelming majority of patients a definite diagnosis can and should be made.

ACOS is characterised by persistent airflow limitation with several features usually associated with asthma, and several features usually associated with COPD.

Making an accurate diagnosis depends on taking a good and detailed medical history to confirm exposure to risk factors, to characterise the symptoms (including provoking and relieving factors and variability) and to confirm that the patient has airflow obstruction using spirometry.

Although initial recognition and treatment may be made in primary care, referral for confirmatory investigations is sometimes required if there is persisting uncertainty about the differential diagnosis or the response to initial therapy is poor.

The treatment recommendation for patients, whose differential diagnosis is equally balanced between asthma and COPD, is to start treatment for asthma until further investigations are performed. Treatment is recommended using an inhaled corticosteroid in a low or moderate dose (depending on level of symptoms); add-on treatment with long-acting beta₂ agonists (LABA) and/or long-acting muscarinic antagonists (LAMA) is usually also necessary. If there are features of asthma, avoid LABA monotherapy. This approach recognises the pivotal role of inhaled corticosteroids in preventing morbidity and even death in patients with uncontrolled asthma symptoms, for whom even seemingly 'mild' symptoms (compared to those of moderate or severe COPD) might indicate significant risk of a life-threatening attack.