Asthma is a common respiratory disease. around the world. Asthma exacerbation is one of the major sequelae and associated with various morbidity and mortality. A satisfactory asthma control can help to bring down the risk of exacerbation and hence hospitalization. However, the real-world evidence on the clinical factors that leads to multiple admissions, when compared with single admission, due to asthma exacerbation is scarce. This study aimed at evaluating the clinical characteristics of patients who had single and multiple hospitalizations for asthma exacerbations, and exploring the risk factors that predict multiple hospitalizations.

Asthma is a common respiratory disease worldwide and in Hong Kong. According to the estimation by the Center for Disease Control and Prevention in 2015, 7.6% adults in the United States have asthma. In Hong Kong, the prevalence of asthma was estimated to be 10.1% among 13 to 14 years old children and 5.8% in randomly selected Chinese elderly aged more than 70. The clinical characteristics varies in different age groups, with elderly asthmatics have higher rates of bronchial hyper-reactivity and more severe phenotypes, when comparing with younger patients. This is more important in an ageing population.

Asthmatic exacerbation is not an uncommon complication and can be fatal. For those patients with near-fatal asthma exacerbation, ICU admission and mechanical ventilation may be necessary to prevent mortality. This is more common among poor controllers and adults. Asthma-related deaths may be reduced if risk factors are recognized and addressed early.

Many predictors for exacerbation had been identified, both endogenous and exogenous, including high eosinophil count, upper airway diseases, gastroesophageal reflux, poor inhaler technique, medication non-compliance, guideline non-compliance. One of the most powerful predictors for future exacerbation in patients with severe or difficult-to-treat asthma is a recent severe asthma exacerbation. In addition, hospitalization for exacerbations requiring ICU care and mechanical ventilation are both predictors for near-fatal asthma. After the acute attack, its unfavourable impact continues and can lead to multiple sequelae. Exacerbation of asthma is associated with a more rapid decline in the post-bronchodilator forced expiratory volume in 1 second and worse quality of life. In addition, individuals with uncontrolled asthma had higher medical expenditures and decreased productivity, contributing to a greater economic burden when compared with individuals without asthma. In contrary, patients with controlled asthma had lower hospitalization rate, mortality rate and less decline in lung function.

Many of these factors are potentially reversible. But, there is still a significant proportion of asthma patients experiencing recurrent exacerbation despite optimization of pharmacological treatment. A model of better asthma care may be established by improving the understanding on these risk factors, leading to less exacerbation events.

Inclusion Criteria:

• All patients with hospitalization for asthmatic exacerbation according to physician's clinical judgement
• Age greater than 18 years old

Exclusion Criteria:

- Inappropriate diagnosis of asthma exacerbation after evaluation

By using the International Classification of Diseases, Ninth Revision, Clinical Modification code 493.xx, each participating hospital identified all primary hospital discharge diagnosis of asthma (exacerbation) during a 12-month period, between 1st January 2016 to 31st December 2016

all IPD that underlie results in a publication

Starting 6 months after publication

upon individual approach for metaanalysis or related study