Should all patients with severe asthma get tested for AAT deficiency?

α1-Antitrypsin (A1AT). Image source: Wikipedia, GNU Free Documentation License.

There is an uncertain relationship between severe AAT deficiency and asthma. Studies have not shown an increased prevalence of AAT deficiency among asthmatics. There is no association of alpha-1 antitrypsin Pi variants such as Pi MZ or Pi MS with more severe asthma.

However, in a large cohort study of over 1000 patients with severe AAT deficiency, 21% of patients had asthma. The presence of asthma was not independently associated with an accelerated decline in pulmonary function, and the results of augmentation therapy appear to be similar for asthmatics and nonasthmatic patients with severe AAT deficiency.

Should patients with severe asthma get tested for AAT deficiency?

For some patients, yes. For the lifelong non-smoker with persistent and irreversible airflow obstruction, a one-time measurement of the serum alpha-1 antitrypsin level is recommended to exclude emphysema due to homozygous alpha-1 antitrypsin deficiency, which is in the differential of chronic and largely irreversible airflow limitation.

References:

Evaluation of severe asthma in adolescents and adults. UpToDate, 2014.
Diagnosis of asthma in adolescents and adults. UpToDate, 2014.
Eden E, Hammel J, Rouhani FN, et al. Asthma features in severe alpha1-antitrypsin deficiency: experience of the National Heart, Lung, and Blood Institute Registry. Chest 2003; 123:765.
http://www.ncbi.nlm.nih.gov/pubmed?term=12628876