Participants

In this case–case–control, the initial study population consisted of 110 controls, 160 asthmatics, and 126 allergic rhinitis patients, for a total of 396 participants. The study population described herein is the aggregate of two previously described samples [19, 20]. Study participants were consecutively recruited from the Jordan University Hospital and the Jordan Hospital during the time period between August 2010 and January 2013. Asthma diagnoses (made by Professor Nathir M. Obeidat) were based on history, physical examination, and pulmonary function tests. Similarly, allergic rhinitis diagnoses (made by Professor Tareq M. Mahafza) were based on history and physical examination. In the latter cases, the skin prick test was used as an ancillary diagnostic tool. Patients with both conditions were excluded from the study. The control group comprised patients’ companions. Controls were unrelated to patients and had no history of respiratory disease, as determined by clinical interview and examination. The chief inclusion criterion for all groups was ethnicity. Participants were exclusively drawn from the same ethnic group (namely Arabs) in order to minimize population stratification. Ethnicity was determined by self-report. Institutional Review Board (Jordan University Hospital, Amman, Jordan) approval was obtained, and all participants or their guardians provided written informed consent.