Definition of respiratory outcomes
We used answers from the parents to determine whether a child had physician-diagnosed asthma. The question was, “Has a doctor ever diagnosed your child as having asthma?” In the prospective survival analysis, incident asthma was calculated by excluding asthma history before the age of 11 years. Incident asthma was determined at age 12 and 17 years by parents’ affirmative answer to whether their child had been physician-diagnosed as having asthma during the follow-up period. The atopic status of asthma was determined by measuring the fractional exhaled nitric oxide (FeNO) by using a portable NO analyzer (NIOX MINO Airway Inflammation Monitor; Aerocrine AB, Solna, Sweden). We categorized children with asthma into the atopic or nonatopic group by using 20 ppb of FeNO as the cutoff value.11
A pulmonary function test was performed in accordance with a previously standardized protocol12 at age 12 years. The forced expiratory volume in 1 second (FEV1)/forced vital capacity (FVC) ratio, which is a marker of obstructive type pulmonary function impairment, was also used as a respiratory outcome.