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.