A case - control study was conducted between March and September 2010 in Urmia city, North West of Iran. To reduce the risk of differential diagnosis between asthma and respiratory tract infections in small children, patients in the first 2 years of life excluded from case group.
Cases (n=207) were selected from asthmatic patients who their disease were confirmed based on the Global Strategy for Asthma Management and Prevention (GINA) criteria. Samples were 2-8 years old children who were referred to the Motahhari children polyclinic and asthma, allergy pediatric center. Two controls (n=401) were selected for each case and they were matched for age and gender using the frequency matching method. To reduce socioeconomic difference and other unknown confounding factors, controls were chosen from area of the city in which the cases were living. Meanwhile, Hospitalized controls were selected from other patient without any allergic or respiratory disease and healthy controls were selected from healthy children that referred to health centers for growth monitoring.
For gathering the required data, the questionnaire prepared by the International Study of Asthma and Allergies in Childhood (ISSAC) phase three was used. It was translated to Persian language and data related to acetaminophen exposure were collected by interviewing parents/ guardians.
Children with positive answer to Acetaminophen use due to fever in the first year of life were compared to children who did not use as the reference group. For Acetaminophen consumption during the last year of study, at least one tablet in a year, once in 2 to 3 months and at least one per month categories were used and compared with children who once or less used as the reference group. Because reported frequency of Acetaminophen intake during the last year in “never use” category was less than 5% it was combined with the “at least once a year” category and new category (once or less use in the last year) was considered as reference group.
The Pearson chi - square was applied to assess effect of Acetaminophen exposure and other potential factors that affected on the association between Acetaminophen exposure and risk of childhood asthma. The univariate logistic regression was used to calculate crude ORs and 95 % confidence intervals and a multiple logistic regression was also applied to calculate adjusted ORs and 95 % confidence intervals for odds of development of asthma following Acetaminophen exposure in the first year of study (non-used as the reference) and during the last year of study (once or less use as the reference) to assess existence of dose-response relationship between Acetaminophen use and development of asthma during the last year of study. The risk of asthma following Acetaminophen use, adjusted for important risk factors in this association when they had significant effect on the risk of development asthma in multiple regression model.
The probable confounding factors such as using antibiotic, maternal smoking, history of asthma and other allergic disorders in the first degree relatives, history of breastfeeding, number of older siblings, number of younger siblings were included in the regression model. The STATA, release 10, statistical software was used for all analyses.