Are environmental risk factors for current wheeze in the International Study of Asthma and Allergies in Childhood (ISAAC) Phase Three due to reverse causation?

BACKGROUND: Phase Three of the International Study of Asthma and Allergies in Childhood (ISAAC) measured the global prevalence of symptoms of asthma in children. We undertook comprehensive analyses addressing risk factors for asthma symptoms in combination, at both the individual and the school level, to explore the potential role of reverse causation due to selective avoidance or confounding by indication.

OBJECTIVE: To explore the role of reverse causation in risk factors of asthma symptoms.

METHODS: We compared two sets of multilevel logistic regression analyses, using (i) individual-level exposure data and (ii) school-level average exposure (i.e. prevalence), in two different age groups. In individual-level analyses, reverse causation is a possible concern if individual-level exposure statuses were changed as a result of asthma symptoms or diagnosis. School-level analyses may suffer from ecologic confounding, but reverse causation is less of a concern because individual changes in exposure status as a result of asthma symptoms would only have a small effect on overall school exposure levels.

RESULTS: There were 131,924 children age 6-7 years (2,428 schools, 25 countries) with complete exposure, outcome and confounder data. The strongest associations in individual-level analyses (fully-adjusted) were for current paracetamol use (odds ratio = 2.06; 95% confidence interval 1.97-2.16), early life antibiotic use (1.65; 1.58-1.73), and open fire cooking (1.44; 1.26-1.65). In school-level analyses these risk factors again showed increased risks. There were 238,586 adolescents age 13-14 years (2,072 schools, 42 countries) with complete exposure, outcome and confounder data. The strongest associations in individual-level analyses (fully-adjusted) were for current paracetamol use (1.80; 1.75-1.86), cooking on an open fire (1.32; 1.22-1.43), and maternal tobacco use (1.23; 1.18-1.27). In school-level analyses these risk factors again showed increased risks.

CONCLUSIONS & CLINICAL RELEVANCE: These analyses strengthen the potentially causal interpretation of previously reported individual-level findings, by providing evidence against reverse causation. This article is protected by copyright. All rights reserved.