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08-07-2010 | Respiratory | Article

Confounding by indication may explain asthma link to antibiotic use


Free abstract

MedWire News: Results from a US study suggest the increased risk for childhood asthma associated with antibiotic use may be a result of confounding by indication.

"Several studies have found a positive association between antibiotic use and risk of asthma in childhood," explain Anne Wright (University of Arizona, Tucson) and team. "However, relations between antibiotic use and childhood asthma are not consistent in all studies."

They add that methodologic issues, such as recall bias in retrospective studies and not accounting for confounding by indication, ie, whether the illnesses for which antibiotics were prescribed were themselves risk factors for asthma, could be responsible to the observed relationship.

To address these issues, the researchers studied 424 children who participated in the Infant Immune Study and who were followed-up for the first 5 years of life.

The use of oral antibiotics by the children was assessed using data from frequent nurse interviews conducted in the first 9 months of the children's lives.

Questionnaires regarding respiratory symptoms and doctor-diagnosed asthma and eczema were completed by an adult caregiver when the children were aged 1, 2, 3, and 5 years, and blood levels of allergen-specific immunoglobulin (Ig)E were also measured at these ages.

Confounding by indication was assessed by considering the relationship between asthma and antibiotic use while controlling for the number of illness visits to a physician in early life.

In total, 136 (32.1%) children used oral antibiotics at least once during the first 9 months of life. By the age of 5 years, 50 (11.8%) children had asthma, 134 (31.6%) had eczema, and 202 (50.8%) had detectable allergen-specific IgE.

The researchers found no significant association between early antibiotic use and doctor-diagnosed eczema or allergen-specific IgE.

Initial results suggested that antibiotic use in the first 9 months of life was significantly associated with an increased risk for asthma by the age of 5 years (odds ratio [OR]=1.5), with a significant dose-response relationship evident for the number of antibiotic courses in the first 9 months.

However, they also found that the prevalence of asthma at the age of 5 years increased with the number of illness visits in the first 9 months of life, regardless of the use of antibiotics. After accounting for the number of illness visits in the first 9 months of life, the asthma-antibiotics relationship was no longer significant.

Wright and team conclude in the journal Clinical and Experimental Allergy: "This study suggests that at least a major portion of the relation between antibiotic use and childhood asthma observed in some studies may be an artefact of the strong relation of illness visits to a doctor to both antibiotic use and risk for asthma diagnosis."

MedWire ( is an independent clinical news service provided by Current Medicine Group, a trading division of Springer Healthcare Limited. © Springer Healthcare Ltd; 2010

By Mark Cowen

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