Atopic status in patients with brittle and non-brittle asthma: a case-control study

Sensitization to inhaled and ingested allergens is an important process in determining the subsequent clinical expression of asthma. Allergen exposure has also been reported to be associated with admission to hospital with acute severe asthma. Patients with brittle asthma, characterized by widely variable peak expiratory flow are at increased risk of life-threatening episodes but the role of atopy in these patients is unknown.

To determine the atopic status of patients with brittle asthma using a case-control design.

We have assessed the atopic status by skin-prick tests to 19 common allergens, and total and specific immunoglobulin E (IgE) in 29 patients with well characterized brittle asthma and an age, sex and treatment-matched control group without brittle asthma.

Mean weal diameters were higher in brittle compared to non-brittle asthma for grass pollen (4.64 vs 2.17; P = 0.01), horse hair (6.28 vs 2.64; P = 0.02), feathers (2.96 vs 1.52; P = 0.01), wheat (1.48 vs 0.66; P = 0.001) and chocolate (1.09 vs 0.41; P = 0.05). Mean radioallergosorbent (RAST) scores to house dust mite were also greater in brittle asthma patients (19.3 vs 7.65; P = 0.05). Patients with brittle asthma also exhibited a significantly greater degree of atopy when weal diameters to all 19 allergens were summated to produce an atopy score (brittle 44.35 vs non-brittle 23.72; P = 0.04). There were no significant differences between the two groups in either the number of positive skin tests (using a 4 mm definition of skin-test positivity), total IgE or RASTs (using a weak +ve score to define positivity). However, the use of differing definitions of atopy (1, 2, 3, 4 and 5 mm skin test weal diameters) resulted in marked intra-group variation in atopic status in both brittle and non-brittle asthma patients.

The greater degree of atopy seen may be an important factor in patients with brittle asthma. The varying interpretations of the classification of what constitutes the presence or absence of atopy, based on mean weal diameters of skin-prick tests, or from IgE or RAST positivity shows that there is considerable potential variation in the degree of difference between the two groups depending on what criteria are used. Although internationally agreed definitions of atopic status have been devised a more rigorous application, or review of these guidelines needs to accompany future epidemiological studies of allergic sensitization.