The predictive value of total serum IgE for a positive allergen specific IgE result.

BACKGROUND Measurement of total serum IgE and allergen specific IgE is often requested to assess possible allergy. As public awareness increases, so do requests for allergy assessment; unless there is a clear "allergen suspect" in the history, several allergen specific IgE requests may be made. This increases the likelihood of detecting borderline increases in allergen specific IgE of uncertain relevance, and has important cost implications for the service. AIMS To provide an evidence base for this observation. METHODS In this retrospective observational study, results from 301 patients under 16 years of age from whom blood was taken for "allergy testing" from March 2001 to February 2003 were studied. RESULTS Allergen specific IgE testing in children with low total serum IgE concentrations (<10 IU/litre) yielded very few positive results (three of 73 children), except in those being investigated for an acute reaction to a single food; when IgE was 11-20 kU/litre, 13 of 73 children had positive allergen specific IgE; in the 21-40 kU/litre IgE group, 16 of 74 children had positive allergen specific IgE and in the 41-80 kU/litre group, 22 of 81 had positive allergen specific IgE. CONCLUSIONS Allergen specific IgE testing in children with low IgE concentrations (<10 kU/litre) produces few positive results in patients with non-specific symptoms. Laboratories should perform allergy testing for specific allergens regardless of total IgE concentration only when there are convincing clinical reasons to do so, and should not proceed with this if the total IgE is <10 kU/litre and the presenting symptoms are non-specific.