DIAGNOSTIC RELIABILITY CONSIDERATIONS OF SPECIFIC IGE DETERMINATION

Total IgE determination constitutes a good method far the screening of atopic diseases, though its actual value is controversial because nor mal values of total IgE do not exclude the existence of atopic disease , and high values of total IgE are not pathognomonic of atopy by thems elves. The first step in identifying an atopic individual as such, aft er doing his anamnesis, can be carried out by means of total IgE deter mination. Most atopic individuals have high IgE values, but a normal r esult must be carefully interpreted: age and season-related variations must be considered. In general, atopic patients with IgE values great er than 1000 Ul/ml, always have positive specific IgE against some all ergen. Antigen-specific IgE will be the next step in the in vitro iden tification of the responsible allergen. Nowadays, there are more than 400 characterized allergens available for in vitro diagnostic tests an d several useful methodologies for specific IgE determination. Specifi c IgE results obtained with the different methods vary significantly, with absolute agreement in 55-65% of the cases, differences in one IgE class in 20-30% of the cases and differences in more than two classes in 5-10%. The specificity of the anti-IgE antibody used in the assay is of critical importance because any contaminant antibody can render unspecific results. On the other hand, it must be pointed out that the re is a compromise between specificity and sensitivity, such that an i ncrease in the sensitivity of a technique leads to a decrease in its s pecificity. It cannot be said that there is one method which is better than the others; if is better to examine them individually, allergen by allergen. Thus, specific IgE determination varies depending on the type of allergen. In general terms, for inhalant allergens, specificit y and sensitivity of the methods are within the range of 85-95%, but t hese values (especially the specificity) decrease in the case of food allergens, and they are still lower when the allergen is a beta-lactam ic drug. There is a good correlation between clinical history and spec ific IgE against inhalant allergens, and a lower correlation in the ca se of food allergens. Due to the fact that most food allergens are not standardized, the definitive diagnosis of food hypersensitivity is ac hieved by means of provocation tests. Nevertheless, negative specific IgE (7-18% of the cases) does not rule out a sensitization against the tested allergen, and a positive specific IgE without symptoms must be carefully interpreted because it can be due to a low degree of sensit ization, unable to express clinical symptoms at this moment, but usefu l in the future as a guide on the disease course. In the evolutive per iod of the disease, specific IgE levels can be modified in a natural w ay (in beta-lactam allergy, 50% of the cases with specific IgE become negative after a year), or as an effect of the treatment (e.g., after immunotherapy in the case of Hymenoptera venom allergy), or it can rem ain positive for a long time, as in the case of pollinosis. On the oth er hand, the cutoff of the method and subsequently the range of values to be considered as positive, will depend on the allergen studied Whi le inhalant allergens (with the exception of some molds) offer relativ ely high mean values of specific IgE, food and drug allergens yield le ss significant values. In general, a class greater than 2 is interpret ed as clinically significant class 1 as dubious or negative (depending on the allergen) and class 0 as negative. in the interpretation of th e results, the possible presence of IgG and anti-IgE antibodies, capab le of modifying the results, will be taken into account When comparing the diagnostic reliability of specific IgE with respect to other alle rgologic diagnostic methods, we find a significant and positive correl ation of this technique with skin tests (but never greater than 90-95% ) and with the histamine release test This last technique is an in vit ro provocation test and offers a quantitative measure of the degree of sensitization of the patients against an allergen, whose seasonal mod ifications are related to the clinical activity of the disease. in sum mary, specific IgE determination is a useful, although not definitive, technique in allergologic diagnosis, whose results must be cautiously interpreted, and always as an aid to the patient's clinical history, but never alone, to finally diagnose an allergic disease.