ALLERGOL. ET IMMUNOPATHOL., 1998;26(5):203-205
EDITORIAL
SKIN TESTS OR IN VITRO TESTS?
Two diagnostic methods, skin tests and challenge tests, and one therapeutic procedure, immunotherapy, are peculiar of our specialty. Although challenge tests are usually reserved for cases in which a diagnostic doubt must be resolved or other available diagnostic tools are insufficient, as occurs with medication or food allergies, skin tests still are the most frequently used diagnostic method.
The reliability of skin tests, which no one questions, depends on their correct realization, the availability of quality allergenic extracts, the absence of medications that inhibit the reaction (antihistamines), an appropriate reading, and, finally, the correct interpretation of results. The technique has drawbacks related to the patient''s skin characteristics, particularly sensitivity (dermographism-factitial urticaria) or dermopathies, such as eczema, when may make it difficult to perform the test or to read and interpret the results. However, the low cost, technical simplicity, and immediate availability of results make skin tests particularly valuable.
The reliability of skin tests is well established for inhaled allergens, such as dust mites or pollens. However, skin tests are much less useful for food allergens, questionable for medications, and may entail risk for hymenoptera venoms.
Since the discovery of IgE and the confirmation of its role in reagin activity, methods have been developed for assessing total serum content (total IgE) and allergen-specific IgE. Circulating IgE has a very short half-life, about 2-3 days, whereas mast-cells-linked IgE persists for months or years. Mast-cells-linked IgE is the active form detected in skin tests, while circulating IgE is the excess not bound by cells. This is why skin tests provide more reliable results than blood serum determinations.
However, technological developments are increasing the precision of measurements of serum levels of specific IgE, as indicated by their sensitivity and specificity. Studies comparing in vivo and in vitro methods (variants of RAST and the CAP System) have shown that these methods can obtain results similar to those of skin tests, although usually not better (1-3). In the study by Cots et al. published in this issue of Allergologia et Immunopathologia (4), the CAP-System was compared with the most recent modality, UniCAP 100, a totally automated system that enhances the results obtained with pneumoallergens and some foods.
The diagnosis of food allergies is a special problem. Skin tests and in vitro techniques produce convincing results in diagnosing allergies to cow-milk proteins or egg proteins because pure antigens are available. However, the variety of proteins present in most foods makes it extremely difficult to standardize extracts for skin tests or laboratory tests. It is estimated that no more than 40% of cases yield frankly positive results, although the results of the CAP-System have recently been compared favorably with those of challenge techniques for certain foods (5). Nonetheless, challenge techniques do not provide a definitive diagnosis of all cases of suspected food allergy (6).
Skin tests with insect venom yield good results, but the risk of anaphylactic shock is high. Therefore, when the patient is suspected to be very sensitive, skin tests should be avoided. Laboratory methods are very sensitive, so serum determinations are preferable (7, 8).
Given these considerations, the question arises as to whether it is justified to evaluate specific IgE in the serum of all patients, given the reliability of skin tests, particularly for pneumoallergens, the allergens most often responsible for allergic reactions (rhinoconjunctivitis, asthma). The cost of laboratory tests is an important argument against their routine use.
In the study by San Martín et al., also published in this issue of Allergologia et Immunopathologia (9), the authors tried to correlate total serum IgE levels with the possibility of obtaining positive results in determinations of specific IgE levels for different allergens. The interest of this study centers on its potential for saving time and expense in routinely requesting allergen series from the laboratory, which might not be requested if total IgE concentration were known. Nonetheless, as noted earlier, total serum IgE level does not reflect reliably what is occurring in the cells to which the reagin binds and low serum IgE levels do not always indicate the absence of sensitization, as this study shows.
However, the most significant may be related with the attendance of supposedly allergic patients, in which general physicians are allowed to request analyses that should be ordered only by specialists. There is no justification for making an initial request to the laboratory for specific IgE determinations for a series of allergens, even if they are common, because it is evident that most will be negative. The correct diagnostic procedure is to perform skin tests and, if it is necessary, then to confirm positivity by serum tests of the specific allergens, particularly if immunotherapy or an exclusion diet is going to be prescribed.
Only in cases in which skin tests are difficult to carry out or sensitization to insect venom is being diagnosed should specific IgE determinations be requested initially.
F. Muñoz-López
REFERENCES
1. Dolen WK. Guest Editorial: allergy diagnosis revised. Ann Allergy Asthma Immunol 1995;74:2-3.
2. Brand PLP, Kerstjens HAM, Jansen HM, et al. Interpretation of skin tests to house dust mite and relationship to other allergy parameters in patients with asthma and chronic obstructive pulmonary disease. J Allergy Clin Immunol 1993;91:650-70.
3. Williams PB, Dolen WK, Koepke JW, Selner JC. Comparison of skin testing and three in vitro assays for specific IgE in the clinical evaluation of immediate hypersensitivity. Ann Allergy 1992;68:35-45.
4. Cots P, Pena JM, Botey J, Eseverri JL, Marín A, Ras A. Determination of total and specific IgE using UniCAP 100. Comparative study with the CAP-system. Allergol et Immunopathol 1998;26:223-7.
5. Sampson HG, Ho DG. Relationship between food-specific IgE concentrations and the risk of positive food challenges in children and adolescents. J Allergy Clin Immunol 1997;100:444-51.
6. Plaut M. New directions in food allergy research. J Allergy Clin Immunol 1997;100:7-10.
7. Leimgruber A, Lantin J-P, Frei PC. Comparison of two in vitro assays, RAST and CAP, when applied to the diagnosis of anaphylactic reactions to honeybee or yello jacket venoms. Allergy 1993;48:415-20.
8. Ford JL, Dolen WK, Feger TA, et al. Evaluation of an in vitro assay for fire ant venon-specific IgE. J Allergy Clin Immunol 1997;100:425-7.
9. San Martín E, Chesa-Jiménez J, Sanmartín M, Ruiz G, Moreno JL. Estudio de la relación entre la concentración plasmática total de IgE y la prevalencia de las distintas clases de RAST de alergia. Allergol et Immunopathol 1998;26:228-33.
