A comparison of skin prick tests, intradermal skin tests, and RASTs in the diagnosis of cat allergy

Background: Skin testing and RASTs are the most commonly used methods for the diagnosis of allergy. Questions remain, however, as to the accuracy of these tests, particularly with regard to the role of intradermal skin tests (IDSTs) in the evaluation of respiratory allergy. Objective: The purpose of this study was to determine the predictive value of skin prick tests (SPTs), IDSTs, and RASTs in the diagnosis of cat allergy. Methods: Patients were challenged with a well-characterized cat exposure model after evaluation by history, SPTs, IDSTs (if SPT results were negative), and RASTs. All patients were evaluated with respect to their upper respiratory responses, although only those patients with asthma were included in the analysis of lower airway responses. Challenge results were considered positive if the mean upper respiratory symptom score was 0.5 or more, the mean lower respiratory symptom score was 0.4 or more, or the maximum fall in FEV₁ value was 15% or more. Results: One hundred twenty patients were evaluated. SPT values were positive in 81 patients; of the remaining 39 patients, IDST values were positive in 26 patients. RASTs were performed in 89 patients; the values were positive in 45 of 51 patients with a positive SPT value and were negative in all patients with a negative SPT value. When any positive challenge outcome was considered, positive challenge results were seen in 38 of 41 patients with a positive SPT score, in 10 of 39 patients with a negative SPT score, in 6 of 26 patients with a positive IDST score, in 4 of 13 patients with a negative IDST score, in 27 of 27 patients with a positive RAST score, and in 12 of 44 patients with a negative RAST score. Conclusion: Although both SPT and RAST values exhibited excellent efficiency in the diagnosis of cat allergy, IDST scores added little to the diagnostic evaluation.