S A T U R D A Y Inmaculada Do~ na, MD, PhD, Francisca G omez, MD, PhD, Tahia D. Fernandez, PhD, Adriana Ariza, PhD, Arturo Ruiz, MD, Maria Isabel S anchez Rivas, Cristina De Leyva, Maria Isabel Monta~nez, Antonio Guzman, Miguel Blanca, MD, PhD, Maria J. Torres, MD, PhD; Allergy Service, IBIMA-Regional University Hospital of Malaga-UMA, M alaga, Spain, Allergy Unit, IBIMA-Regional University Hospital of Malaga, M alaga, Spain, Research Laboratory, IBIMA-Regional University Hospital of Malaga-UMA, Malaga, Spain, Allergy Unit, Regional University Hospital of M alaga-IBIMA, UMA, M alaga, Spain, Allergy Unit, Regional University Hospital of Malaga-IBIMA, UMA, Malaga, Spain, Research Laboratory, IBIMA, Regional University Hospital of Malaga, UMA, Malaga, Spain, Pharmacy Unit, Regional University Hospital of Malaga-IBIMA, UMA, M alaga, Spain, Allergy Unit, Regional University Hospital of M alaga, IBIMA, UMA, M alaga, Spain. RATIONALE: Pyrazolones are the most common cause NSAIDs hypersensitivity. We studied a large group of patients with immediate and nonimmediate selective responses (SR) to dipyrone. METHODS: Patients with suspiction of hypersensitivity to dipyronewere evaluated. We verified acetilsalicilic acid-tolerance and classified patients as immediate or non-immediate responders if they showed symptoms less or more than 1hour after dipyrone administration. Skin tests were performed and if negative, basophil activation test (BAT) for immediate responders. If negative results, we performed drug provocation test (DPT) with dipyrone. RESULTS: A total of 137 patients were included: 114 reacted within one hour (single NSAID-induced urticaria/angioedema/anaphylaxis; SNIUAA); 23 after one hour (single-NSAID-induced delayed hypersensitivity reactions; SNIDHR). More specifically, 73.72% reacted within 30 minutes; 9.48% 30-60 minutes; 6.56% 1-2 hours; 3.64% 2-8 hours and 6.56% after over 24 hours. Most SNIUAA patients developed anaphylaxis (66.66%, p50.0001); for SNIDHR urticaria was more frequent (52.17%, p50.014). Skin testing was positive for 62.04% of all cases and BAT for 35.89% of SNIUAA patients with negative skin tests. In 5.1% cases DPT with dipyrone was needed. In 22.62% of cases DPT was not indicated, being diagnosed by repeated allergic episodes in spite of negative skin and BAT. CONCLUSIONS: SNIUAA to dipyrone is the most frequent group with anaphylaxis being the most common clinical entity. It may occur with interval longer than 1hour. The SNIDHR occur in a very low percentage. The low sensitivity of diagnostic tests may be due to incomplete characterization of the chemical structures of dipyrone and its metabolites.