Background: Metamizole is a pyrazolone derivative, and its most common reactions are IgE-mediated reaction and idiosyncratic reactions. Non-immediate reactions are poorly described and there are very few reports on non-immediate reactions to pyrazolones.
Materials and methods: We evaluated 12 patients (nine men) who consulted for a non-immediate reaction after metamizol administration. We performed cutaneous tests (skin prick tests and immediate and delayed intradermal tests) and epicutaneous tests, and, if necessary, an oral challenge test.
Results: All skin prick and intradermal tests, if necessary, were negative in immediate reading. Delayed intradermal tests were positive in six of 10 patients (60%) and epicutaneous tests were positive in four of 11 patients (36%). Three cases (25%), were diagnosed by a positive oral challenge test.
Discussion: Delayed-reading intradermal tests and patch tests are useful tools in the diagnosis of nonimmediate reactions to pyrazolones and should be considered the first step when evaluating these type of reactions. Intradermal test appears to be more sensitive than patch test. The positivity of skin tests suggests an immunological reaction, probably mediated by T lymphocytes, but further studies are required.