Proposal for Use of Two Concentrations for Metamizol to Intradermal Testing

Metamizol, a pyrazolone derivative, is commonly implicated in IgE-mediated allergic reactions. Skin tests have different sensitivity in drug allergy diagnosis and false positive or false negative results render their interpretation difficult, especially because the ranges of the tested concentrations have not been adequately investigated in all drugs. The objective of our study was to determine the non-reactive concentrations for metamizol in healthy subjects using prick and intradermal tests. We studied the wheal and flare response to prick and intradermal tests in 20 Caucasian nonallergic volunteers for metamizol. We selected more clustered concentrations that resulted from decimal dilutions, identical for both prick and intradermal testing (500, 250, 100, 50, 25, 10, 5 and 1 mg/mL). For intradermal testing we focused on reading wheal/initial wheal ratio (RW/IW) at different metamizol concentrations. For prick testing the maximal concentration which did not produce wheal and flare was 500 mg/mL. For intradermal tests, we defined some test concentrations (C) important in the interpretation: C1 represents the highest concentration for which the test is not modified (RW/IW ratio ≤ 1); C2 represents the highest concentration for which the test is modified but the RW/IW ratio is < 2; Cmax represents the concentration for which half of the subjects surpass positivity criteria RW/IW ≥ 2. For metamizol C1 is 5 mg/mL, C2 is 50 mg/mL and Cmax is 250 mg/mL. Using two reading concentrations, C1 and C2, it might be reduced the number of errors due to false positive and false negative results and it might improved the interpretation of intradermal tests.