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Drug hypersensitivity results from interactions between a pharmacologic agent and the human immune system. These types of reactions constitute only a small subset of all adverse drug reactions. Allergic reactions to medications represent a specific class of drug hypersensitivity reactions mediated by IgE. Immune-mediated drug reactions may be discussed generally in the Gell and Coombs classification system, a widely accepted conceptual framework for understanding complex immune reactions. However, some reactions involve additional, poorly understood mechanisms that are not easily classified. Identifiable risk factors for drug hypersensitivity reactions include age, female gender, concurrent illnesses, and previous hypersensitivity to related drugs. Drug hypersensitivity is a clinical diagnosis based on available data. Laboratory testing may be useful, with skin testing providing the greatest specificity. Treatment is largely supportive and includes discontinuation of the offending medication, symptomatic treatment, and patient education. Patients with penicillin allergy should avoid carbapenems, and caution should be used in prescribing cephalosporins in these patients. Reactions to radiocontrast media can be limited by pretreatment with prednisone, diphenhydramine, and either ephedrine or a histamine H^sub 2^-receptor antagonist. (Am Fam Physician 2003;68:1781-90. Copyright© 2003 American Academy of Family Physicians.)
Adverse drug reactions are common. Identifying true drug allergy, however, can be challenging. Complicating factors of drug reactions include the myriad clinical symptoms and multiple mechanisms of drug-host interaction, many of which are poorly understood. In addition, the relative paucity of laboratory testing that is available for drug allergy makes the diagnosis dependent on clinical findings.
Definitions and Classifications
The terms "drug allergy," "drug hypersensitivity," and "drug reaction" are often used interchangeably. Drug reactions encompass all adverse events related to drug administration, regardless of etiology. Drug hypersensitivity is defined as an immune-mediated response to a drug agent in a sensitized patient. Drug allergy is restricted specifically to a reaction mediated by IgE.
Drug reactions can be classified into immunologic and nonimmunologic etiologies (Table 1). The majority (75 to 80 percent) of adverse drug reactions are caused by predictable, nonimmunologic effects.1 The remaining 20 to 25 percent of adverse drug events are caused by unpredictable effects that may or may not be immune mediated.1 Immune-mediated reactions account for 5 to 10 percent of all drug reactions and constitute true drug hypersensitivity, with IgE-mediated drug allergies falling into...