Drug Allergy

Extract

[...] Drug allergy has a great variety of clinical manifestations and has been attributed to most categories of therapeutic agents. Introduction Because specific diagnostic tests are not usually available, physicians most often base decisions on probabilities and the patient's need for treatment. This presentation provides an overview of drug allergy with an emphasis on pathogenic mechanisms, diagnostic considerations, and preventive measures. Epidemiology and Etiology Complications of drug therapy are the most common adverse events among hospitalized patients, and 10 to 14% of drug reactions have an allergic basis. [...]

[...] The history should be adequate to allow classification of the type of reaction experienced, and physicians should educate patients to distinguish allergy from other types of adverse reactions. Reactions attributable to drug toxicity or side effects do not preclude future use of the same or chemically similar agents. Recommendations Before prescribing any new drug, physicians should inquire again about past drug reactions. Skin tests can predict a type I hypersensitivity response to some drugs and should be done routinely before heterologous antisera is given. [...]

[...] Patients suspected of having drug allergy are often receiving multiple drugs, and identification of the agent responsible can be difficult. Diagnosis It sometimes is helpful to make a flow chart listing the starting dates and times of all medications, including drug therapy that has been discontinued recently. The likely allergen may be recognized by considering the above-mentioned criteria and the drug categories most commonly implicated in allergic reactions. An allergic reaction to drugs that have been given continuously for long periods is much less likely than a reaction to recently introduced therapy. [...]

[...] This testing method can be used only to predict or confirm drug reactions of the immediate hypersensitivity type, such as urticaria or systemic anaphylaxis. To obtain valid results, testing must be done with relevant antigens, which for most low-molecular-weight drugs are unknown metabolites. Diagnosis The lack of knowledge of the immunochemistry of most drugs severely limits the usefulness of skin testing. Negative tests are often uninterpretable, and false-positive reactions can result from nonspecific skin irritation. Skin testing has proved useful for evaluating penicillin allergy in cases in which the relevant antigens are well known (see later) and for allergic reactions associated with anesthetic agents. [...]

[...] In some cases, a careful history reveals that allergy has been confused with other types of adverse drug reactions, such as side effects or drug toxicity. Some patients who have experienced severe allergic reactions become fearful of all drug use and experience reactions attributable to anxiety. Treatment The possibility of true allergy to multiple, chemically dissimilar drugs also must be considered, however. A prospective study showed that patients with a history of allergic reactions to any antimicrobial agent were 10 times more likely to react to unrelated antimicrobial drugs than were history-negative controls. [...]