The term drug allergy should be considered to encompass any reaction due to a drug that has clinical features compatible with hypersensitivity. Therefore, anyone presenting with a rash, angioedema, bronchospasm, hypotension should be considered to have drug allergy regardless of the nature of the underlying mechanism. In most cases, the mechanism of a drug reaction is due either to a Type 1 IgE-mediated hypersensitivity or a Type 4 T-cell mediated reaction. Reactions are due to non-steroidal anti-inflammatory drugs (NSAIDs) often present with urticaria, angioedema or bronchospasm. The mechanism of reactions due to NSAIDs is usually due to inhibition of Cyclooxygenase-1 and not a classical Gell and Coombs hypersensitivity reaction. Nevertheless, they should be classified as a drug allergy just as the term anaphylaxis now encompasses a clinical syndrome rather than a pathological mechanism.
The original Gell and Coombs Type 4 T-cell mediated classification has been further subdivided by Pichler into four subtypes a-d representing the different cellular mechanisms of delayed-type hypersensitivity. An allergy specialist who investigates patients with drug allergy must become familiar with each mechanism of allergic reaction that can occur with each drug class and indeed with each individual drug.
The investigation of drug allergy relies heavily on a detailed clinical history which must include a description of the rash, the onset of symptoms and the number of doses of each drug taken before the reaction starts. Therefore, documentation of an acute drug-allergic reaction is key to accurate identification of the culprit. The NICE drug allergy guideline (CG183 2014) provides guidance on documentation and recommends that the patient receives written communication detailing the nature of their drug allergy and a list of drugs to avoid. It is essential that all primary and secondary care organisations incorporate a template onto their electronic patient record systems in order to fulfil the requirements for documentation and communication of drug allergy.
The website provides comprehensive guidance on drug allergy and we hope it will fulfil a much-needed resource which is frequently accessed by specialists managing and investigating patients with drug allergy. We greatly value feedback on how the website can be further improved.
NICE drug allergy guideline (2014), click here to view.
BSACI guidelines for the management of drug allergy (2008), click here to view.
International Consensus on Drug Allergy (2014), click here to view