Drug Allergies
Description Classification Diagnosis ICD-10 code: T88.7 DESCRIPTION Drug allergy is an immune-mediated reaction to the drug. Reactions are idiosyncratic and, unlike side effects, cannot be predicted by physiological action of the pharmaceutical agent. Common drugs involved include penicillin, sulphonamides, non-steroidal anti-inflammatory drugs, anticonvulsants, and chemotherapeutic agents. CLASS
ICD-10 code: T88.7
DESCRIPTION
Drug allergy is an immune-mediated reaction to the drug. Reactions are idiosyncratic and, unlike side effects, cannot be predicted by physiological action of the pharmaceutical agent. Common drugs involved include penicillin, sulphonamides, non-steroidal anti-inflammatory drugs, anticonvulsants, and chemotherapeutic agents.
CLASSIFICATION
Drug hypersensitivity reactions are simply classified as:
- immediate (≤ 1 hour after exposure): anaphylaxis, urticaria, angioedema; or
- delayed (≥ 6 hours): often involving rash with or without systemic symptoms.
DIAGNOSIS
Drug allergies are diagnosed clinically, based on symptoms and signs, and their timing relative to drug exposure, as well as exclusion of other potential causes.
In the acute setting, laboratory tests help to confirm the diagnosis and to determine the extent of systemic involvement (e.g. eosinophil counts, liver or renal function tests).
Tryptase measurement
An elevated serum tryptase concentration can help to confirm the diagnosis of anaphylaxis in cases where this is in doubt, but normal measurements do not necessarily exclude it. Serial tryptase measurements are the most helpful, with sampling at 1 - 2 hours, 4 - 6 hours, and 24 hours after the start of the reaction.
No serum biomarkers are currently available to identify delayed hypersensitivity reactions.
Specific diagnostic testing to identify the causative drug
Do tests to confirm the causative drug only if the benefit to the patient outweighs the risk, and only in consultation with a specialist.
- Skin tests: Includes subcutaneous skin prick tests, intradermal tests, and patch tests:
- Should be performed in specialised units. Safety equipment is required as significant reactions can occur.
- Have variable sensitivity and specificity depending on the drug.
- Serum specific IgE against the suspected drug:
- Available for a few drugs only.
- The majority have low sensitivity but high specificity.
- Cellular antigen stimulation tests for selected drugs:
- Useful for non-IgE mediated reactions.
- Either measures basophil activation markers (via flow cytometry) or sulpholeukotrienes (via ELISA).
Drug provocation testing
- Is the gold standard to identify the causative drug and is often required for a number of drugs due to the limited diagnostic accuracy of in vitro and in vivo testing.
- Perform only in specialised units. Safety equipment must be available as they can provoke significant reactions.
Standard Treatment Guidelines and Essential Medicines List for South Africa, Hospital Level, Paediatrics, 2023 Edition
Chapter 25: Drug Allergies, 24.1 Drug Allergies