Drug allergy laboratory findings
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Charmaine Patel, M.D. [2]
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Overview
Drug allergy and its associated conditions is primarily a clinical diagnosis based on the patient history, and through physical exam. Certain laboratory findings may be seen during the acute phase of the reaction, but are not always specific. Skin testing and biopsies can be performed when there is not a clear diagnosis.
Laboratory Findings
- Erythrocyte sedimentation rate (ESR) may be increased.
- White blood cell (WBC) may be increased.
- Urine eosinophils may be increased, especially in cases of allergic interstitial nephritis.
- Blood eosinophils may be increased, especially in cases of drug induced TEN.
- Liver function tests (LFT)'s may be increased.
- Elevations in tryptase may be seen detected in serum or plasma within several hours after an acute allergic event, and is consistent with anaphylaxis.
- Histamine levels may be elevated after an acute reaction, but is unreliable for diagnosis.
Other Tests
- Skin testing- skin prick testing (SPT) pricks the skin with a tiny amount of the suspected allergen, and leads to the diagnosis of IgE mediated type I hypersensitivity reactions. These tests are standardized for penicillin, and are also useful for local anesthetics, muscle relaxants. These tests are also very sensitive for high-molecular-weight protein substances such as insulin and monoclonal antibodies. A negative test is useful for ruling out a penicillin allergy, however with other tests (except for with high-molecular-weight proteins), a negative test is not always useful for ruling out the presence of serum specific IgE
- In-vitro tests for immediate drug reactions are available, but are largely considered investigational.
- Patch testing to test for a type IV reaction where drugs are mixed into petrolatum and applied to the skin for 48 hours. This test is useful in evaluating patients with maculopapular exanthema, acute generalized exanthematous pustulosis, and flexular exanthema. It is not to be used in patients with a history of Stevens-johnson syndrome or toxic epidermal necrolysis.
- Intradermal testing with delayed readout is more sensitive than a patch test, and involves injection of a small amount of the allergen dissolved in water, under the skin. A prick test should be done beforehand, and the concentration used should be non-irritating.
- Skin biopsy may be useful to distinguish between Stevens-johnson syndrome and toxic epidermal necrolysis, and also to rule out other conditions on the differential diagnosis list.