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.
- Intradermal tests – these tests involve the injection of a small amount of allergen under the skin, into the dermis. Intradermal testing with delayed readout is more sensitive than a patch test, but carries a slightly higher risk of adverse allergic reactions when compared to skin testing. It tests for the same compounds as a skin prick test, and also leads to the diagnosis of IgE mediated hypersensitivity reactions. A prick test should be done beforehand, and the concentration used should be non-irritating.
- In-vitro tests for immediate drug reactions are available, but are largely considered investigational.
- Patch testing – this type of testing is useful for the diagnosis of various delayed type IV cutaneous reactions such as exanthemata, acute generalized exanthematous pustulosis, and flexular exanthema. It involves placing an aluminum disc containing allergens mixed with petrolatum on a patient’s back for 48 hours. The patient is then assessed for any reactions that may have occurred. This type of testing is not helpful to the diagnosis of Stevens-Johnson syndrome or toxic epidermal necrolysis and is contraindicated in anyone with a history of these conditions.
- 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.
- Histamine and tryptase levels- these test have shown to be useful in confirming the diagnosis of acute IgE mediated reactions, in particular anaphylaxis. Negative results do not rule out an anaphylactic reaction.
- Complete blood count – a complete blood count (CBC) is useful in ruling out hemolytic type II drug reactions, such as hemolytic anemia, thrombocytopenia or neutropenia.
- Coomb’s test- the indirect and direct Coomb’s test is used to determine the presence of antibodies on red cell membranes. This test can also be useful in confirming hemolytic anemia. [1]
References
- ↑ Schnyder B (2009). "Approach to the patient with drug allergy". Immunol Allergy Clin North Am. 29 (3): 405–18. doi:10.1016/j.iac.2009.04.005. PMID 19563988.