Fixed Drug Eruptions Workup

Updated: Oct 09, 2020
  • Author: David F Butler, MD; Chief Editor: Dirk M Elston, MD  more...
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Laboratory Studies

Blood studies are not useful for the diagnosis of fixed drug eruption (FDE), although eosinophilia is common with drug eruptions.


Other Tests

Patch testing and oral provocation have been used to identify the suspected agent and check for cross-sensitivities to medications. [58, 59, 60, 61] A refractory period has been reported in fixed drug eruption; therefore, a delay before and between patch testing and oral provocation is recommended. One study used an 8-week time window after lesion resolution and between tests, which yielded positive results. [62] Patch testing must be performed on a previously involved site; otherwise, a false-negative result is likely. [59] Some locations may be inappropriate for patch testing; thus, clinical discretion is advised. Once patch testing is complete, oral provocation should follow, with the least likely culprits and the negative patch test agents first, followed by more likely causes. Oral provocation is thought to be the only reliable way to diagnose fixed drug eruption.

Patch testing is particularly efficacious in identifying a putative cause of the reaction when nonsteroidal anti-inflammatory agents are suspected, but patch testing is not helpful in discerning reactions to antibiotics and allopurinol. [63]



Skin biopsy is the diagnostic procedure of choice.


Histologic Findings

Histological examination of inflammatory/acute lesions shows an interface dermatitis with vacuolar change and Civatte bodies [13] (see the image below).

Acute interface dermatitis with prominent vacuolar Acute interface dermatitis with prominent vacuolar change and individual necrotic keratinocytes within the epidermis (X10).

The overall pattern may mimic that seen in erythema multiforme. Dyskeratosis and individual necrotic keratinocytes within the epidermis may be a prominent feature (see the image below).

Interface dermatitis, vacuolar change, necrotic ke Interface dermatitis, vacuolar change, necrotic keratinocytes, and incontinent pigment in the dermis (X40).

On occasion, the lymphocytic infiltrate can be prominent enough to obscure the dermoepidermal junction. Spongiosis, dermal edema, eosinophils, and occasional neutrophils may be present. Pigmentary incontinence within the papillary dermis is a characteristic feature and may be the only feature seen in older, noninflamed lesions. Chronic or inactive lesions may also show mild acanthosis, hyperkeratosis, and relatively few inflammatory cells.