Approach Considerations
The most sensitive methods for detecting protein contact dermatitis (PCD) include prick and scratch tests, although open application testing may also be performed. Patch-test results are usually negative. If the prick testing is negative, some authors suggest performing a rub test by applying the allergen to the area affected by protein contact dermatitis. [24] If available, specific recombinant IgE testing to the allergens revealed by prick testing may be performed. This allows for specification of epitopes and evaluation of potential cross-reactants. [5]
Systemic symptoms, including anaphylaxis, have been reported with prick and scratch tests. Therefore, these tests should be performed only in settings where appropriate resuscitation equipment is available.
For prick testing, a commercial extract should be used for the testing when available. If none is available, a prick-by-prick method may be used, whereby the allergen is pricked and immediately afterward, the forearm is pricked. [19]
Go to Irritant Contact Dermatitis, Allergic Contact Dermatitis, and Pediatric Contact Dermatitis for complete information on these topics.
Fungal Testing
Testing should be performed to exclude a tinea infection. This may consist of potassium hydroxide (KOH) testing, fungal culture, or periodic acid-Schiff staining
Open Application Testing
Open application testing involves placing or rubbing the allergen on intact skin and/or damaged, eczematous skin.
Prick Testing
Prick testing involves placing one drop of diluted test allergen, vehicle (negative control), and histamine (positive control) onto the volar forearm of the patient. The test sites are pierced with lancets to introduce the allergen into the dermis. The results are read at 15-minute intervals over 1 hour. A positive reaction is a wheal of at least 3 mm in diameter that is also at least half the size of the histamine control in the absence of a reaction in the vehicle control.
Scratch Testing
Scratch testing involves placing one drop of diluted test allergen, vehicle, and histamine onto the volar forearm and scratching the skin lightly with a needle. The test sites are read at 15-minute intervals over 1 hour. A positive reaction is a wheal that is at least half the diameter of the histamine control in the absence of a reaction in the vehicle control.
Radioallergosorbent Testing
A radioallergosorbent test (RAST) with a particular allergen may be performed to measure allergen-specific IgE in the patient's serum, although a negative test result does not rule out protein contact dermatitis. RAST testing measures only circulating antibodies and does not assess tissue-bound antibodies.
Imaging Studies
If the patient is experiencing extracutaneous symptoms, appropriate imaging studies may be performed. No specific imaging study is required in the evaluation of protein contact dermatitis.
Histologic Findings
Histology findings are relatively nonspecific. Biopsy may reveal perivascular lymphocytic infiltrate with eosinophils or spongiosis with lymphocytic exocytosis. One report of protein contact dermatitis describes a clinical presentation of a fixed pigmented eruption, and the biopsy revealed a dense polynuclear neutrophilic infiltrate. [19]
