Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.
A 32-year-old woman is evaluated for a recurrent pruritic rash on her hands. She works as a dental hygienist and notices that it improves when she goes on vacation. Topical corticosteroids alleviate the rash, but it quickly recurs.
On physical examination, she has scaling, erythematous patches on her dorsal hands, fingers, and wrists. She has scaling, peeling, and scabbed erosions on several fingertips.
Which of the following tests should be done next?
A: Patch testing
B: Potassium hydroxide (KOH) preparation
C: Prick testing
D: Radioallergosorbent testing
MKSAP Answer and Critique
The correct answer is A: Patch testing.
Patch testing is done to determine if a patient’s eczema is from a contact allergy. Allergic contact dermatitis (ACD) is a type IV or delayed hypersensitivity reaction caused by allergen-specific T-lymphocytes. The first reaction to an antigen may occur several weeks after exposure, but subsequent reactions usually develop within 24 to 48 hours of reexposure. Reactions often become more intense with repeated exposure. ACD is usually intensely itchy. In acute reactions, the skin is red, edematous, weepy, and crusted, and there may be vesicles or bullae. Chronic exposure results in a chronic eczema. Pruritus is prominent. The hands are a common site for allergic contact dermatitis, especially in health care workers. Exposure history and pattern of the rash may provide clues as to the causal allergen. Epicutaneous patch testing is the gold standard for diagnosis of ACD. Patch testing should be performed to evaluate recurrent or recalcitrant dermatitis. Patch testing is performed by applying a number of standardized chemicals on small discs to the patient’s back. It typically requires three visits within a 1-week period. Patients should diligently avoid any identified allergens. Topical corticosteroids are used for pruritus and inflammation. Systemic corticosteroids may be used for severe cases for 2 to 3 weeks but should not be a long-term therapy.
The potassium hydroxide (KOH) test is a common bedside diagnostic test performed on skin scrapings to detect fungi or yeast. An expanding, ringlike lesion with a slightly scaly, erythematous, advancing edge and central clearing suggests a fungal infection, findings that are not present in this patient.
Prick testing and radioallergosorbent testing (RAST) are useful in diagnosing immediate-type hypersensitivity reactions; however, these tests are less appropriate in the setting of ACD, which is a delayed-type hypersensitivity reaction.
- Epicutaneous patch testing is the gold standard for diagnosis of allergic contact dermatitis and should be performed to evaluate recurrent or recalcitrant dermatitis.
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