CDT Allergy Testing & Patch Testing 1 — Questions and Answers
Question 1: What is the primary purpose of patch testing in dermatology?
- To diagnose psoriasis
- To identify contact allergens causing allergic contact dermatitis (Correct answer)
- To test for systemic drug allergies
- To screen for autoimmune skin disorders
Correct answer: To identify contact allergens causing allergic contact dermatitis
Patch testing is the gold standard procedure for identifying specific contact allergens responsible for allergic contact dermatitis.
Question 2: Where are patch test panels typically applied on the body?
- Inner forearm
- Abdomen
- Upper back (Correct answer)
- Behind the ear
Correct answer: Upper back
Patch tests are applied to the upper back because it provides a large, flat surface area with relatively uniform skin for multiple allergen applications.
Question 3: How long should standard patch tests remain in place before removal and the first reading?
- 24 hours
- 48 hours (Correct answer)
- 72 hours
- 96 hours
Correct answer: 48 hours
Standard patch tests are left in place for 48 hours before removal and initial reading per international patch testing guidelines.
Question 4: Which of the following is a contraindication for patch testing?
- History of atopic dermatitis
- Active widespread dermatitis on the upper back (Correct answer)
- Previous positive patch test result
- Patient age over 60 years
Correct answer: Active widespread dermatitis on the upper back
Active widespread dermatitis on the upper back prevents proper placement of test chambers and accurate interpretation of results.
Question 5: Which standardized patch test system is most commonly used and FDA-approved in the United States?
- European Baseline Series
- TRUE Test (Thin-layer Rapid Use Epicutaneous Test) (Correct answer)
- RAST panel
- Prick test panel
Correct answer: TRUE Test (Thin-layer Rapid Use Epicutaneous Test)
The TRUE Test is the FDA-approved, ready-to-use standardized patch test system most widely used in US dermatology practices.
Question 6: What instruction must be given to patients during the patch testing period?
- Apply moisturizer to the test site daily
- Avoid getting the patch test area wet (Correct answer)
- Exercise vigorously to confirm adhesion
- Remove patches if mild itching occurs
Correct answer: Avoid getting the patch test area wet
Patients must keep the patch test area dry throughout the testing period to prevent allergen chambers from loosening or becoming contaminated.
Question 7: A true positive (allergic) reaction on patch testing is best described as:
- Erythema only at 48 hours that clears by 96 hours
- Vesicles, papules, and erythema that persist or worsen at 72–96 hours (Correct answer)
- Any skin color change visible at 48 hours
- Scaling with mild redness only at 48 hours
Correct answer: Vesicles, papules, and erythema that persist or worsen at 72–96 hours
A true allergic reaction shows vesicles, papules, and erythema that persist or intensify at the later 72–96 hour reading, distinguishing it from an irritant response.
CDT Allergy Testing & Patch Testing 1
What is the primary purpose of patch testing in dermatology?