A patch test is a method used to determine whether a specific substance causes allergic inflammation of a patient’s skin. Any individual suspected of having allergic contact dermatitis or atopic dermatitis needs patch testing.
Patch testing helps identify which substances may be causing a delayed-type allergic reaction in a patient, and may identify allergens not identified by blood testing or skin prick testing. It is intended to produce a local allergic reaction on a small area of the patient’s back, where the diluted chemicals were planted. The chemicals included in the patch test kit are the offenders in approximately 85–90 percent of contact allergic eczema, and include chemicals present in metals (e.g., nickel), rubber, leather, formaldehyde, lanolin, fragrance, toiletries, hair dyes, medicine, pharmaceutical items, food, drink, preservative, and other additives. It can detect delayed allergic reactions, which can take several days to develop.
Interpretation of results
The dermatologist will complete a record form at the second and third appointments (usually 48 and 72/96 hour readings). The result for each test site is recorded. One system used is as follows:
- Weak positive
- Strong positive
- Negative (-)
- Irritant reaction (IR)
- Equivocal / uncertain (+/-)
- Weak positive (+)
- Strong positive (++)
- Extreme reaction (+++)
Irritant reactions include miliaria (sweat rash), follicular pustules, and burn-like reactions. Uncertain reactions refer to a pink area under the test chamber. Weak positives are slightly elevated pink or red plaques, usually with mild vesiculation. Strong positives are ‘papulovesicles’ and extreme reactions have spreading redness, severe itching, and blisters or ulcers.
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