Patch Test (medicine) - Interpretation of The Results

Interpretation of The Results

The dermatologist or allergist 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:

• 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.

Relevance is determined by exposure to the positive allergen(s) and is rated as definite, probable, possible, past or unknown. For an allergen to have definite relevance, the product the patient is exposed to must be tested and also be positive in addition to the test allergen. Probable would be used to describe a positive allergen ingredient which is in a product the patient uses (i.e. quaternium-15 listed in a moisturizing cream used on the sites of dermatitis). The interpretation of the results requires considerable experience and training. A positive patch test(s), might not explain the present skin problem since the test only indicates that the individual became allergic during the encounters with that chemical(s) at some point in their life. Relevance, therefore, has to be established by determining the casual relationship between the positive test(s) and the eczema. The confirmation of relevance will occur after the patient has avoided exposure to the chemical(s) and after they have noticed that the improvement or clearance of your dermatitis is directly related to this avoidance. This outcome usually occurs within four to six weeks after stopping the exposure to the chemical(s).

If all patch tests are negative, the eczema is probably not due to an allergic reaction to a contactant. It is possible, however, that you were not tested to other chemical(s) that can produce allergic reactions on the rare occasions. If the suspicion is high in spite of negative patch testing, further investigation might be required. This can be discussed during the final evaluation of the patch test procedure.