What is a patch test?
A patch test is a test designed to find out the cause of an allergy. It is used when an allergic contact dermatitis is suspected.
What is an allergic contact dermatitis?
When a substance comes into contact with the skin, it can either be ignored by the skin or the skin could react to it and become red, scaly and itchy. The latter reaction is called an allergic contact dermatitis. Such an allergic reaction is dependent on the immune system. Without immune cells arriving at the scene to 'attack" the perceived offending substance, an allergic reaction cannot occur. Examples of allergic contact dermatitis are reactions to nickel (jewellery, watches, paper clips etc) and fragrance (cosmetics, deodorants, soaps, washing powders, fabric softeners etc).
Not all contact dermatitis are allergic in nature. Some contact dermatitis may be irritant in nature. In this variety of contact dermatitis, the immune system does not get involved. An example is contact with petrol. It may irritate the skin and cause a contact dermatitis. This type of reaction is termed an irritant contact dermatitis.
When should a patch test be performed?
A patch test should be performed in:
ny eczema has not cleared in a reasonable time (6 weeks)
all cases of hand dermatitis
all cases of facial dermatitis
eczema that is localised to one or few areas of the body
eczema of the feet (when shoe contact dermatitis is suspected)
Hair dye allergy - paraphylamine diamine
Deodorant allergy - Fragrance
Shoe contact dermatitis - leather dye - potassium dichromate