Patch testing is done for –

Patch test is the only effective means of confirming contact dermatitis. Now the question arises for which type of contact dermatitis (Irritant or allergic) the test is done. You know that allergic contact dermatitis is type IV hypersensitivity against particular antigen, where as irritant contact dermatitis is due to irritant action of the chemical. In patch test reading is taken after 48-72 hours to see the signs of delayed hypersensitivity, i.e. induration. So, patch test relies on the principal of type IV (delayed type) hypersensitivity and type IV hypersensitivity is the pathogenic mechanism in allergic contact dermatitis.