Why should skin-prick testing with Soluprick Positive Control (histamine dihydrochloride) be performed only on healthy skin without lesions?

Soluprick Positive Control (histamine dihydrochloride) requires healthy skin without lesions because disorders such as dermographism, atopic dermatitis or eczema can affect interpretation of the result; where applicable, the back may be used or testing postponed until the disorder is stabilised.