A patch test is commonly used to determine which allergen is causing a skin reaction. It’s considered the “gold standard” for determining allergy contact dermatitis.
For a patch test, a doctor applies tiny amounts of different allergens directly to the patient’s back and covers them with a dressing that keeps the air out, or uses small wells and aluminum patches that are taped to the back.
In both methods, the patches are removed after two days and then reapplied and removed again after a few more days. A reaction—such as skin redness or swelling—will indicate an allergen. The patch test works best on metal and chemical allergens but may still take repeat applications and several weeks for a usable result.
Bringing suspected materials to your doctor’s appointment can speed up the process.
Skin Lesion Biopsy
Repeated, severe cases of contact dermatitis might prompt your doctor to perform a skin biopsy. This includes taking a sample of skin and sending it to a lab for testing.
Should you have lesions, the doctor may take a small sample from the sore, or from another area of skin. The sample is usually collected in one of three ways:
The least invasive, a doctor removes skin from the outermost layer. No stitches are needed.
A doctor takes a pencil eraser-sized sample of skin using a sharp, hollow instrument. Stitches may be needed, depending on the size of the sample.
During this procedure, a doctor surgically removes the entire lesion and closes the wound with stitches. For large sections, a skin graft may be needed.