Contact dermatitis is a skin condition in which the skin reacts to an allergen or irritant by developing a rash. There is usually no test required for diagnosing the condition. It can be diagnosed by clinical observation and medical and social history. In some instances, tests can be helpful and necessary.
A patch test can be used in diagnosing allergic contact dermatitis. It helps determine which allergen is causing a skin reaction. It usually takes five to seven days for patch testing to be completed.
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 five to seven days. A reaction will indicate an allergen. Reactions can include a rash, bumps, or blisters. If the reaction disappears after the allergen is removed, the test was likely positive.
Bringing suspected materials to your doctor’s appointment can speed up the process. Ask your doctor if you should bring anything to your appointment.
A skin biopsy is usually not done to diagnose contact dermatitis. It’s used to rule out anything else your doctor might suspect, like a fungal infection. Repeated and severe cases of contact dermatitis might prompt your doctor to perform a skin biopsy. This involves taking a sample of skin and sending it to a lab for testing.
If you have lesions the doctor may take a small sample from the sore or another area of skin. The sample is usually collected in one of three ways.
This is the least invasive type of biopsy. It involves a doctor removing 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.
This is less common than the other types of biopsies. 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.
Most of the time, you will not need a test to diagnose contact dermatitis. Your doctor will be able to identify it from the history you give and an examination of the rash and surrounding skin. These tests are to make sure it’s not something other than contact dermatitis, or to identify a particular allergen. Talk with your doctor about whether testing is necessary and what would be the most appropriate choice.