Health Professionals
What is patch testing?
Patch testing is a process used to diagnose allergic contact dermatitis (a Type 4 delayed type hypersensitivity reaction). Allergic contact dermatitis (ACD) occurs when a person becomes allergic to something that is coming into contact with their skin, causing a rash. During patch testing, small amounts of chemicals or products that are used at work or home are diluted and placed onto discs mounted on hypoallergenic tape and then placed on the back of the patient. These are left on the back for 2 days and then removed. The patch tests are “read” by a dermatologist at 2 days, and again at 4-7 days after the patch tests were originally applied. A positive patch test causes a small itchy red mark that is still present at the second patch test reading and is used in conjunction with the patient history and relevant exposures, to make a diagnosis of ACD.
When should patch testing be considered?
There are several reasons a person should be referred for patch testing.
- Persistent rash and not responding to treatment
- Eczema and starting biologics
Special note:
Testing for prosthetic joint components
Although ACD to a prosthetic joint is uncommon, there is a growing trend to patch test patients, particularly those with a history of metal allergy, prior to surgery to help guide the choice of implant used. This often arises because people describe a history of nickel allergy, however nickel is RARELY used in prosthetic joints. However, some patients are referred for patch testing AFTER implants, because of a poor outcome. In our experience, only those with a rash over the joint are likely to have a relevant allergy. There’s occasionally a handful of cases in the literature where people with an allergy have presented with swelling over the joint. Sometimes testing is undertaken for medicolegal reasons. It is important to assess the listed ingredients in the implant to evaluate their potential role in ACD.
Other sources of skin allergens should be considered such as adhesive skin glues used to close wounds (Dermabond), latex, topical medications, rubber and antiseptics.