Patch testing is an investigation that allows the dermatologist to determine what is causing conditions like urticaria or pruritus. The results allow the dermatologist to confirm what is causing conditions like contact dermatitis.
Patch testing is conducted by analysing the skin’s reaction to substances commonly known to be allergenic (cause reactions), as well as other substances the individual is regularly exposed to. Some common allergens include;
These substances will generate a ‘positive’ or ‘negative’ result, meaning that they either caused a reaction (typically redness and itching), or that they did not. It is possible to have an allergy to one type of a substance (like a metal) and not another, so the results will allow patients to know exactly which of the tested substances to avoid.
Sometimes a patch test may also need to be exposed to ultraviolet (UV) light, if a sensitivity to sunlight is suspected. This is called photopatch testing.
The results are monitored and interpreted by the dermatologist, who will use them either to confirm a suspected allergen; or to make a diagnosis in the event that the skin reaction could be the result of multiple conditions.
Depending on the skin condition, it may be necessary to get further diagnostic information, so it’s possible to require additional tests alongside patch testing.
Patients who are pregnant or breastfeeding; have moderate to severe eczema in the area to be tested; have had recent sun exposure or have used artificial tanning creams may not be suitable for patch testing until a later date.
Patients taking steroid tablets or immunosuppressive drugs wanting to undergo a patch test should first consult with the dermatologist, however, should be advised that they may not be medically suitable to undergo patch testing.
What is the Benefit of Patch Testing?
While many allergens may be detected through other investigations such as blood tests, they can only determine allergens that are airborne, inhaled, or ingested.
Only a patch test can indicate and confirm an allergic reaction in the skin, and if it is not conducted, the allergy cannot properly be treated.
Who is Patch Testing Suitable For?
Patch testing is suitable for:
What is the Process of Patch Testing?
If you are undergoing patch testing, you will need to attend the clinic for three consecutive appointments in a week so that the test can be conducted, and the results monitored by your dermatologist.
Prior to your first appointment, which may take up to 1 hour, you will be asked to bring specific items with you, such as personal toiletries, medications, or certain substances you come into contact with at work. This will allow the dermatologist to either confirm, or rule out suspected triggers.
Each substance to be tested will be applied to the skin on your back, arms or thighs, using small discs which are kept in place by hypoallergenic tape. The dermatologist will also make markings in ink to designate particular substances. Itchiness or redness within the test areas is normal, and part of the test, but you will be strongly advised against scratching. The test is not painful.
Once the discs are in place, you can leave the clinic, and more or less carry on with normal activities. You will be advised about any activities to avoid – such as swimming, or doing anything that will cause you to sweat, which would cause the patches to move and render the test ineffective.
The discs and the substances remain taped in place until your next visit, when the taping is removed, the skin examined and any reactions to the skin are noted. Sometimes, the dermatologist will add additional patches on the second visit, and consider ski n prick testing if it is appropriate. They will remain on until the final appointment.
On your third visit, the dermatologist or dermatology nurse will remove the disks and any remaining substances from your skin, and then the dermatologist will examine the reactions and the notes before discussing the findings with you.
You’ll be able to ask the dermatologist questions about any specific concerns you might have and any treatment that might be proposed for your skin condition.
If your patch tests come back negative, it is still helpful, as the dermatologist will be able to rule out contact allergy as the cause of your skin condition.
Although most allergies are obvious by your final reading, a reaction can occasionally take up to 2 weeks, so if you find your skin does have a belated reaction, contact the clinic to let us know.
What are the Risks and Complications of Patch Testing?
There are minimal risks associated with patch testing, and it is expected to see the skin reddening and becoming itchy as part of a positive test result.
Some patients who have a positive reaction may get what’s called a persistent reaction, meaning that the area stays irritated or itchy for up to a month. Patients who have eczema may sometimes experience a flare-up of their condition following a patch test.
It’s also possible to see a change in the pigmentation of the skin in the area that has been tested, and this could last for several months. In rare cases, this could be permanent.
Infection is another rare complication, and scarring from a patch test is very rare, affecting just 1 in 10,000 individuals.
In a small number of patients, an allergy to one of the substances applied in the patch test can be developed, but this should not cause significant long-term problems.
What is the Recovery Time and Outcome for Patch Testing?
Your dermatologist will be able to give you the results of the patch test at your final appointment, but you may need to attend further appointments for the treatment of your skin condition.
Patch tests don’t typically cause any significant side effects for people who are generally in good health.
There may be slight swelling and itchiness in the application sites in the days after a patch test, and some people with underlying skin conditions might have a flare-up, but usually, the reaction subsides within 24 hours.
The follow-up care instructions provided by the clinic will let you know when you can resume normal activities, and if there are any special considerations, your dermatologist will advise you. If you have any questions after the patch test, you can email our team, who will endeavour to reply as quickly as possible.
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