Patch testing finds out whether a rash is caused by an allergy to something touching your skin, known as contact allergy (allergic contact dermatitis). It is not used for hives (urticaria) or food allergy, which are investigated with skin prick testing or allergy blood tests.
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:
- Certain types of chemicals or preservatives;
- Substances commonly used in hair or skin products;
- Perfumes and cosmetics;
- Metals;
- Natural rubber latex
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, but should be advised that they may not be medically suitable to undergo patch testing.
Patch Testing
Before patch testing, avoid steroid creams on your back for at least a week, and avoid sunbathing or sunbeds on your back for a few weeks, as both can stop reactions showing. If you take steroid tablets or other medicines that dampen the immune system, tell your dermatologist, as they can affect the results.
You will need to keep your back dry between visits, so plan to avoid swimming and heavy exercise during the test week.
What are the benefits 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:
- Patients who are experiencing the symptoms of contact allergy or other dermatitis, and who need to identify what is triggering the condition; and
- Patients whose skin condition looks like an allergic reaction, which requires ruling out to confirm a different diagnosis
What is the process of patch testing?
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 skin 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 discs 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?
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.
Further reading: Patient leaflet from the British Association of Dermatologists
Related Specialists at Derma
Your consultation
Your consultation is with a consultant dermatologist, who will plan your patch testing.
Dr Ling Li
Consultant Dermatologist
General dermatologySkin surgery
Specialist interest in patch testing and cutaneous allergy.
Dr Catriona Wootton
Consultant Dermatologist
Skin surgery
Specialist interest in contact dermatitis and patch testing.
Your treatment
Our nursing team applies and removes your patches.
Diane Murphy
Nurse Team Leader
Leads our nursing team, making sure every patient receives safe, high-quality care.
Sarah Peett
Senior Dermatology Nurse
Specialist dermatology nurse with a background in surgical, medical and midwifery care.
Aleesa Dela Vega
Dermatology Nurse
Dermatology nurse with experience in respiratory and intensive care, known for her caring approach.
Roberta Cattolico
Dermatology Nurse
Works alongside our consultants in clinic, with a focus on patch testing and phototherapy.
Sandra Marcikic
Healthcare Assistant
Supports our patients and clinicians in clinic, drawing on her medical background.