Dandelion Patch Testing
Find out what may be causing your skin to react.
Patch testing helps identify allergic contact dermatitis, a delayed allergic reaction caused by substances that come into contact with your skin.
Common triggers can include fragrances, metals, preservatives, cosmetics, personal care products, rubber chemicals, adhesives and topical medications.
What is patch testing?
Patch testing is different from regular allergy skin prick testing.
Skin prick testing looks for immediate allergic reactions that usually happen within minutes. Patch testing looks for delayed reactions that can take hours or several days to appear. Small amounts of potential allergens are placed on your back and left in contact with the skin for about 48 hours.
This can help your allergist determine whether something you regularly come into contact with is contributing to eczema, itching or recurring rashes.
What can patch testing look for?
Depending on the testing panel being used, allergens may include:
- Fragrances
- Nickel and other metals
- Preservatives
- Cosmetics and skin-care ingredients
- Hair products
- Rubber chemicals
- Adhesives
- Topical medications
- Ingredients found in soaps and cleaners
- Workplace or occupational exposures
Your allergist will determine which allergens should be included based on your symptoms and history.
How does patch testing work?
Patches applied
Allergens placed in chambers and secured to your back.
Patches removed
About 48 hours later, your skin is checked for early reactions.
Doctor reads results
Delayed reactions are examined and interpreted with you.
Monday: patches are applied
Small amounts of different allergens are placed into special testing chambers and secured to your back.
The locations are marked so we know exactly where each substance was placed. You then go home with the patches in place.
Wednesday: patches are removed
Approximately 48 hours later, you return to the clinic. The patches are removed and your skin is checked for early reactions.
The testing areas may be marked again so they can still be identified at your final appointment.
Friday: final reading with the doctor
Your doctor examines the testing areas for delayed reactions and interprets the results.
This final appointment is important because some allergic reactions do not become visible until several days after the allergen was first applied. A Monday, Wednesday and Friday schedule is a commonly used patch-testing schedule.
What does a positive result look like?
If you are allergic to one of the substances, the skin where that allergen was placed may become:
- Red
- Raised
- Itchy
- Irritated
- Occasionally blistered
Your doctor looks at the type and strength of the reaction when interpreting the test.
A positive patch test does not automatically mean that substance is responsible for all of your skin symptoms. Your doctor will compare the result with your history and the products or materials you are exposed to.
What happens if I test positive?
Knowing the allergen is only the first step. Your doctor can help you understand:
- What substance caused the reaction
- Where it is commonly found
- Other names it may appear under
- Which products may contain it
- What you should try to avoid
- Whether it may be related to your current skin condition
Avoiding the identified allergen can significantly reduce symptoms in some patients.
What should I do during patch testing?
It is very important to keep the testing area intact during the week. While the patches are on, you should generally:
- Keep your back dry
- Not shower or swim if it will wet the patches
- Avoid heavy exercise
- Avoid activities that cause significant sweating
- Not remove or loosen the patches
- Avoid scratching the testing area
- Avoid excessive heat
- Avoid sunlight on the testing area
After the patches are removed on Wednesday, you may still need to avoid rubbing, scratching, applying creams or exposing the testing area to sunlight until after the Friday reading.
Getting the patches wet, loosening them or excessive sweating can interfere with the accuracy of the test.
How should I prepare?
Before your appointment:
- Tell us about all medications you are taking
- Tell us if you are using steroid creams or ointments on your back
- Tell us if you take prednisone or other medications that suppress the immune system
- Avoid applying creams or moisturizers to the testing area on the day of testing
- Avoid tanning or significant sun exposure to your back before testing
- Wear comfortable, loose clothing
Unlike regular allergy skin testing, antihistamines usually do not interfere with patch testing, because patch testing measures a different type of immune reaction.
About your medications
Do not stop any prescription medication unless instructed by our clinic.
Patch testing vs. skin prick testing
| Patch Testing | Skin Prick Testing | |
|---|---|---|
| Looks for | Delayed contact allergies | Immediate allergies |
| Common uses | Dermatitis, eczema, skin reactions | Environmental, food, venom and some drug allergies |
| Where performed | Usually the back | Usually forearm or back |
| Needles or pricks | No | Small superficial prick |
| How long for results | Several days | Usually 15 to 20 minutes |
| Number of visits | Usually 3 | Usually 1 |
Curious about the other kind of testing? See allergy skin testing.
Frequently asked questions
Does patch testing hurt?
There are no needles involved, and most patients do not find the application painful.
If you react to one of the allergens, the area may become itchy or irritated while the test is being performed. Try not to scratch the area, as this can affect the results.
Can I shower?
While the patches are attached, your back needs to stay dry. You may need to wash using a cloth or carefully shower while keeping the testing area completely dry.
After the patches are removed, follow the instructions provided by the clinic until your final reading.
Why can't the result be read on Wednesday?
Contact allergies are delayed reactions. Some reactions may be visible when the patches are removed after approximately 48 hours, while others take longer to develop.
That is why the Friday appointment is important.
Why does patch testing take an entire week?
Because allergic contact dermatitis develops slowly. We need to see how your skin changes over several days.
Can I go to work while wearing the patches?
Usually yes, as long as your work does not involve heavy sweating, getting your back wet or activities that may loosen the patches.
What if a patch starts coming off?
Contact the clinic for instructions rather than removing or replacing it yourself.
Can I exercise?
It is best to avoid strenuous exercise and excessive sweating during the testing period because this can loosen the patches and affect the results.
Can patch testing diagnose food allergies?
No. Patch testing is primarily used to investigate allergic contact dermatitis.
Food allergy is assessed using your medical history and, when appropriate, skin prick testing, blood testing and oral food challenges.
What if all my results are negative?
A negative test does not mean your skin symptoms are not real.
It means that we did not identify a contact allergy to the substances that were tested. Your doctor can then discuss other possible causes and treatment options.
Ready to find out what may be causing your rash?
Patch testing can help identify substances you may be coming into contact with every day without realizing they are contributing to your skin symptoms. The process requires three short visits over one week: patches applied Monday, removed Wednesday, and read by the doctor on Friday.
Assessments are by referral, and patients cannot book directly. Ask your family doctor, walk-in clinic, or another healthcare provider to fax a referral to 416 850 9993. Our team reviews it and contacts you to arrange the appointment.
If you are unsure whether this service applies to you, you are welcome to ask us a question first.