Patch testing skincare: how it works and what it tells you

What can patch testing tell you?

Patch testing investigates whether something touching your skin is causing an allergic reaction. Contact dermatitis means skin inflammation after contact with a substance; the skin can become red, sore or swollen. A dermatology patch testing service is the appropriate route when contact allergy is suspected.13

Clinicians test a standard set of common allergens, meaning substances that can trigger allergies, and add others suggested by your history. These can include preservatives, perfumes, rubber, metals and plants. The aim is to identify what you need to avoid, rather than simply confirm that one cream upset your skin.2

How do you patch test skincare products?

For allergy diagnosis, arrange testing through dermatology rather than copying a hospital patch procedure at home. Clinical testing uses selected substances in small chambers, controlled contact times and repeat readings. A quick dab followed by a look in the mirror cannot replace that process. A home check also cannot tell you which ingredient caused a reaction.4

A typical clinical test follows this sequence.

1. 1At the first visit, discuss your symptoms and products with the clinician. Selected substances are placed in small chambers, taped to your back and marked so each test site can be identified.
2. 2The chambers usually stay in place for 48 hours. Follow the clinic's instructions about removal; some services remove them in clinic, while others give you directions to remove them yourself.
3. 3Attend the first reading, often on day 2 or 3. Keep the identifying marks and tape in place as instructed.
4. 4Attend the later reading. Some services assess again on day 7, and an additional reading is sometimes needed. The clinician then explains the results.

Where should you patch test skincare?

Clinical patches usually go on the back. Arms or thighs are occasionally used, and extensive eczema on the back can mean another site is needed. The clinician chooses the test location. The back is a clinical testing site, not an instruction to tape undiluted skincare products there yourself.2

Bring the products, not just their names

Bring suspected creams and soaps in their normal labelled containers so the clinician can identify their ingredients. The packaging has a useful job here, beyond making the bathroom look like a shop. Not every product you bring can be tested directly. Ask whether the clinic wants your regularly used cosmetics, toiletries and medical creams too; a product list is useful if you cannot bring samples.45

What should you do before the appointment?

Read your own clinic's preparation instructions before changing anything. Clinics give different directions for hair removal, creams on the back and sun exposure. For example, Cambridge University Hospitals asks for no creams or ointments on the back for 48 hours because the chambers may not stick well, and no sunbathing for four weeks. Do not substitute another hospital's timetable for yours.45

Tell the clinic about prescription creams, steroid tablets and medicines that suppress the immune system. Some treatment can delay testing or affect the response. Do not stop prescribed medicines on your own: Cambridge University Hospitals tells patients to continue oral steroids, azathioprine and ciclosporin unless the doctor advises otherwise. Ask specifically about any strong steroid cream you use and where you apply it.45

Tell the service if you are pregnant or breastfeeding before booking. Cambridge University Hospitals advises postponing clinical patch testing until after pregnancy and breastfeeding. This concerns the allergy investigation, not permission to use a particular skincare ingredient during pregnancy.5

How do you look after the patches?

• Keep the back dry for the period your clinic specifies, including after the chambers are removed if instructed.
• Avoid strenuous exercise, excessive bending and twisting, or other activity that could loosen the patches.
• Wear an old dark shirt or vest because the marking pen can stain clothing.
• Leave the identifying marks in place until the readings are complete.
• Contact the clinic if a patch comes loose or the test becomes difficult to tolerate.

Clothing that opens at the front can be easier to remove without shifting the patches, which gives the humble button an unusually specific skincare job. If an entire patch falls off, note the time and date and contact the dermatology department.2

When will you get an answer?

Do not interpret the first reading as the final result. Later readings help distinguish persistent allergic reactions from doubtful early findings and detect reactions that appear later. The appointment calendar is doing diagnostic work, rather than collecting return visits. Tell your clinic if a new reaction appears after the final appointment; some reactions take up to two weeks to develop.24

A negative result means no reaction was detected to the tested substances, but an allergy can occasionally exist despite a negative test. A positive result also needs to fit your symptoms and actual exposure. The clinician combines those details with the test findings to decide what you should avoid.3

What if the test causes a reaction?

Patch testing involves no injections, but the patches can feel strange or itchy. If itching becomes severe, removal may be necessary: note the chamber number and contact the department. Positive reactions usually settle over a few days, although some last longer. Contact the clinic with problems rather than waiting for the next scheduled reading.45

What if lip balm keeps causing a rash?

Lip cosmetics can cause allergic or irritant reactions. Persistent redness, scaling, dryness and itch can occur with allergic contact cheilitis, meaning inflammation of the lips caused by contact allergy. Testing can include the standard allergens, additional lip product ingredients and your own products; some people react only to their own cosmetics. Avoiding an identified allergen usually resolves the inflammation. If symptoms persist despite stopping the identified product, seek reassessment because more than one factor can contribute.6

What is the difference between a patch test and a skin test?

"Skin test" is a broad term: patch testing is one type. Skin prick testing checks immediate allergy, while patch testing checks delayed contact allergy. Choose the investigation with the clinician based on the reaction being investigated, not which test sounds more thorough.1