Patch Testing

What is patch testing?

Patch testing is a specialist skin test that can reveal whether a rash is being triggered or worsened by an allergy to a substance touching the skin. This type of allergy is called contact allergy, and the rash it causes is known as allergic contact dermatitis. Dr Mounsey offers patch testing to help identify the cause.

Small amounts of common allergens are placed on the skin of your back and left in place for two days. Any area that reacts over the following days points to a substance you are allergic to. Patch testing looks for delayed skin allergies only, so it is different from skin prick or blood tests, and it cannot detect food allergies or hay fever.

What is it used for?

Patch testing is helpful when contact allergy is suspected as the cause of a rash. Common examples include eczema of the hands, face, eyelids, lips, feet, genital or anal skin; a rash that keeps returning in the same place; reactions to hair dye, cosmetics, fragrance, jewellery or other metals; and eczema that has not settled as expected with treatment.

It is also used when a rash may be linked to work, for example in hairdressers, healthcare and beauty workers, builders, cleaners and anyone who handles chemicals, rubber gloves or glues.

A standard set of the most common allergens, known as a baseline series, is usually tested. Further allergens are added depending on your job, hobbies, the products you use and where on the body the rash appears. Some of your own products may also be tested, after being diluted where needed.

Before your procedure

Testing involves three short visits over one week, and you will need to attend all three. Please plan your week around this, as the patches must stay dry and in place throughout.

Please bring details of the medicines you take, along with all the creams and ointments you apply. Bring the products you use on your skin, hair and nails, such as soaps, moisturisers, cosmetics, perfumes, hair products and sunscreens, in their original packaging so the ingredients can be checked. For household products, the packaging alone is enough. If you think something at work is involved, ask your employer for the safety data sheets (sometimes called COSHH sheets) and bring these too.

Please let us know beforehand if you are pregnant or breastfeeding, if you take steroid tablets or any medicine that dampens the immune system, if you have recently had sun exposure or used a sunbed on your back, or if the skin on your back currently has eczema. Testing may need to be postponed in these situations. Stop using steroid creams or ointments on your back three or more days before the first visit, and keep your back free of any cream on the morning of the appointment. Antihistamine tablets can be continued as normal.

If you have private medical insurance, please check with your insurer before booking whether patch testing is covered and whether any excess applies.

What happens on the day?

First visit: the allergens are applied to your back in small chambers mounted on strips of hypoallergenic tape, and the position of each strip is marked on your skin with a pen. Occasionally the arms or thighs are used. This visit takes longer than the others, so please allow extra time.

Second visit, two days later: the strips are removed and the skin is checked for early reactions. The marks are refreshed and must stay on your skin until the final visit.

Third visit, usually two days after the second: the skin is checked again, as many reactions take several days to appear. Any positive results are explained, and you will be told which substances to avoid and where they are commonly found.

What to expect afterwards

Mild itching under the patches is common and normal, but try not to scratch. A positive reaction appears as a red or pink, itchy and sometimes raised patch at the site of the allergen, and usually fades over a few days. A strong reaction can occasionally form a small blister.

Some allergens are coloured, such as hair dye and textile dyes, and may leave a temporary stain on the skin that is not a reaction. Occasionally a reaction appears after the last visit. If this happens, take a photograph and contact the clinic.

Aftercare

Keep your back dry throughout the week of testing. Do not shower, bathe or swim, as water can loosen the strips and wash off the markings; a shallow bath or a strip wash that keeps your back dry is fine.

Avoid exercise, sport and heavy physical work that makes you sweat, as this can cause the strips to come away. Keep your back out of the sun and away from sunbeds.

Wear old, dark clothing during the week, as the ink and some allergens can stain fabric permanently, and wear a T-shirt or vest in bed to protect the strips. Clothes that open at the front are easier to take on and off.

If the edge of a strip starts to lift, tape it down with the tape provided. If a whole strip comes off, note the date and time, take a photograph of your back, and let the clinic know.

What are the risks?

Patch testing is safe, and side effects are uncommon. Itching is expected, and positive reactions may take a few weeks to fade completely. Your usual eczema may flare for a short time, particularly if you have a strong reaction.

Lighter or darker marks can be left where the patches were applied, and these can take several months to fade. Testing works best when your skin is calm. When eczema is flaring, the back can become over-sensitive and react to several patches at once, making the results difficult to read.

Rarely, a person becomes newly allergic to one of the substances tested, estimated at about 1 in 5,000. Scarring is very rare, at around 1 in 10,000. Occasionally, people react to the tape itself.

Results and follow-up

Your results are discussed with you at the final visit. A positive result is only useful if it fits with your rash, so each reaction is considered alongside your history to decide whether it explains your symptoms now, explains a past problem, or is unlikely to be relevant.

You will be given written information about any allergen you react to, including the names it may appear under on ingredient labels and the products it is commonly found in. Once a relevant allergen is avoided, the skin frequently improves a great deal, though full settling may take a number of weeks.

A negative result is also helpful, as it makes contact allergy an unlikely cause and points towards other treatments. A letter summarising the results can be sent to your GP.

More information

British Association of Dermatologists patient leaflet: https://www.bad.org.uk/pils/patch-testing

This page gives general information only and is not a substitute for a consultation. Everyone's skin is different, and what is right for you will depend on your own circumstances. If you are worried about a skin problem, please book an appointment with Dr Mounsey or speak to your GP. If you have a mole or skin lesion that is changing, bleeding or growing quickly, please seek medical advice promptly.