Skin Patch Tests Allergies

Facts about the skin from DermNet New Zealand Trust. Topic index: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Dermatologists apply patch tests in patients with dermatitis, to find out whether their skin condition may be caused or aggravated by a contact allergy. Patch tests are not the same as skin prick tests, which are used to diagnose hay fever allergy house dust mite, grass pollens and cat dander. Skin prick tests have very limited value for patients with skin rashes.

The patch testing described here is as it is undertaken in Hamilton, New Zealand. There may be slight differences in methods used at other centres - if you are having patch tests done, ask your dermatologist to explain.

A range of substances can be used for patch testing. A baseline series such as the European Standard Series of allergens or similar is applied to nearly every patient, together with specific tests appropriate to the individual. Each substance known as an allergen has been tested to find the best concentration to demonstrate an allergic reaction without causing irritation to those who are not allergic to the material.

Sometimes the results can be inconclusive or misleading. Instead of one or two positive reactions, sometimes nearly all test areas become red and itchy. This is known as angry back and is most likely to occur in those with very active dermatitis false positive result. At other times, there may be little or no apparent reaction to a substance that regularly causes dermatitis in that person false negative result.

Further testing may be necessary. Patch tests do not always explain the cause of a dermatitis.

The first appointment will take about half an hour. Tiny quantities of 25 to 150 materials in individual square plastic or round aluminium chambers are applied to the upper back. They are kept in place with special hypoallergenic adhesive tape. The patches stay in place undisturbed for 48 hours.

At the second appointment, usually two days later, the patches will be removed. Sometimes further patches are applied. The back is marked with an indelible black felt tip pen or other suitable marker to identify the test sites.

These marks must still be visible at the third appointment, usually two days later 4 days after application. The back should be checked and if necessary remarked on several occasions between the 2nd and 3rd appointments.

The dermatologist will complete a record form at the second and third appointments usually 48 and 96 hour readings. The result for each test site is recorded. The system we use is as follows:

Irritant reactions include sweat rash, follicular pustules and burn-like reactions. Uncertain reactions refer to a pink area under the test chamber. Weak positives are slightly elevated pink or red plaques. Strong positives are papulovesicles and extreme reactions are blisters or ulcers. The relevance depends on the site and type of dermatitis and the specific allergen. The interpretation of the results requires considerable experience and training.

- reaction hair dye discolouration

Essentially negative patch test reactions

Do not expose your back to the sun for four weeks before your patch tests

Wear old clothing; felt tip pen marks can stain clothes

Do not swim, rub, or exercise, as the patches may come off

Keep the back dry, so no baths, showers or unnecessary sweating

Arrange for someone to remark the test sites with indelible felt tip marker

Bring Your Own Materials For Testing

Discuss the particular substances with which you come into contact with your dermatologist; you may be asked to bring materials from home or work.

Provide your dermatologist with data sheets of industrial items with which you are in contact.

Bring or send all chemical items for testing at least a week before the first appointment so that they can be prepared for testing if necessary.

Only small quantities are required eg. a few drops or grains.

Label items carefully with their common and chemical names - provide data sheets if available.

Identified food items and plants if relevant should be brought fresh to the first appointment; ice block trays are useful to separate items.

Bring a selection of cosmetics to be tested up to ten items including nail varnish, moisturiser, sunscreen and perfume. Shampoo and soap are not usually tested these are intended to be washed off and may irritate the skin if left on the skin for two days.

Bring all prescribed and non-prescribed ointments, creams and lotions that you have used.

Relevant clothing including rubber gloves and footwear can be tested; about one centimetre of material is needed, taken from seams or other unimportant areas in contact with the affected skin.

Some patients have photopatch tests because their dermatitis develops on skin exposed to the sun photosensitivity. Two sets of perfumes, antiseptics, plant materials and sunscreens may be applied. After removal, one set is exposed to a small dose of ultraviolet radiation UVA. This is not enough to cause a photosensitivity reaction on its own.

Adverse reactions to patch tests

Positive patch test results are small areas of active eczema / dermatitis. They will be itchy and may require treatment with topical steroid.

Occasionally patch test reactions persist for several weeks.

Patch tests may provoke other areas of dermatitis to recur or to appear for the first time.

Although hypoallergenic tape is used, occasionally people react to all areas in contact with the tape.

An angry back reaction may arise, particularly in a patient with active dermatitis at the time of testing, or in someone who has multiple positive reactions. Angry back refers to false positives to many or all of the tested allergens.

Rarely, sensitisation to a new allergen may occur as a result of the test – this is revealed as a reaction occurring around 10 days after the test was applied.

Re-testing may be required, sometimes one allergen at a time, to confirm or clarify a reaction.

Extension of dermatitis to surrounding skin

Adverse reactions to patch testing

Johansen JD, Aalto-Korte K, Agner T, Andersen KE, Bircher A, Bruze M, Cannavó A, Giménez-Arnau A, Gonçalo M, Goossens A, John SM, Lidén C, Lindberg M, Mahler V, Matura M, Rustemeyer T, Serup J, Spiewak R, Thyssen JP, Vigan M, White IR, Wilkinson M, Uter W. European Society of Contact Dermatitis guideline for diagnostic patch testing - recommendations on best practice. Contact Dermatitis. 2015 Jul 14. doi: 10.1111/cod.12432. Epub ahead of print PubMed PMID: 26179009.

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If you have any concerns with your skin or its treatment, see a dermatologist for advice.

Patch Testing

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A patch test is used to find allergic causes of eczema/dermatitis and some other types of skin reactions such as reactions to drugs.

Your doctor may refer you for a patch test:-

If you suspect an allergy as a cause of your eczema such as to cosmetic ingredients or hair dye

If you have eczema that is proving difficult to treat to ensure you are not allergic to ingredients of the treatment for your eczema

If you have a work related eczema/dermatitis

Every patient is tested to the common chemicals known to cause allergy such as nickel in metals, fragrances other cosmetic ingredients, rubber chemicals found in gloves etc together with additional allergens related to your specific problem e.g. work chemicals or extra ingredients of cosmetics or treatments. Each chemical is applied to a disc about 1cm in diameter and then taped to your back in strips of 10. Each patient may be tested for allergy to up to 100 different substances. You will also be asked to take a small sample of any material that you think is causing a reaction such as a cosmetic to be tested as well.

The hospital will need normal skin to apply the tests so any eczema on your back will need to be under control. Ideally, to avoid suppressing a reaction steroid creams should not have been used on your back for 3-4 weeks before the tests and you should not have had a lot of sun exposure. High doses of steroid tablets prednisolone more than 10mg may also interfere with the results.

Your skin is marked to identify the location of the tests and you will be asked to keep the skin dry. The tests shouldn t prevent you from going to work. The patches are left in place for 48 hours. After this time, the tests are removed and the skin is examined to see if any reactions red itchy areas have appeared. Additional tests may be applied to help confirm doubtful reactions and it may be necessary to shine light on some of the tests if a sun induced reaction eg to sunscreen is suspected. A final reading is taken after a further 2 days and the results discussed with you.

If an allergy is found and it is established that it is contributing to your eczema avoidance should result in a lasting improvement in your eczema. Even if the tests are negative this can be helpful in reassuring your doctor that we are not missing an allergic cause and that treatments given should be effective at controlling your skin disease.

Last updated: June 2013         Next review date: June 2015              Version 3.