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Patch testing for skin allergies

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Patch testing for skin allergies

Written by Dr Virginia Hubbard, specialist registrar in dermatology and Dr Malcolm Rustin, consultant dermatologist

What is patch testing?

Patch testing is a way of identifying whether a substance that comes in contact with the skin is causing inflammation of the skin (contact dermatitis). There are two types of contact dermatitis: irritant contact dermatitis and allergic contact dermatitis .

Irritant contact dermatitis An irritant substance is one that would cause inflammation in almost every individual if it was applied in sufficiently high concentration for long enough. An irritant reaction is caused by the direct contact of an irritant substance with the skin and does not involve the immune system.

Allergic contact dermatitis

An allergic reaction is specific to the individual and to a substance (or a group of related substances) called an allergen. Allergy is a hypersensitivity (oversensitivity) to a particular substance, and always involves the immune system. All areas of skin that are in contact with the allergen develop the rash. The rash will disappear if you avoid contact with the substance.

Why is patch testing done?

If you have a dermatitis that started recently or if you have a persistent or unusual eczema, your dermatologist may suspect you have an allergic contact dermatitis. If you have been using a medication on the lower legs, hands, face, ears, eyes, anal or genital region for a while and subsequently developed dermatitis, your doctor may suspect that this is aggravating the dermatitis.

Patch testing is the only way that your doctor can prove that a substance is causing or aggravating your dermatitis. Once an allergen is identified, avoiding it should help cure your dermatitis.

How is patch testing done?

First your doctor will discuss your skin problem with you. Subjects discussed include:

the site where your rash began and how it developed.

the treatments you have tried.

previous skin disease.

the general health of your and your family, especially any tendency to get one or a combination of asthma, hay fever or eczema.

cosmetics and toiletries used.

your occupation - this will focus on materials used at work and the effect of weekends and holidays on your dermatitis (if it settles during these times, it is likely that you are in contact with an allergen at work). If other workers are affected with a similar rash then tell your doctor.

your hobbies.

If you can think of anything that you were in contact with around the time the rash first appeared then tell your doctor. Do not assume that just because you have been using something previously without a problem, it will not be the cause. Sometimes a cosmetic that you have been using for some time can become the cause of dermatitis.

Your doctor will then examine your skin. The dermatitis is usually most severe at the site of exposure, but can be widespread (for example, if a patient with an allergy to a substance in nail varnish touches her face, the dermatitis may spread).

Which allergens are tested?

Your dermatologist will suggest which allergens you should be tested for. The standard selection of allergens used is the European Standard Battery, which consists of the commonest allergens. Together these cause 85 per cent of all allergic skin reactions. In addition, the dermatologist may suggest additional patch tests using other allergens specific to your occupation or site of the rash as well as your own cosmetics.

Patch testing

Patch testing should be done on a skin site where the dermatitis is not apparent. The allergens are mixed with a non-allergic material (base) to a suitable concentration. They are then placed in direct contact with the skin, usually on the upper back, within small aluminium discs. Adhesive tape is used to fix them in place, and the test sites are marked. The patches are left in place for 48 hours, during which time it is important not to wash the area or play vigorous sport because if the adhesive tapes peel off the process will have to be repeated.

The patches should not be exposed to sunlight or other sources of ultraviolet (UV) light. After 48 hours the patches are removed and an initial reading is taken one hour later. The final reading is taken a further 48 hours later. Additional readings beyond 48 hours increase the chance of a positive test patch by 34 per cent. The patient should refrain from washing until the last reading is taken.

Interpretation of results

Any reaction seen is scored according to the International Contact Dermatitis Research Group system, as follows:

+? = doubtful reaction: mild redness only.

+ = weak, positive reaction: red and slightly thickened skin.

++ = strong positive reaction: red, swollen skin with individual small water blisters.

+++ = extreme positive reaction: intense redness and swelling with coalesced large blisters or spreading reaction.

IR = irritant reaction. Red skin improves once patch is removed.

NT = not tested.

The distinction between allergic and irritant reactions is of major importance. An irritant reaction is most prominent immediately after the patch is removed and fades over the next day. An allergic reaction takes a few days to develop, so is more prominent on day five than when the patch is removed.

A substance that causes an irritant reaction may exacerbate any underlying dermatitis such as atopic eczema, but this will not get worse with time and can be prevented by wearing a barrier cream or plenty of moisturiser.

A substance that causes an allergic reaction will cause a dermatitis and should be avoided completely if possible. The more times the skin is exposed to the substance, the worse the allergic reaction can become.

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