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PATCH TESTING FOR ALLERGY


Definition:
Allergic contact dermatitis occurs when your skin comes in contact with an allergen that your skin is sensitive or allergic to. The reaction usually appears within 48 hours after the initial exposure to the allergen. Symptoms that are commonly seen include the following: redness, swelling, blistering, itching, and weeping. The allergen can be a substance in a product that you have used for many years; it does not have to be a new product. Allergic contact dermatitis occurs more commonly in adults. The most common types of allergic contact dermatitis are allergy to poison ivy and poison oak.

Cause:
· Whether or not you are allergic to something is determined by your genes.
· Many people believe that you cannot be allergic to a product that you have used everyday for many years. This is not true. At any time your skin can become allergic to one of the specific substances in the product, even though you have used it for a long time.
· Often, a rash may break out on your face even though an allergen did not directly come in contact with your face. If you get something on your hands that you are allergic to and then touch your face, the allergen can cause a reaction on the face even though there is no reaction on your hands. This is because the skin on our hands is thicker than that of the face, especially around the eyelids.

Treatment:
· You have to discontinue all products that you have been using on the exposed area, and only use the treatment the dermatologist has prescribed for you. The most common treatment for allergic contact dermatitis is a cortisone ointment preparation.
· It is important that you return to your dermatologist at the end of your one to two week treatment. The treatment used is strong and you should not use it for more than one to two weeks without being under the supervision of your doctor. If overused, the medication can cause permanent thinning of the skin. When you come for the second visit, if all is going well, your doctor will then taper down to a weaker medication.
· Two weeks after your symptoms have cleared, you can begin the search for the chemical that you are allergic to. You do this by re-introducing the products you were using prior to your reaction, one at a time, one week apart. It is often best to do a small test area first.
· If you have a good idea what you are allergic to, you simply try to avoid it in the future and you should not have any further problems.
· If your problem persists and you are unable to determine the cause, patch testing may have to be done. This will be explained in greater detail if it is required.


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.

Patch testing can help to differentiate between the two. The test involves the application of various test substances to the skin under adhesive tape that are then left in place for 48 hours. The skin is then examined a further 48 hours later for any response. This can help the doctor decide which allergens you are allergic to and identify those that could be aggravating your dermatitis. The doctor will then be able to advise how you can avoid the allergens.

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.

Photo-patch testing
Some chemicals produce an allergic reaction only when exposed to light (usually ultraviolet type A light, UVA). Patients who are oversensitive to light and those with a rash that appears on parts of the body normally exposed to light (mostly the face, the 'V' of the neck and the hands) but that does not appear in areas shielded from the light (eg under the chin and the triangle between the nose and the mouth) should have a photo-patch test.

With photo-patch testing, two identical sets of allergens are applied to the back on day one. One of the sets is exposed to UVA light, and the sites are then examined as usual. A positive photo-patch test is recorded when an allergic reaction appears only on the light-exposed site.

IN POSITIVE REACTION
· You should be given detailed information about sources of the allergen.
· Scrupulously avoid any further contact with the allergen.
· Carefully read ingredients of new products, especially cosmetics.
· Use barrier creams and protective clothing to avoid the allergen.
· Use alternative products that do not contain the allergen.
· If the allergen is at work then discuss the options with your employer. They should provide materials to protect you from the allergen or if this is not possible, consider how to change your work.