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.

What should I do if a reaction is positive?

• 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.

Common allergens tested

• Balsam of Peru: an aromatic mixture made from resins and essential oils. It is found in the haemorrhoid preparation Anusol, some perfumes and certain spices.
• Caine mix: local anaesthetics found in preparations for sore throats, sunburn remedies, haemorrhoid preparations, Wasp-eze. Used by dentists and doctors for minor surgical procedures.
• Carba mix: rubber 'accelerators' (chemicals used to speed up the polymerisation process in the manufacture of rubber). It is found in rubber gloves, shoes, bandages and elastic. Of those allergice to carba, 85 per cent are also allergic to thiuram.
• Chlorocreosol: a substituted phenol preservative that kills bacteria. It is widely used in medications and some cosmetics. It cross-reacts with Dettol, which you should also avoid if you have a chlorocreosol allergy.
• Chromate: a metal used for plating other metals to prevent rusting and in the manufacture of stainless steel. It is also found in cement and tanned leather.
• Cobalt: found in jewellery, dental implants, artificial joints, jet engines. Most patients are also allergic to nickel, and some are also allergic to chromate.
• Colophony: present in adhesives, plasters, paper, printing inks, medicated creams, glue tackifiers (stamps, labels), and cosmetics.
• Epoxy resin: plastics, used mainly as adhesives in the industrial setting but also by DIY enthusiasts. Found in two-component glues, such as Araldite.
• Formaldehyde: preservative frequently used in household products and in industry. Often found in cosmetics and shampoo.
• Fragrance mix: used in patch testing, this collection of eight individual fragrances detects about 75 per cent of patients allergic to perfume. If you have perfume allergy, you will not be allergic to all fragrances, but you cannot tell from the label which fragranced cosmetics are safe. Avoid all cosmetics listing 'parfum' as an ingredient on the label. Also found in air fresheners, washing powders and candles.
• Lanolin: produced by sheep to protect the fleece from the results of weathering. It is widely used in cosmetics, medical creams and bandages.
• Mercapto mix/thiazoles: a rubber accelerator found in rubber shoes, insoles, gloves and elastic. It is also a component of balloons and bandages.
• MBT (mercaptobenzothiazole): another rubber accelerator.
• Neomycin: an antibiotic commonly used in ear and eye drops and creams to treat infected skin problems. Cross-reacts with other antibiotics.
• Nickel: 10 per cent of women and at least 1 per cent of men are affected by nickel allergy. Nickel is released from metals such as alloys or electroplated items. Found in jewellery, keys, coins, zips and buckles, pacemakers and batteries.
• Parabens: preservatives found in cosmetics and topical medical products to inhibit the growth of fungi and prevent slow deterioration. They are commonly used in cosmetics, household products, glue, shoe polish, shampoos and conditioners, sunscreens and medical creams.
• PPD (paraphenylenediamine): a permanent hair dye that is very frequently used in hair salons and at home. Dyed hair cannot cause an allergy but the dye may do during application. Also found in skin paints and occasionally in fur and leather dyes.
• Primin: a substance produced by the plant Primula obconica, a common houseplant.
• PTBPF resin (para-tertiary-butylphenol-formaldehyde): a synthetic polymer used as an adhesive. It is often combined with leather or rubber to make shoes, handbags, watchstraps, hats and belts.
• Thiuram (tetramethylthiuram disulphide or TMTD): another rubber accelerator that is also found in pesticides. People who are carba allergic often react to thiuram. Patients who react to the drug disulfiram, used for alcohol dependence, may also be allergic to thiuram.
• Toluene sulphonamide formaldehyde resin (TSF resin): the commonest polymer in nail polish and a frequent allergen.