Skin Care World

These details are only a general guide. Individual circumstances differ and the National Eczema Society does not prescribe, give medical advice or endorse any products or treatments. We hope you find this information useful but they do not replace, and should not replace, the essential guidance which can be given by your doctor or nurse.

Dr G Lowe, Consultant Dermatologist at Ninewells Hospital, Dundee, examines the role of patch testing in eczema.

WHAT IS PATCH TESTING?

Patch testing is a well established method for detecting allergies to substances which come into direct contact with the skin. This reaction is known as allergic contact dermatitis and is a form of ‘delayed’ allergy because the reaction takes two or three days to develop. It is of value for the investigation of any persistent eczema and, in particular, where there is clinical suspicion of contact allergy. It is important to remember that most contact reactions are due to irritant factors such as water friction, abrasives, acids or alkalis and detergents. Patch testing is not designed to detect these, neither is it designed to test for ‘immediate’ allergies to things like house-dust mites, animals or foods, which are detected by other methods.

CLINICAL SUSPICION OF CONTACT ALLERGY?

Sometimes the distribution of the eczema makes us fairly certain of the diagnosis such as a patch of eczema under a watch-strap buckle in a patient with a history of reacting to cheap earrings (nickel allergy) or a streaky eczema on the hands and arms in a patient who carefully tends the indoor plants (primula allergy). Eczema on certain parts of the body such as hands and face always makes us suspicious, but contact allergic dermatitis can affect any part of the body so we are always thinking about the possibility of allergic factors being present.

Allergy to creams and ointments used to treat the eczema can also occur and if the skin rash deteriorates or does not clear up as it should do with the prescribed treatment, allergy to medicines should be considered.

In many cases irritant contact dermatitis cannot be distinguished from the allergic type simply by the appearance of the rash and some contact allergens are very difficult to diagnose clinically. Therefore, sometimes it is easy to diagnose but at other times it is not so easy - a high level of suspicion is important.

SUBSTANCES TESTED?

We are aware of about 3000 substances that can cause contact allergic reactions. Fortunately, for practical reasons, most reactions can be detected using a standard battery of allergens, some of which are grouped into ‘mixes’. These batteries are subject to local variation and change with time if new products and chemicals are introduced into the environment and old ones are replaced or removed.

In addition to the standard battery, other groups of allergens are available for testing in special circumstances. For example, patients in occupations such as hairdressing, dentistry and engineering are exposed to certain substances which can on occasion trigger an allergic reaction and these can be tested for. There are also special tests available where there is a suspicion of allergy, for example, to cosmetics and perfumes, to medicines applied to the skin and to sunscreens.

PERFORMING A PATCH TEST

The allergens to be tested are first diluted to minimise their irritant potential in an appropriate vehicle which is usually petrolatum/white soft paraffin, and sometimes water or another solvent. These are then normally placed in small aluminium cups and fixed to the skin - usually on the upper back which is the most reliable site for obtaining allergic reactions. These are then fixed with hypoallergenic adhesive tape.

The tests are generally left on the skin for 48 hours to allow sufficient penetration of allergen to provoke a reaction in sensitised people; however, reactions may take longer than this to develop, so the final reading is usually interpreted at 72 to 96 hours. Results are scored depending on the intensity of reaction, with positive ones being red, swollen, itchy and occasionally blistered. This sounds easy enough but, in fact, there are several potential problems related to the performance and interpretation of the test.

PROBLEMS

Results of patch tests are not always easy to interpret and false reactions can occur for a number of reasons. A false-positive reaction is a reaction when there is no contact allergy and can result from testing with too high a concentration of allergen producing an irritant reaction. It can also be triggered by adjacent strong reactions or be due to active or recent eczema. The concentration of allergen applied is very important - it should be dilute enough to remove irritant potential but not so dilute as to miss an allergic reaction. This is not always easy to achieve and some people, such as those with atopic eczema, are more susceptible to the effects of irritants.

Some materials brought to the patch test clinic by the patient can be tested ‘neat’ or ‘as is’, such as ‘leave-on’ cosmetics and clothing. Others, such as ‘wash-off’ cosmetics and detergents, require appropriate dilution to avoid false results. Industrial products have to be tested with great care with reference to appropriate safety data sheets. It is very important never to patch test an unknown substance in case it is a very strong irritant.

False-negative reactions are negative reactions in the presence of contact allergy. This may be due to insufficient penetration of allergen caused by too low a test concentration, not enough being applied, allergen not released by the vehicle or loosening of the test strip. Other reasons are: reading the test too early (some allergens can give delayed reactions), or recent exposure to ultraviolet radiation or potent topical corticosteroids which can inhibit the reactions. Adverse reactions to patch testing can occur even with the most careful technique, but the use of standarised allergen concentrations, avoidance of powerful irritants, not testing during acute phases of eczema and the use of hypoallergenic adhesive tape means that any such reactions are generally mild when they do occur.

Deciding on the appropriate tests to perform and evaluating the relevance of the results also requires skill, experience and curiosity. A positive patch test result does not necessarily mean that it is the cause of or is contributing to the current eczema, as it may be explained by a previous unrelated episode. Detailed lists are available on the occurrence of many of the allergens in the environment but, in fields such as cosmetics and skin-care products, ingredients are forever changing which leads to difficulties in allergen detection and avoidance.

COMMONEST ALLERGENS

Nickel is the commonest allergen in the standard battery series and the usual cause of sensitisation is ear piercing. Nickel is used in a large number of alloys and chemical compounds and is everywhere in the environment leading to constant exposure and persistence of the allergy. Avoidance of jewellery and metal clothing accessories containing nickel may be sufficient to keep the skin of sensitised persons settled, but sometimes more rigorous measures are required.

Another common allergy is to perfumes or fragrances which are present not only in cosmetics, soaps, shampoos and bubble baths, but also in products such as detergents, fabric softeners, air fresheners and polishes. Perfume allergy evaluation is difficuqt because there are several thousand different fragrance components in existence. Each commercial product may contain up to several hundred of these and perfume manufacturers tend to be secretive. The standard battery mix tests a mixture of eight fragrances which will detect about 75% of all cases of fragrance sensitivity and is a useful screening agent. Fragrances are the most frequent cause of cosmetic allergy, but cosmetics also contain other potential sensitisers such as preservatives to inhibit bacterial growth, emulsifiers such as lanolin and UV filters. In Europe ‘hypoallergenic’ cosmetics usually means that they are just fragrance free and are still potentially allergenic.

A further very important contact allergy is to topically applied medicines. This may lead to obvious local aggravation of the eczema, or to spread of the eczema to other parts of the skin, or it may simply show as failure to respond to treatment. The allergy may be to the active constituent of the medicine, such as an antibiotic, or to one of the vehicle constituents or additives such as lanolin.

Almost all medicaments applied to damaged skin are capable of causing allergic sensitisation -even apparently bland preparations like emulsifying ointment. A high level of suspicion is often required to pick these up, but the potential benefit to be gained is obvious. Topical corticosteroids are well known to cause a variety of complications such as thinning of the skin and masking of infections. Allergic sensitisation is becoming increasingly recognised as a complication and should be considered in any eczema apparently resistant to treatment.

Rubber allergy is usually caused by accelerators and other chemicals used in its manufacture. Allergy to rubber gloves is an important cause of deteriorating hand eczema. Other sources include shoes and elastic.

LIMITATIONS OF PATCH TESTING

False reactions, as mentioned earlier, are one drawback. Knowledge of sensitising potential and occurrence of allergens in the environment are other limiting factors. Sometimes positive reactions are unable to be interpreted in the light of current knowledge. Inevitably, time (for staff and patients) and costs (of staff and materials) are other limiting factors in the performance of patch testing.

CONCLUSIONS

Potential consequences of a missed contact allergen include persistence of the eczema, repeatprescriptions, recurrent visits to doctor or hospital, absence from work, and loss of wages. History and clinical examination are not enough for accurate diagnosis as contact allergies can be very difficult to diagnose. The performance and interpretation of patch testing should be carried out by a dermatologist who can assess all the factors involved in the patient’s eczema. If atopy, irritants or infection are also operating, avoidance of identified allergens will not clear the eczema, but should confer some advantage in subsequent management. Even a negative patch test, although causing some patient disappointment, is helpful for clinical management.

Patch testing is only one facet of the management of patients with eczema. Irritant avoidance, treatment of secondary infection, regular use of emollients and use of topical steroids remain cornerstones of management. In the past there has been some debate on the benefits of this test, but it is now generally seen as a necessary and helpful investigation which should become even more so as further knowledge is gained in the future.

Typical standard battery of allergens for patch testing.

Control

White soft paraffin

Nickel Jewellery, clothing accessories, coins etc Benzocaine Topical anaesthetic in medicaments Cobalt Contaminant of nickel Thiuram mix (rubber) Gloves, shoes Dichromate Cement, leather, matches Mercapto mix (rubber) Shoes, gloves Fragrance mix Perfumed products IPPD Black rubber, eg tyres Balsam of Peru Perfumes, flavours Phenylemediamine Hair dye Neomycin Antibiotic in topical medicaments Epoxy resin Adhesives Tixocortol Marker for hydrocortisone allergy Formaldyhyde Cosmetics, clothing Wool alcohols Lanolin in cosmetics, medicaments Colophony Varnishes, adhesives, paper, ink Parabens Preservative in cosmetics, medicaments Phosphorous sesquisulphide Matches Quinoline mix Antiseptics in topical medicaments Primin Primula obconica (houseplant) Ethylene diamine Medicaments BPF resin Adhesive for shoes/watch straps

FURTHER INFORMATION

This fact sheet is one of a series provided as a service by the National Eczema Society to give up-to-date, practical help. More detailed information on this and other eczema-related subjects is available to members of the National Eczema Society. Membership of the Society costs just £20 per year.

Members receive a comprehensive information pack and copies of the Society’s quarterly journal, Exchange. They also have unlimited access to the Society’s Information Service. Membership of the National Eczema Society also offers people affected by eczema the opportunity to talk with others who share similar problems and experiences.