Patch testing for skin allergies

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

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.

Chlorocresol: 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 chlorocresol 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.

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