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The role of skin prick testing in eczema

From Dr H Morrow Brown, Derby

The advice on eczema given by teaching hospital dermatologist Dr Malcolm Rustin (Clinical, 2 February) is seriously flawed.

He states that both skin prick and RAST tests are unreliable in eczema, yet recent studies show these tests can accurately identify the causes in most children.

The atopy patch test for foods and for inhalants, which is widely used in Europe and yields much more significant results, is not mentioned, but patch tests for contact dermatitis are.

The role of food allergy in eczema causes much controversy between allergists and dermatologists, so it is encouraging that 40 per cent of eczema in children is considered due to food. Referral to a dietitian may not help because very few British dietitians are knowledgeable about food allergy and intolerance. An elimination diet for 10 days cannot lead to nutritional deficiency.

I cannot understand why dermatologists so often refuse prick testing even when requested by patients.

Further information may be found on my new website www.allergiesexplained.com

Dr Malcolm Rustin replies:

Dr Morrow's letter raises many issues. Identification of allergy to aeroallergens by patch tests can indeed identify positive reactions. House dust mite avoidance in children with atopic eczema may subsequently provide clinical benefit but similar avoidance in adults does not seem to provide benefit despite reducing allergen levels.

With regard to pet danger, there is no good evidence showing removal of cats or dogs from the home environment results in an improvement of the atopic eczema and indeed there is an increasing body of opinion suggesting that exposure to cats and dogs in early life might actually reduce the subsequent prevalence of asthma and atopic eczema.

This probably results from the generation of regulatory T cells after allergen exposure.

There is no doubt that parents would like to know what causes their child's atopic eczema but the multifactorial nature of this disease should not allow commercial exploitation to promote unnecessary RAST tests whose interpretation is extremely difficult.

With regard to the duration of an elimination diet, I had not mentioned 10 days. I would recommend a single dietary exclusion for one month together with optimum topical treatment so that any improvement can be identified.

A subsequent reintroduction may then confirm that the offending food can aggravate the eczema. Such dietary manipulation requires a methodological approach and I was warning against blanket exclusions of foods.

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