Eczema, food allergy and tests

Author: Dr R Turner, Allergy Specialist based in the North Hampshire Hospital

Date: Oct 2010

It is always a difficult question to answer: "is my child’s eczema caused by something he/she is eating?"

A week or two ago a mum brought her child with severe eczema to see me. She reported apparent improvement by leaving one or two foods out of the child’s diet that the mum had worked out logically by trial and error. Then she had taken her child to a homeopath who had performed a vega test. Having given the history to the therapist she was relieved that this test ‘confirmed’ her child’s allergy and had thrown up other foods, house dust mite and polyester that ‘went off the scale’. Skin prick testing did not confirm any of these allergies- more of which later.

Let's first look at vega testing - now widely available in the private sector but not within the NHS. Two recent reports, one in Health Which (not conducted by doctors) and the other in the British Medical Journal from Dr George Lewith, Head of the Complimentary Medical Centre in Southampton said that this was not a reliable test - there is no good science to support it and if we think about it logically the whole basis of it is improbable. It was originally developed to guide homeopathic dosing and not as an allergy test. A friend who is a medical homeopath used it on me - he told me it was a bit like medical dousing - and so some testers might intuitively find an answer but there is no guarantee at all it will be the right one - which it wasn’t incidentally in my case. The point is, he told me, the amount of pressure the tester applies to the skin using the acupressure probe influences the result; so knowing the history before testing, the tester can either consciously or unconsciously influence the result: ‘confirming’ a known allergy or the most likely allergens for a particular condition.

Now most doctors reckon that an accurate medical history contributes 80% of the diagnosis, and testing the other 20%; and yet patients often think, or would like to believe, it’s the other way round: which may be true in some branches of medicine but it is certainly not true in allergy at the moment.

We would certainly like an accurate test for food allergy. The skin prick tests (SPTs) that I mentioned earlier are a very accurate test but only unfortunately for acute types of allergy that include foods such as nuts, eggs, and fish, for example. In practice we have found quite a lot of patients allergic to potatoes by this method and cutting them out has proved effective in relieving symptoms. Unfortunately though if eczema is associated with a food it is often not an acute reaction but rather a delayed one and delayed reactions will not give positive SPTs.

Blood tests are no better either: the so called RAST (CAP) tests are similar and less accurate than SPTs. IgG4 tests available only privately maybe more helpful but give many false positive results- more trials looking into their value are being planned.

We have had some success with patch testing for foods- but cannot draw any conclusions on reliability of this test yet.

Where does this leave us?

Unfortunately in a somewhat frustrating situation. The only accurate way at the moment of diagnosing food allergy in eczema is a trial of an exclusion diet for foods suspected either by the history or just empirically tried because certain foods are more likely to be a problem in eczema than others. From currently available studies about 40% of children with eczema may be benefited by specific food exclusions- note mostly benefited by not cured by. I have yet to see a patient with vega testing with a negative result! I have seen children started on quite restrictive diets as a result of these tests with no lasting benefits: there can be an immediate benefit (which is an interesting manifestation of any intervention): the diet however is often continued because of the initial response and positive tests.

Finally remember good parental observation and common-sense can lead you to the right conclusion with children’s eczema more often than any tests - for the time being at least.

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