Why Results from an Allergy or Intolerance Test May Be Misleading: Part 1

There are significant problems involved with direct-to-consumer (DTC) testing for allergies and intolerances. Dr. Scadding characterised some of these self-tests as a waste of money and in response to a question about whether she considered that these tests were sufficiently regulated, she answered, "They should be banned". She went on to outline some of the harm that is experienced by children when they are misdiagnosed or if they pursue unnecessary allergen avoidance.

Dr. Scadding discussed a recent example that illustrates some of the potential problems for consumers who have access to direct-to-consumer tests.

Dr. Scadding: For example, I saw a child this morning before coming here and she had...We did skin tests that are well recognised and she had skin tests to house dust mite and also to tree pollens. Two kinds of tree pollen. She has absolutely no symptoms referable to the tree pollen whatsoever. She does not have Spring Hayfever. She has good symptoms related to the house dust mite, so I treated her with house dust mite avoidance and anti-allergy therapy.

If she had got a kit, then she would have felt that she was tree pollen allergic as
well and something had to happen about that. She has sensitisation but not clinical disease. And if you do a test, only about half the people with that positive test will have clinical disease.

So, you can not have self-testing kits: they're going to lead to mis-diagnosis, mis-allergen avoidance. You need both the test and a detailed history taken by somebody who has some experience of allergy history taking and interpretation of tests. [Emphasis added.]

There are additional problems with blood tests which argue against their usefulness as a sole diagnostic aid for allergy. It is well-known that some people have cross-reactivity among allergens: e.g., an allergy to birch pollen, resulting in Springtime Hayfever, may co-exist with an oral allergy syndrome response to raw apple.

The same reservations expressed for skin prick testing-namely, poorly standardised food allergen extracts-are also true for the various methods for determining serum concentrations of allergen-specific IgE against food. Another major problem with the newer and technically highly sensitive methods is that they detect the many clinically insignificant serological cross reactions, in which IgE raised against and directed towards epitopes on, for example, grass pollen, also binds to wheat proteins, but without any clinical significance of the finding.

It is a common finding that the results of allergen-specific IgE blood tests should be interpreted with caution, particularly any weakly positive results in individuals with high serum concentrations of total IgE.

Dr. Adrian Morris provides a very helpful overview of some of the pitfalls of various allergy tests. He reports that:

The negative predictive value of food allergy testing is good - if a test is negative, then there is a 90% chance of there being no allergy to that food., but the positive predictive value is less specific - a positive test indicates sensitisation to that food but does not necessarily confirm a clinical allergic response will occur...Some foods such as milk and certain fruits contain a large number of different potential allergens and one has to be aware that a specific RAST may only test a few of the potential allergens in the food. Some people who are perhaps sensitised to a minor allergen in a particular food may RAST test negative, when in fact they are allergic to that food. [Emphasis added.]

Dr. Adrian Morris discusses an unusual presentation of food allergy in which

patients develop itching, urticaria, angioedema and upper airway obstruction with bronchospasm during or shortly after strenuous exercise taking place within a few hours of eating certain foods. This allergic condition may occur up to 12 hours after eating wheat, celery and shellfish especially prawn. These patients have no reaction to the foods if at rest. It is most commonly seen in female athletes usually under the age of 30years. [Emphasis added.]

It is not possible for any of the conventional allergy testing methods to identify exercise-induced food anaphylaxis and it highlights why it is essential that individuals who are investigating possible allergies need the guidance of appropriately qualified and experienced clinicians.

Dr. Morris also discusses foodstuffs that don't contain histamines but that may trigger histamine release in some people that may resemble classic food allergy symptoms although no IgE mechanism is involved.

Foods that have been implicated in this type of reaction include Egg white, shellfish, strawberries, chocolate, citrus fruit, tomatoes, fish and pork meat. The histamine liberated in this reaction will cause symptoms that may mimic true food allergy.

Dr. Morris discusses the natural history of food allergy in children and adults. A higher percentage of children have classic, IgE mediated food allergies but 80% outgrow them, leaving 1-2% of adults with these allergies. However, it is notable that:

If foods are completely avoided, up to 30% of adults will become clinically non-reactive to an offending food over a 2-year elimination period. They will however remain atopic - that is maintain a positive Skin Prick Test or retain specific IgE antibodies to the food, but have no reaction if they eat the food.

So, even for an accepted testing methodology such as IgE, the results of these tests need to be interpreted appropriately by an expert who has taken a good clinical history. An expert can advise individuals as to whether their tests results and and clinical history indicate the need for allergen avoidance which can be gruelling, disruptive and expensive. Expert clinicians can also give appropriate guidance on allergies which can be life-threatening.

Commentators such as Patrick Holford praise the availability of self-testing kits and ascribe dubious motives to others:

some health professionals just haven’t kept up to date. Perhaps it’s because a ‘home test’ takes the power away from the professional and puts it in your hands.

However, it is more plausible that experienced clinicians such as Dr. Scadding have justifiable reservations about the suitability of such tests for the home market.

As I've outlined in other posts, there may be a range of consequences for children who follow a restricted diet, particularly ones that eliminate foods that are a significant supply of calories or nutrients or are so restrictive that they cause significant social difficulties.

Related posts:

Truthiness and referenciness make the case for IgG food intolerance tests
More allergy and intolerance testing nonsense: part 1
More allergy and intolerance testing nonsense: part 2
Quote Mining and Misrepresentation: Poor Ways to Claim Clinical Validation or Sound Science
What is the Significance of IgG Antibodies and Testing?
Why IgG Testing for Food Intolerance Is Not As Simple As ABC or Doh Ray Mi

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