New safer test to spot food allergies

AneW blood test promises to diagnose food allergies more accurately and offer a safer alternative to current testing practices. A trial by doctors at guy’s and st thomas’ Hospital in London has found that the test — called the basophil activation test (BAT) — is close to 100 per cent accurate at spotting nut allergies.

Allergic reactions occur when the immune system overreacts to a trigger — usually food, medication or an environmental irritant — causing the release of histamines, substances that try to rid the body of the allergen. this leads to symptoms such as itchy rashes and swelling.

the most severe form of allergic reaction, anaphylaxis, can lead to life-threatening symptoms such as breathing difficulties and loss of consciousness.

Diagnosing allergies usually involves skin prick tests, where a drop of liquid containing a substance the patient may be allergic to is placed on their forearm. the skin is then scratched, so the liquid seeps into it.

Blood tests, which check for ige, an antibody produced when there is an allergic reaction, can also be used.

But both of these tests are far from perfect and can lead to a high rate of false positive results, meaning an allergy is indicated when there isn’t one.

in skin prick tests, for instance, that’s because the amount of the food protein on the arm is often larger than the amount you would absorb from digestion (which may not cause a problem).

Because these are inaccurate, to confirm the diagnosis, allergy patients may be given an oral food challenge (OFC), in which the patient eats a food they may be allergic to in tiny amounts, while they are under supervised hospital conditions. this can be risky as it may provoke anaphylaxis.

it’s hoped that the basophil activation test will offer a safer option to OFCs. it involves mixing a small amount of the food the patient is thought to be allergic to — for example, peanut protein — with a sample of their blood.

the sample is then analysed to check the basophils — a type of white blood cell involved in food allergies. in particular, the test detects the presence of a molecule called CD63 on the basophils’ surface, which is a sign that the patient is allergic to that particular type of food.

Results are available in three hours. the test is 96 to 100 per cent accurate at diagnosing nut and sesame seed allergies, a study of 92 children by guy’s and st thomas’ Hospital found.

it also reduced the number of OFCs needed, the Journal of Allergy and Clinical immunology reported recently. the researchers suggest the technique could be used routinely within years.

graham Roberts, a consultant in paediatric allergy and respiratory medicine at University Hospital southampton NHS trust, said: ‘this is exciting as a lot of the time skin prick testing and ige tests are inconclusive. BAT gives you more ( and different) information, which is actually complementary.

‘However, you need blood that has just been taken from the patient, and it needs to be run within a few hours of the blood being taken, so hospitals will need the right set-up to do this.

‘But it will save children from doing challenges which can put the patient at risk.’