How allergy tests can get the result horribly wrong
. . . and cover up the dangerous problems that are REALLY causing your symptoms
HARRY Maplethorpe used to cough so much at night that he was often too exhausted f or school. Sometimes he coughed so hard he was sick, but his symptoms were put down to common childhood chest infections.
Then, three years ago, his parents thought they’d got to the root of his persistent problems when, at the age of eight, it seemed Harry was allergic to dairy products.
He was diagnosed via a skin-prick test after being referred to an NHS allergy clinic. The test involves placing drops of common allergens, such as pollen, house dust mites and food, on the underside of the forearms, then pricking through the drops with a lancet so that i mmune cells i n the skin are exposed to the substance.
The idea is that if the patient is allergic to the substance in question, a large, itchy red bump will appear on their skin within 20 minutes.
‘All the other allergens produced a slight bump, but the dairy one was really raised and the redness spread around his arm,’ says Harry’s mother Leanne Kendrick, of Fleetwood, Lancashire.
Harry was diagnosed with a dairy allergy, and advised to avoid dairy products for several months — a harder task than you’d imagine.
‘I thought it was milk, cheese and cream, but I started reading food labels and dairy was everywhere,’ says Leanne, 31, a full-time mother.
Harry even had to give up his favourite cheesy Quavers crisps — but his new diet made no difference to his symptoms.
At his follow-up appointment three months later, it was found Harry did not have an allergy after all — during a repeat skin-prick test, none of the substances, including dairy, provoked a reaction.
‘I was really annoyed — I’d spent months making sure he didn’t eat dairy, for nothing,’ says Leanne. Further tests revealed that Harry, in fact, had tracheobronchomalacia. This is a rare condition in which the walls of the airways are softer than usual, and narrow whenever Harry breathes, making it harder to clear sputum from his lungs and leaving him susceptible to chest infections.
Skin-prick testing is the most common allergy test carried out on the NHS, but the results are not quite as accurate as you might assume.
If you get no reaction to a substance, there’s a 95 per cent chance you’re not allergic to it. But only half of people who have a positive reaction are actually allergic to that substance, according to the Association for Clinical Biochemistry and Laboratory Medicine.
Blood tests, which look for specific antibodies against the substance, are also available on the NHS, but are similarly open to misinterpretation.
The good news is that new tests are on the horizon, which could remove some of this uncertainty — and are already available in some UK clinics.
PRECISELY why skinprick tests can produce a reaction i n people who aren’t allergic is unclear. The idea behind the test is that by introducing a potential allergen, you are asking the immune cells that live in the skin whether they recognise it and have begun to co- ordinate a response against it.
If they previously encountered the protein and have committed i t to memory, they are considered ‘sensitised’ to it.
They will t hen produce immunoglobulin e (Ige) antibodies, which trigger the release of histamine, the substance responsible for allergic symptoms such as a swollen mouth. With the skin-prick test, histamine will cause l ocal redness and swelling.
The problem is, just because you produce antibodies against a substance, it doesn’t mean you’ll have allergic symptoms.
Furthermore, a local skin reaction doesn’t necessarily mean they’d react if the allergen was inhaled or swallowed.
Although the same antibodies and cells are involved in both sensitisation and allergy, an ‘allergy means that the immune system both sees and reacts to a substance upon exposure, whereas a sensitisation means they just see it,’ says Tariq elShanawany, a consultant clinical immunologist at Cardiff and Vale NHS Trust. The person conducting the test will look at the size of the raised bumps and rate them against a standard scale.
‘ With skin- prick t esting, there’s a range above which you’re pretty confident that someone is allergic; a range below which you’re pretty confident they’re not; and a fairly large grey area in the middle,’ says Dr el-Shanawany.
Careful interpretation of the test is therefore essential.
‘I come across patients who have been misdiagnosed with allergies all the time, but it’s not the test’s fault,’ explains Adam Fox a consultant paediatric allergist at Guy’s and St Thomas’ Hospital in London.
‘Wherever there are people who aren’t well experienced in using the test, there is an inherent danger the results will be over-interpreted.’
Although skin-prick tests are conducted in NHS or private allergy clinics, these aren’t necessarily staffed by allergists — doctors who specialise in allergy diagnosis.
‘There are 30-35 adult allergists in the UK, and around 20 paediatric allergists,’ explains Dr Fox.
Besides the test results, doctors should pay attention to whether the patient has noticed symptoms such as a rash or swelling in the mouth when they eat or come into contact with the substances that are under suspicion.
‘A skin-prick test should only ever be a small part of trying to assess what someone’s allergic to,’ says Moira Austen, helpline manager at the Anaphylaxis Campaign, which supports people who are at risk of developing severe allergies.
Sometimes, being wrongly told you’re allergic to something could make you more ill.
‘I had a case where a mother was given a huge list of things that her child should avoid and he became malnourished,’ says Maureen Jenkins, clinical director of Allergy UK. ‘He was allergic to just one of them.’
OF COURSE, the only way to know for sure if you’re allergic to a food is to eat it, but this can be dangerous because of the risk of anaphylaxis — a potentially life -threatening, reaction. So this should be done only under close medical supervision.
Some larger hospitals offer ‘oral food challenges’ — these involve exposing the patient to increasing doses of allergen over a day, to see if they react.
However, these are costly, time-consuming and do carry a slight risk of anaphylaxis.
There may soon be other options, though. Dr Fox has been conducting clinical trials of a test which does the same as an oral food challenge, but outside the body, so it’s safer.
The basophil activation test ( BAT) involves extracting immune cells (basophils) from the blood. These are packed with granules containing substances that trigger allergic symptoms.
These cells are then mixed with the suspected allergen and watched under a microscope to see whether any granules are released.
In a study published in the Journal of Allergy and Clinical Immunology in 2014, Dr Fox and his colleagues demonstrated that their test could diagnose peanut allergy with 97 per cent accuracy when tested i n 104 children who either had a confirmed peanut allergy, were sensitised to peanuts, or who were unresponsive to peanuts.
The test, which could be available within several years, proved useful in cases where the skinprick tests were ambiguous.
But they’re still not perfect. ‘I would still do an oral challenge test if there had been no accidental exposure to the allergen after two years,’ says Dr Fox.
As f or Harry, now 11, he remains susceptible to infections, but intravenous infusions of antibiotics several times a year make these less frequent.
His mum, Leanne, urges other parents to query the results of skin-prick allergy tests if they have any doubts. ‘I don’t take anyone’s word now,’ she says.