Allergy tests are very confusing. Even to doctors. After several conversations with other allergy moms on tests and what they mean, I decided to write a blog post about it because there is so much confusion out there.
Now first things first – Even doctors struggle to interpret allergy tests sometimes, unless they are trained in the field of allergy. When I worked as a family physician, my primary care colleagues used to either a) not order them so they don’t have to interpret them (and refer all suspected allergies to allergists) or b) send the results to me cos they have no idea what to tell patients. So you are not alone!
The reason they are so hard to interpret?
BECAUSE THEY ARE TERRIBLY UNRELIABLE!!!
There are two types of allergic reactions:
- IgE mediated – usually immediate or within few hours, hives, swelling, airway compromise, rash, anaphylaxis etc
- Non-IgE mediated – takes longer to manifest, you don’t get the above acute symptoms but rather you get slower onset symptoms with eczema, gut issues like reflux, painful swallowing, constipation or diarrhoea, tummy aches. The symptoms can be indolent & hard to recognize as allergies and both blood test and skin prick testing are usually negative.
Now the tests. There are 4 main ways we test for allergies, and I will talk about each with the pros and cons and what to look out for:
- blood test – this tests for the antibody that would attack the food allergen should it enter the body. (You sometimes might see this listed as RAST, or ImmunoCap). It has a false positive rate of around 50% i.e. it is positive in 50% of people who are NOT allergic to a certain food. It is useful to rule out IgE-mediated food allergies because a negative test means the person is likely NOT allergic (see caveat above on Non-IgE food allergies, these usually don’t show up on blood test or skin prick testing). However, a positive result is not so helpful unless you have a clinical history to go with the test result. This is why an interpretation by an experienced physician is VITAL because the test alone is really not that helpful.
I see a lot of people talk about numbers, and levels or Classes as if it meant anything – Sorry folks, the numbers HELP allergist get an idea of how LIKELY a reaction is to occur, but not how severe. So someone with a high number/level/class has a high probability of reacting to a food but we don’t know how severe it will be. NUMBERS/levels/classes DO NOT correlate with severity, this is the most common misunderstanding. I hear a lot of moms say, oh my son’s number is off the chart, so that means he is anaphylactic – not true. It just means he is highly likely to REACT, but how he will react is anyone’s guess.
- Skin prick – where a little pin prick is made on the skin, and a drop of allergen is placed on the pin prick, and you watch for a reaction on the skin. You measure how big the swelling is on the skin and give it a number. Usually you have to have stopped taking antihistamines for 2-3 days prior to the test or it comes up falsely negative. Again, 50% false positive result. So this test is only helpful when interpreted with a medical history, by an experienced physician… If it is negative, it is likely that the person does not have an IgE mediated food allergy (but non-IgE mediated allergy still possible), however, a positive skin prick does not always indicate an allergy. For example, the allergen from those vials are usually whole food proteins. However, if your body has digested and broken down the protein, it may not react. This is one of the many reasons why a test might come up positive on the skin when you are actually NOT allergic. Because when your immune system sees the actual food you have eaten, it is normally in a digested form.
- Oral food challenge – this is the gold standard, but the most time consuming and expensive way of testing. It seems that allergists vary hugely in whether they use this to diagnose allergy or not. Some only use the two methods above and advice avoidance if both positive or numbers are high, whereas others will see a positive result on skin/blood test, but advise a challenge just to confirm the diagnosis (more likely to happen if numbers are low). An experienced allergist will be able to use the above two tests to guide which foods to challenge with, because challenging with something someone is highly allergic to is dangerous. However, some have a blanket rule about not challenging at all. Ultimately, a physician takes on a risk when they decide to challenge a patient so some are just not comfortable with that risk.
- Elimination diet – this is more for the delayed type reactions (non-IgE based). The Skin prick and blood test mainly detect IgE mediated allergies – these are immediate, with reactions often seen within minutes/hours. However, there is another type of allergy called delayed allergy (Non-IgE mediated) – the symptoms take up to 3 days to manifest and usually it’s much more subtle symptoms like flare in eczema, reduced appetite, tummy aches, constipation. So the only way to make a diagnosis is to take the suspected food allergen out of the diet (for 2-3 weeks), and then reintroduce it.
There is yet to be a reliable test for allergies… all the modalities western doctors adhere to (with the exception of food challenge which IS the GOLD STANDARD) have high false positive rates…