Allergy testing wasn’t something I learned in school, likely because the tests that exist right now are admittedly not very good. So when I ended up on the Allergy Bench I was a little bit boggled that there was a whole bench for it. Hopefully this will be an acceptable crash course in it.

Introduction

Allergies are complicated! A lot (it varies by region, but about 1/4) of people think they have allergies but it is actually closer to 1/50 according to studies and tests. Since the range of actual allergic symptoms is so huge, from sensitized but not reactive to severe lifethreatening anaphylaxis, current testing makes it kind of tricky to estimate whether someone will be fine or if they need an epi pen on them at all times. But we’re working on it.

Actual allergies (as opposed to intolerances) can be a “true” allergy in that your body is really reacting to the allergen, but they can also be cross reactive to another allergen (someone with a pollen allergy may be “allergic” to peanuts when there is a lot of pollen in the air, for example), sensitized (the antibody is made, but no response happens), or tolerant (your body has grudgingly learned to accept the allergen as a way of life). Responses can also be IgE antibody mediated (your classic allergy symtoms like anaphylaxis, hives, itchy palate, runny nose), cell mediated (dermatitis and celiac disease), or both (things like eosinophilic enteritis). To be an allergy though, reactions have to be reproducible with exposure to the allergen and not dose dependent.

Of course, since the tests are not as informative as some others may be, patient history is very important in diagnosing an allergy. You can see how it may be a problem when someone has an allergy that is just psychological. 

Skin Prick Testing

The skin allergy test is the more sensitive (fewer false negatives) of the two main ones and is a great negative predictor (ie, you probably don’t have an allergy if your test is negative). All they do is scratch the allergen into your skin and see if a bump (wheal) forms. Then they measure the wheal size (average of the longest diameter and the diameter perpendicular to that) and compare it to a chart for wheal sizes. So for example, if your wheal is >7mm in an egg prick, you’re considered positive. It is a very quick test, taking about 15 minutes altogether.

Being positive doesn’t mean you are going to have a reaction and you may just be sensitized, but the larger the wheal, the more likely you are going to have an adverse reaction to the allergen. That being said, the extracts they use aren’t standardized and most are pretty crudely produced, so the chances of a false positive are high, and no one wants to be told they are allergic to something when they are not. So far, skin prick testing is not overly recommended.

Serological testing

This is the one we use! We actually just bought a second machine because we started doing all the celiac testing for the region and the workload on the instrument has spiked. But serological testing is the more specific (fewer false positives) of the two. There are a couple of iterations of this, named RAST after the first serological test for allergies which was not very good. I disagree with calling them all RAST because some methods are better than others, but we’ll not go there. Ours is the ImmunoCAP method:

The instrument makes tiny cups that have the allergen stuck to the walls. Patient serum is added and any antibodies to the allergen will bind. Then everything unbound will be washed out. Conjugate is added (anti-Human IgE with an enzyme tag on it), and again, unbound conjugate will be washed out. A proprietary development solution is added with will react with the conjugate to make it flouresce. Stop solution is added after a certain amount of time, and the amount of flourescence is measured and plotted against a calibration curve to give you the concentration of antibody.

Again, the result doesn’t tell you too much about sensitization vs true allergy, but a higher antibody level is correlated to a stronger response, and antibodies <0.35 kUA/L is still considered negative. For reference, most results seem to  linger around 0.8-2.00 kUA/L here, though there’s no actual reference range established. Positive levels are graded on a scale that depends on the allergen from 1+ to 6+ for reporting ease since not all doctors really care about the antibody level itself. 

Double Blinded Placebo Controlled Oral Food Challenge:

Considered the gold standard for food allergy testing, but like most gold standard tests, it is very rarely done. Partly because it is pretty dangerous! I mean, you are giving someone something that may give them severe anaphylaxis. And even if you don’t have a severe reaction, any reaction can be pretty inconvenient. But it is also incredibly time consuming (takes several hours and usually 2 appointments) and requires pretty extensive dietary and medicinal preparation (especially cutting out anti histamines if you are taking them). The placebo and allergen are also hidden in food like cake and mashed potatoes so you can’t taste them. This may or may not be an upside depending on your tastes.

Anyways. Being a positive result requires at least two reproducible symptoms from a list of symptoms such as hives. 

Component Resolved Diagnostics:

Something new! Using this for nut allergies is something our lab is working on and I am part of the team trying to develop the assay. There’s not a whole lot out there on CRD other than the theory behind it and a couple of studies here and there.

Basically, the allergens used in current serum IgE tests are rather crudely prepared and you get a lot of cross reaction. Take the peanut, for example. Patients with allergies to the protein Ara2 have pretty profound reactions to peanuts alone and will likely need to carry an epipen. But people who have antibodies to a different component that can be found in several tree nuts have very mild reactions like an itchy palate and will otherwise just take an antihistamine (if needed) and go on with their day. Being able to predict the severity will help with a lot of quality of life issues that come with unpredictable reactions to allergens.

And that’s the gist of it in a nutshell! Questions can be tossed into my askbox as usual. :)