Skin Scratch Tests and MCAS

Disclaimer: This is one of those times where I actively disagree with the standard veiws held by many physicians. I'm not a doctor, so don't take my advice, don't not take my advice, don't blindly follow my thinking, stop and think things through; most importantly, err on the side of caution.

I will never forget my first skin prick tests. I was about 12. I'd started reacting to peanuts, in addition to anything with fur, feathers, dust or pollen… so off we went to the allergist. At the time, they didn't test for peanuts if you'd had a confirmed reaction. It was undoubtedly peanuts, though my reaction wasn't severe, it had the potential to be a serious allergy.

So, the doctor spoke with my Mom and I, (much to the consternation of the doctor who was not used to having kids participate in those discussions…) and I was taken to another room for the skin test. It was a teeny room with no space for parents, so my mom went to sit in the waiting room. 

The nurse began the tests, and it was much, much worse than I thought it would be. The serums stung and burned before she even pricked them. The test serums were much more concentrated then, and they actually used a needle to prick the skin, instead of a scratch with a plastic thingie (not pleasant either, but a whole lot better). By the time she had done all of them, they were already insanely itchy and hot. She left the room, and I started to feel nauseous, dizzy and flushed. My arms had begun to swell, the pricks themselves, and my whole forearm- they were growing in front of my eyes. By then they were throbbing. Every fiber of my being was screaming warnings at me by that point. I tried to get the nurse's attention a couple of times. I finally got her, and I politely said, "This is really itchy, it doesn't feel right." She didn't look. The patronizing reply was "We've only just put it on. It's been five minutes, you have a long more time to go. You're just going to have grin and bear it." I tried SO hard to wait, and then instinct overcame politeness. I (very firmly) said "I CAN'T wait, something isn't right. I have to wash it off!" My mother had heard my tone- she flew back to where I was. She knew that If I spoke like that to an adult, something was very wrong. The nurse hadn't clued in, she was meandering over. My mother came in, looked at my arms and said to the nurse "That isn't right. Get the doctor. NOW." The nurse began to protest, and then she saw my arms.  

A look came across her face. A look that I had seen a once before, and many, many times since then. It says "Holy Shit!! She was right! " My arms were turning purple, the skin stretching tight in places. She called for the physician, started washing my arms (to his credit, he'd heard my mom and was already 3/4 of the way there). They gave me some medication- one I hadnt taken before. Benadryl… (my first Benadryl, what a precious moment…. ) They had me lie down. Once I felt better (mercifully quickly), the doctor spoke with my Mom and I. He said "I don't think I have ever seen anyone's arms do that before… Well, you're allergic to everything, cats, dogs, grass, ragweed, dust, although it's tricky to tell which are the worst ones. You are allergic enough that to do allergy shots you would be seeing me once a week for the rest of your life… not exactly practical. There are new medications coming onto the market, let's try a new long acting antihistamine.". It was the kind of event that makes an impression on a child. I hadn't really thought about how my Mom felt when she saw me. 

This week I saw the exact same look. It's one thing when they give me that look, another thing altogether when they have it directed at my child. The nurse knew it could be interesting- she knows I have MCAS, and she helped hold Daughter down when she was 2… She knows how allergic she can be. A few minutes in, Daughter started getting very itchy.. I  looked at her back- it wasnt too bad. A couple of minutes later, her demeanor changed, she got wiggly, agitated, and the stress in her voice was apparent – (I know that feeling well- that's the body going into fight or flight.) . I looked at her back, and nodded to Hubby to get the nurse. I tried not to make her alarmed, but it was foiled by her big brother. "Woah! Holy cr…, Yikes! That's really bad!"(subtly and 13 year old rarely co-exist).

Even though the nurse was reasonably mentally prepared, the blood drained from her face. "Wow. That's special!". We washed her back, got some benadryl into her, and she recovered quickly. It was a week ago- I'm still recovering.

The Kids' Results
My son had never seen an allergist. He gets some pretty nasty seasonal allergies, and we were wondering about cats and dogs, as well as a couple of foods (citrus and grapes). As we had expected, the foods were all negative, and he reacted to grass and tree pollen, cats and dogs. (He adores cats- his eyes and nose don't! ). This is what his arm looked like- a pretty 'typical' scratch test. 

A 'typical' skin test result, showing a few positive reactions. The water control has no welt, the histamine has a clear welt. Anything larger than the histamine control is positive.

Can you look at this and NOT get itchy? #3 is shrimp.

So, skin tests are where someone puts a concentrated serum or even a fresh food onto the skin, and the skin is scratched. The skin is monitored for 20 min to half an hour, and then the welts are measured. They use one serum that is basically histamine and another that is water. If the water wheal is as big as the histamine, then the test is invalid, as it means youre reacting to the scratch, not to histamine. The classic case is when someone has dermatographia, where scratching the skin creates a reaction. As long as the water is less reactive than the histamine, Anything that makes a wheal bigger than the histamine control is considered 'positive'. 

False negatives and positives
When you look online, the information all says that false potisitve are common, false negatives are not. My experiences with myself and my daughter have convinced me that the advice is the wrong way around for us. A false positive means that you react on the skin, but can eat it. A false negative is when they scratch the skin, no welt forms but you have an anaphylactoid or systemic reaction when exposed to it. 

In MY world, false negatives are VERY common, false positives are extremely rare. If you put something on my skin, and i swell up, how could I possibly eat it? Mucosal membranes are more permeable than skin (more stuff goes through, the more permeable the tissue, the more prone to reactions it is). If a drop does that, imagine what it would do to my intestines! Any delayed reactions, IgG mediated responses, etc., also don't shown with this type of test. If citrus gives my son bleeding sores in his mouth, I'm not going to give him and orange juice…

What Scratch Tests Can and Can't tell You

20 years ago, allergists thought that if you had an allergy, you would always have that allergy. Some still assert that you can't develop new 'true'(i.e. IgE mediated) allergic reactions as an adult. They also say that it's extremely rare to lose an allergy once you're past about age 10 (my peanut allergy- has come and gone several times in my life). As well, they still have trouble wrapping their brains around being a 'little bit' allergic. (I can eat about 10 grapes a day right now- more, and I start to react.) So, if you have a test and react, and then six months later you don't react, they'd say you had a false positive. Here's the fundamental flaw…people outgrow and develop new allergies all the time. If you have MCAS, it's even more likely. If putting a drop of something on my skin makes me get a huge welt, I'm not putting that food in my mouth! No way, no how, not gonna happen. Six months later, however, might be very different. In practice, allergists don't challenge foods that have a positive skin test, even though false positives are said to be common. There are a LOT of people who have delayed reactions, IgG reactions, etc etc. where eating that food can lead to anaphylaxis- even though nothing showed up on their skin test. And yet, false negatives are thought to be rare… (a lot of that comes down to the definition of allergy rather than differing opinions about what is happening).

Many people are very nervous about doing a food challenge. It seems scary and huge the first time we do it. Doctors encourage people to do it in the doctor's office, in case they have a severe reaction. One thing I have never understood- why do they start with putting something in your mouth? I start much further back.. I start with smelling it. If it smells moldy, spoiled, or fermented, I stop. If not, I rub the food on my inner arm first. Then I touch it to my lips, and wait, then start to move to actually in my mouth. 

Better tests

So, what IS helpful? Different methods may be more accurate then others, depending on the individual, which food is tested, in which form, and the goal of the test (diagnosis for diagnosis sake isn't worth it, IMO). Ultimately, if it's helpful, then do it, if not, don't. Skin tests really aren't helpful for me- I get pressure urticaria and dermatographia. I'm going to react to everything or nothing, and I'd never make it to the allergist's office if I had to be off antihistamines for a week prior. My daughter's skin test had clearly reacted to the histamine control, but not the water. When only a few things are reactive, skin scratch tests can actually be helpful. There are better skin tests that use a variety of serum and food extracts, a combination of scratch, a drop under the skin or patch testing. There is a recognition that certain foods, temperature that it's cooked at can make a difference, They can be done ever increasing concentrations of an allergen, of the which tells you how sensitive you are to it. That type of testing may be harder to 'measure' in research, but I think it's much more descriptive, it corresponds to symptoms much more accurately, and can guide treatment/trigger reduction, etc.

How I use scratch tests

Skin tests in the traditional sense are not useful for me, but I do occasionally use them. I use them to assess how I will react to a new medication. Rather than tossing it straight into my IV, I do a skin test first. I try to do it aheadof time, if I can, but regardless, I make sure I test it that day with that medication, in that space, before it goes into my IV. I'm high enough risk that it's worth the extra 15 minutes. (not if you've been in a car wreck- but if you've waited 4 months for a CT scan, another 15 minutes won't matter) If I put a drop on my skin and I get a massive welt, it's not safe for me to take it. It is also helpful in reducing anxiety in those situations- mine and my caregivers!  It's not going to tell me of something is 'safe', but it has helped me avoid some potentially scary encounters.

Skin tests tell you one thing- what you are having an immediate reaction to on your skin TODAY, to that particular item. It could be totally different next week. And what is used for testing can be more important than many allergists think. One of those positive tests for my daughter was corn- a big welt. Since then, she had popcorn at the movies. The 'prevailing' wisdom is that 'corn is corn', it shouldn't matter what type of corn it is. I know that for me, sweet corn versus hard kernel corn doesn't make any difference, but that isn't the case for my daughter. It will be good to figure out why, to understand the full mechanism, but in the meantime, popcorn is okay for my daughter, corn on the cob isn't. Knowing that they used actual fresh sweet corn helped us understand what was going on. It would be a shame for her to miss out on movies and popcorn if she didn't need to.

What is the BEST Test for Food Allergies?

There's only one way to know what your body will do if you eat a particular food. Eat it. I know that sounds flippant, but it really is the most reliable way to do it. I have a remarkably successful time in identifying triggers. I have learned enough that taste alone tells me if I can tolerate a lot of things. I rarely get more than one bite in- my body sends a 'stop' signal before any symptoms appear. Ultimately, the best source of information about what is going on in your body is YOUR BODY.

Any test is only as valuable as the skill of those who administer it, and those who interpret it. Skin tests can be revealing, but only if you know exactly what they mean, and their limitations.

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