Taking the sting out of diagnosing contact dermatitis

“Have a high clinical suspicion for contact allergens or irritants in anyone presenting with eczematous rash.” —Dr. Rebecca Pratt

Creams that sting and rashes that won’t quit —welcome to the complex world of contact dermatitis. Dr. Rebecca Pratt joins Dr. Mariam Hanna to unpack why diagnosing and managing this condition is anything but straightforward.

Dr. Pratt is an allergist and clinical immunologist in St. Catharines, Ontario, with a passion for dermatoimmunopathology and practical tools that empower patients. She breaks down when to patch test, what to suspect, and how to help patients take control.

On this episode:

• Why 80% of contact dermatitis cases are irritant—not allergic
  
  
• Clues that point to allergic contact dermatitis, especially in recurring facial, hand, or foot rashes
  
  
• How to approach patch testing in adults and kids, including when immunosuppressants or UV exposure may interfere
  
  
• What to do when results are negative—and why that still matters
  
  
• The reality of managing allergens found in foods, not just topicals
  
  
• Why there’s no one-size-fits-all answer to the sunscreen question—and how physical blockers stack up

There may be no cure for contact allergy, but there’s power in knowing what to avoid.

Visit the Canadian Society of Allergy and Clinical Immunology

Find an allergist using our helpful tool

Find Dr. Hanna on X, previously Twitter, @PedsAllergyDoc or CSACI @CSACI_ca

The Allergist is produced for CSACI by PodCraft Productions

Transcript

Dr. Mariam Hanna

Hello, I'm Dr. Mariam Hanna and this is The Allergist, a show that separates myth from medicine, deciphering allergies and understanding the immune system. Hey, I have a totally unrelated question but I promise it's just like a quick question for you. What's the best kind of cream or sunscreen for me?

I've tried every kind and it burns and stings my skin so what should I use? Quick, eh? Simple, eh?

The art of navigating the dermatitis question is fascinating to me. As an allergist, do you suggest empiric trials, test site application and see how you do, or patch testing for every sunscreen question? Listen, there's a pro and a con to which way you go and this is not so quick of a question afterall. And the art of what that patch test contains and what you interpret the positives to mean and even the entirely negative test is actually still a challenge.

And then, what should they buy? What should they avoid? Will we still just be prescribing medications when the inevitable dermatitis flare occurs?

It's actually an interesting topic and one that's particularly fun to discuss as we start addressing the age-old question, am I reacting to something or is it just my disease? Well, today I'm delighted to introduce to you a dear colleague of mine who has been passionate about this topic from early on in her fellowship. Dr. Rebecca Pratt is an allergist and clinical immunologist in St. Catharines, Ontario. She completed both her internal medicine residency and allergy and immunology fellowship at McMaster University where she is also an assistant clinical professor. Her interests include dermatoimmunopathology, delabeling medication allergies and optimizing efficiency in the office. Dr. Pratt, thanks for taking the time out of your busy but highly efficient practice to join us today and welcome to the podcast.

Dr. Rebecca Pratt

Very excited to be here, Mariam, thank you.

Dr. Mariam Hanna

All right, What exactly is allergic contact dermatitis?

Dr. Rebecca Pratt

Yes, a question that could be very short or very long depending on how much depth we want to get into but the long and short of it is it's any dermatitis, which is essentially inflammation of the skin that occurs in response to contact with an exogenous substance. So as dermatitis can be acute or chronic, it can be caused by a lot of different things including genetics, including fungal infections, including psoriasis, etc. But this particular dermatitis we're going to talk about tonight has to be the   result of being in contact with something.

Dr. Mariam Hanna

And how do we differentiate allergic contact dermatitis from more of an irritant contact dermatitis? What's the difference there?

Dr. Rebecca Pratt

It can be actually hard to differentiate just by looking at the skin. So clinically, even histologically, it could be hard to differentiate. But 80% of contact dermatitis is irritant contact dermatitis and irritant essentially means direct chemical damage leading to inflammation and skin damage, whereas allergic contact dermatitis is a delayed type 4 T cell mediated hypersensitivity reaction to an allergen.

So again, they can look similar, but they're clinically quite different. As far as actually how we differentiate them, really patch testing, which we'll get into is the only way of doing that. But then again, there's lots of other differential diagnoses here we have to think about as well like atopic dermatitis, or like I said, psoriasis or more worrisome rashes like mycosis fungoides, etc.

But for the big two for contact is irritant versus allergic.

Dr. Mariam Hanna

Okay, and like interesting number to say 80% of the time, it's actually just going to be irritant, whereas 20% it's more an allergic picture. So when should we have clinical suspicion for allergic contact dermatitis?

Dr. Rebecca Pratt

Yeah, I really do mention that to patients before patch testing is the more likely thing is that it is an irritant contact dermatitis. Just to set the stage for that, we may not find an allergen here, but that doesn't mean that being in contact with substances could result in this dermatitis. But as far as the clinical suspicion goes, anyone with an acute or chronic dermatitis, you should have it as a possible differential diagnosis.

So this can be generalized, but it also can be local. And I think when it is local to certain sites like a perioral or periorbital dermatitis, colitis, hand and foot dermatitis, you may think about ACD or allergic contact dermatitis a bit more. Obviously, if someone comes in saying they were in contact with some product that resulted in this, that's fairly easy to recognize, or if they're working at a job that potentially is now causing them sensitization.

So you really want to get that good history about what different things they've been in contact with. But unfortunately, sometimes this can be a chronic thing where they develop it after years of exposure. So even that is not fully foolproof in making the diagnosis.

Dr. Mariam Hanna

So I see a lot of patients that have recurrent perioral dermatitis or periorbital dermatitis. You're saying all of these should at some point be considered to have potentially contact dermatitis, allergic contact dermatitis. Did I catch that right?

Dr. Rebecca Pratt

I think they should be considered, yes. Now, obviously, atopic dermatitis eczema can present in these areas. And especially when they're younger kids and they're lip-licking or they're eating messily, of course, that could just be more of an irritant or atopic dermatitis.

But yeah, I think it is definitely worth keeping in mind those certain distributions that may be a hint that it could be more of an allergic contact dermatitis.

Dr. Mariam Hanna

OK. And is there a particular population that is most at risk for allergic contact dermatitis?

Dr. Rebecca Pratt

So it's tough because there's conflicting evidence in the literature that people with atopic dermatitis are more at risk. So you would think it would make sense because they have that impaired skin barrier. But really, in studies, it's not clear on that.

So I think you've got that right immune system coupled with people being exposed more frequently to some allergen than other people. And that just makes them more at risk. So, for example, they looked at people that have more piercings.

And the more piercings you have, the more likely you are to be nickel allergic. Certain industries, if you're in the dental industry, greenhouse, spa, hairdressers may be more at risk just because of their occupations and being exposed to allergens more frequently. But, you know, you really have to have that clinical suspicion with anyone who presents, like I said, with a dermatitis.

Dr. Mariam Hanna

OK. And then it's been peculiar, but every year or two, we have like the most common trigger for the year. And it's always like a topical of interest thing to know what's the most common contact dermatitis allergen of the year.

Do you happen to know what this year's most common allergen is? And is there a rhyme or reason to what it is that's being reported here?

Dr. Rebecca Pratt

OK, yes, I did go to the meeting in March in Orlando. It is toluene 2,5-diamine-sulfite this year. And just to be clear, this actually isn't the most common allergen that they found that year or anything.

It's actually more, they just every year pick an allergen of the year to raise awareness of that allergen. And that's why this was picked.

But it's by no means the most common contact allergen. So toluene is a PPD alternative, so paraphenylenediamine alternative in hair dyes. And so they really just wanted to raise awareness about this allergen.

So that's why they picked it. But in fact, nickel has been the most common contact allergy for years upon years ever since I started practice. So probably before that.

And again, probably because of the jewelry sensitization, females are more likely to have this than males. But the usual suspects always pop up on the top 10 list, fragrances, preservatives, again, PPD and hair dye, topical antibiotics, etc. So those you see year after year.

So that yearly one, yes, is simply just them picking one to raise awareness.

Dr. Mariam Hanna

My awareness has been raised. And thank you for clearing that up. I was always curious why it changed each year.

Dr. Rebecca Pratt

No worries.

Dr. Mariam Hanna

OK, just a sidebar here. If most of the patients that we're going to patch test have irritant versus allergic contact dermatitis, does that matter? Or is there a way of telling them that this is only an irritant dermatitis versus atopic dermatitis versus allergic contact dermatitis?

Dr. Rebecca Pratt

Yeah, I mean, I do grade the severity of the reaction when they have the patch test. And the grading system does show questionable, I say plus minus result versus irritant reaction. We say IR versus 1 plus, 2 plus, 3 plus, with 3 plus being like a severe vasicular or sometimes bullous reaction.

So you do grade it with irritant reaction as well. But I think the issue is that not all irritants are going to be on there by any means. So, for example, people who are bartenders do a lot of wet work and their hands are always soaking wet.

And that can be an irritant in itself. And it's not an allergen, right? So you're not going to catch all the irritants on the panel.

But yes, we do grade some of them as an irritant reaction if there's no palpable kind of infiltration there. But yes, that you will not have all the irritants on the patch test panel.

Dr. Mariam Hanna

And to grade an irritant, that's done immediately upon removal of the patch? Or is it more as part of your like 48 or 72 hour read?

Dr. Rebecca Pratt

It's the 48 and 72. So essentially how we do the patch testing is they come in my clinic on a Monday. I apply the 80 most common allergens on the back in the form of stickers.

They can choose if they want just the paper-based patches or waterproof patches. And then 48 hours later, they have their first read day. And at that point, I will look at the back and grade some reactions.

But the more useful read day where more allergens would develop would be at the 72 to 96 hour time point. So some allergists would do the day three read. Some would do day four read.

Some even bring them back on the next Monday and do a day seven read. So I think obviously the more read days, the much more labor intensive for both patient and clinician. But yeah, all of those are feasible read days actually.

Dr. Mariam Hanna

Okay, interesting. And because this is like a topical type reaction, are there any systemic medications or conditions that would interfere with being able to read your patch test accurately?

Dr. Rebecca Pratt

Yes, for sure. So first of all, topical agents. So any of the creams we use generally for eczema, like topical corticosteroids or topical immunosuppressants, should be avoided at least a week on that site.

They can use it in other sites, but not on the back. And then any systemic agents we would use again for dermatitis, like systemic corticosteroids, ideally no more than 10 max 20 milligrams. We'd really want them to hold that higher doses for about a week.

Any other immunosuppressants, some data conflicting that like methotrexate, cyclosporine, et cetera, could blunt the patch response. And then same with kind of JAK inhibitors or biologic agents. There’s I'd say conflicting data, but we'd ideally want to hold those.

And then UV light is a big one. So someone just came back from vacation, their skin is very tanned, or if they're going through UV light therapy, that really can change the effect of the patch test results. So you would really want them to hold that for a few weeks, ideally before.

But it's all a patient discussion, because of course, if someone has a severe dermatitis, they may not be able to get off their immunosuppressant or their biologic to do the patch test. And then again, I would still do it, but I obviously counsel that we may miss some allergens if we do it on immunosuppressants. So if there's that option to go off, that's ideal.

Dr. Mariam Hanna

And what about for your pediatric population? Is there a special panel for those guys with their patch testing?

Dr. Rebecca Pratt

Okay. So very much, you can offer patch testing to kids. Sometimes I hear this myth that there's a certain age you can and cannot do it.

I think it'd be very rare for a baby or something to have contact dermatitis. I personally haven't done it, I think, in age probably three and under, but I've done four to five-year-olds and have seen multiple sensitizations. Of course, you're not offering this to everyone.

You're not offering this to people with very classic atopic dermatitis, which would be much more common in that age group. But for some patients, it is definitely worthwhile to do. From a logistical standpoint, just because of the lack of room, doing the full panel of 80, you have to somewhat maneuver where you're going to put the patches.

So that can be hard on really small kids. But also, they wiggle a lot, and they have jutting scapula. So I do recommend the waterproof.They tend to stay better in the little ones.

You want to be aware of certain sensitizers like PPD and acrylates that potentially could, you know, with time, induce that allergen in kids. So sometimes we remove those from the panel.

But yes, I still do the same panel in the kids as I do in the adults. I don't have a specific child panel. The sensitization profile hasn't shown to be different in kids.

It's not like they have all different allergens than adults. So yeah, it's the same panel.

Dr. Mariam Hanna

Okay. And what are some most common challenges or pitfalls in interpreting your patch test results?

Dr. Rebecca Pratt

Yeah. So there is a somewhat high false negative rate of up to 30%. And again, maybe part of that is that not doing the late phase readings and looking for those late phase reactions.

I think that you have to have a high level of training to do patch testing in the sense of even certain trainees or even patients will go, what's that I'm reacting to? And when I look at it, it's just the dye. So if you don't know which ones can just discolor the skin, you could potentially be calling it a reaction when it isn't.

And so then you could be obviously over calling. The patch testing can peel. And so results can, of course, be compromised if they aren't on the back for the full 48 hours.

So really counseling patients to not get the patches wet if they're the non-waterproof patches and to just take it easy. If they do see any peeling immediately, tape it back up. As well, there's something called angry back syndrome where patients can actually get a strong reaction around one particular allergen.

And because of that, it just irritates the whole area, if you will. So all the surrounding allergens just really look positive as well. But it may just be because of that one allergen.

So if you're doing it properly, then if that happens where all of them around one particularly angry reaction react, you really should probably separate that panel and repeat to know that you're not over-calling other multiple other ones.

Dr. Mariam Hanna

Interesting. And in whom do you specifically do a delayed read or suggest with like phototherapy or UV exposure to do their patch testing? Who are those guys that get that?

Dr. Rebecca Pratt

Okay, so certain allergens certainly are the ones that we know can react late, like topical corticosteroids or some antibiotics. So if they do have that history of reacting to actually creams you would prescribe, it may be worth getting them back in on the Monday to read the panel again. Like I said, I already have complaints, people coming in three days in the week, let alone if I got them to come back four or five times.

So I think it is logistically tough and I don't do it a lot. I'm more likely to tell the patient that if they could come, if they could take pictures of the back and call me or come in, if they notice a different reaction occurring, like just say I've marked them as negative, but they notice a square shaped dermatitis developing on their back, they take pictures and I will locate which one it is and tell them. I think that can help as well.

But of course, you have to have someone that's motivated to do that. But yeah, certain allergens we know can be late. And so if you're suspecting those, you could ask them to come in.

But if they're only getting the dermatitis when they're on vacation, and maybe not light alone does it, but if they're in the sun, plus they have a sunscreen on, they're getting the reaction. But if either of them are alone, they don't, that would be where I'd suspect photodermatitis and consider sending to your tertiary center for testing for that.

Dr. Mariam Hanna

Perfect. I had this interesting experience during training, where at some point in our journey of managing EOE or understanding how to manage eosinophilic esophagitis, there was non-standardized food patch testing that was being done. Is that at all useful?

Dr. Rebecca Pratt

Yeah, I agree. I've seen it more for that kind of presentation, the eGIDS, eosinophilic GI diseases, as opposed to for contact dermatitis to foods or anything like that.

Everything I've read on it, just the data is not clear that it's very useful, so I don't do that. It's also labor intensive to prepare the patches with foods.

Like how we used to skin test for EOE, and it's really not showing to be very beneficial. I go down the same path with patch testing for that. Same with medications.

Like for some of these delayed drug eruptions, people are doing patch testing for medications. And again, you'll find some data on that, but it's tough because it's, again, not standardized. So I think we need better data on that to say for sure.

Dr. Mariam Hanna

Perfect. Okay, so you've gone through the process of considering allergic contact dermatitis. We've tested the patient in the appropriate manner.

Hopefully, they're not the 80% that was an irritant or the 30% that got a false negative, and you actually have a positive result. How do you end up counseling the patient, especially when it involves like a substance that's hard to avoid?

Dr. Rebecca Pratt

Yeah, it can be tough. Sometimes I can hardly pronounce the names, let alone counsel them on how to avoid it. It's everywhere.

So I'm a member of the American Contact Dermatitis Society, and I highly recommend, if you patch test, to be a member because they supply us with a lot of high-yield handouts. So not just the quick two-line sentence of what the allergen could be called otherwise, because that's another problem with these allergens. It's not just one name.

A lot of them have 5, 10, 20 names for that same chemical. So good luck reading labels, etc., with that kind of confusing picture. So yeah, the American Contact Dermatitis Society has nice handouts on a lot of the key allergens that really tell you where the product can be found and potential alternatives of what else you could use without it.

But the most useful thing about it is it also generates a safe list that we could put in the patient's allergens, and then it emails them this safe list of 150-plus pages of products that don't contain their allergens. So instead of having to be, like I said, reading every label, especially if you have multiple allergens, it could be a full-time job trying to work out what products are safe. Instead, they can get that safe list and even download an app on their phone, and then when they're in the shop, they can just pick products on that safe list.

So that's usually what I tell them about. The other thing is, to make it even more confusing, is some substances we test for, including nickel and preservatives and some naturally occurring chemicals, are found in foods. And so then, you know, you are counseling potentially on food avoidance, because that could cause a systemic eruption of their dermatitis as well.

It's hard to study, and I think it varies in people if that's going to help or not. Certainly the very mild allergics, the questionable reactions or the irritant or 1-pluses, I'm less strong about the dietary avoidance. But for your 2-3-pluses, it can actually make a difference depending on their distribution.

So it's something to consider. But the patient has to be motivated, and certainly the dermatitis has to be severe enough to warrant that avoidance.

Dr. Mariam Hanna

I have many patients that would quickly jump on, so you're saying that there is a food that is causing my dermatitis. What are some examples of culprit foods that are found here? 

Dr. Rebecca Pratt

Yes, because patients all the time say to me, well, are you testing for foods? I think I'm allergic to foods. And this is how I word it.

So I say, you're not allergic to a food in the sense of anaphylaxis to foods. That's when we would skin test you and in 15 minutes be able to tell you if you're allergic, if you had the right symptomatology, but you don't. You're having dermatitis.

You're not having anaphylaxis. So you're not in the sense, as we know it, an allergy to food. You're not allergic to a food.

But there's substances within foods that can cause dermatitis. And that's what we're testing for on the patch test panel. So there's no dairy or peanut on the patch test panel.

What there is nickel, balsam of Peru, like I said, certain preservatives that are added to foods that then would be in foods that they can avoid. So again, like my handouts have a list of foods that nickel so I'm using nickel because it's by far the most contact allergen I see. So nickel, huge list of foods that nickel is in.

So again, if the person had a dermatitis once since never come back, am I really going to suggest they avoid all these nickel-containing foods? There's tons of food that nickel is in, including the good stuff like chocolate and wheats and coffee and tea, etc. So to have someone overhaul their life like that, I better have a really good reason to do that.

But in your people that have systemic dermatitis, and it's really affecting their quality of life, to trial a nickel-free diet for four to six weeks? Absolutely, especially if topical avoidance doesn't help. That's what I usually start with is topical avoidance, then we go to the food avoidance.

But yes, it's not a specific food. It's the allergen that's found in food that we're testing for.

Dr. Mariam Hanna

Very interesting. And I like the very practical approach. So now I'm going to pick on your practical approach with my quick curbside consult, Dr. Pratt. Do you have a recommendation for the right sunscreen question that I always end up with in clinic?

Dr. Rebecca Pratt

Yes. Well, maybe not. So the sad fact of the matter is, as usual, there's no one-size-fits-all for most patients.

So if they really want something that's the most likely substance that isn't going to irritate or cause allergy, it would be the physical UV blockers like titanium dioxide or zinc oxide. So, you know, those ones that you look like a ghost and very white when you put them on. And I think they're getting better at formulating some of those.

But yes, those really haven't shown to be allergenic. Once you get into the easier sunscreens that are more palatable to actually wear, it gets tricky because people can be allergic to the actual chemical in sunscreens, the oxybenzone, but they can also be allergic to a lot of the additives in sunscreen like fragrances and preservatives. So all I can say about those ones, if you're not going to use physical blockers, is to keep it with the kind of fragrance-free, hypoallergenic, tried-and-true ones like we talk about a lot in allergy and dermatology.

But nothing is a perfect science, and it is to some degree trial and error to find what you tolerate.

Dr. Mariam Hanna

An imperfect science filled with trial and error. I like that. That's a perfect way to wrap up.

Okay, time to ask today's allergist, Dr. Rebecca Pratt, for her top three key messages to impart to patients and physicians on today's topic, contact dermatitis. Dr. Pratt, over to you.

Dr. Rebecca Pratt

Okay, so my key messages would be, number one, I think have a high clinical suspicion for contact allergens or irritants in anyone presenting with eczematous  rash. And remember that these sensitizations can occur years into a job or exposure. So it is important to get a really good history, including hobbies and occupations, and not just if they came into something new, in contact with something new within a week or two or something.

Number two is patch testing can be very helpful and satisfying to both us as clinicians and the patients if we find an allergen or allergens that are causal. But I also really think it's important, if it's negative, to rule out this as a differential diagnosis. So I always try saying that to patients if they're disappointed when they don't have a bunch of allergens come up, is at least we've ruled this out.

And number three is there's no cure for contact allergies. The treatment really is avoidance at this point. But it is nice to put some control back to the patient instead of just saying you're on this chronic eczema cream for the rest of your life.

They have some control about what they can avoid to hopefully help their skin.

Dr. Mariam Hanna

Perfect. Thank you, Dr. Pratt, for joining us on today's episode of The Allergist. 

Dr. Rebecca Pratt

Thanks so much for having me, Miriam.

Dr. Mariam Hanna

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