Nailing the Patch Test: The Basics

In the Nailing the Patch Test podcast series, Jonathan Silverberg, MD, will discuss various aspects of patch testing, diagnosing allergens, and more. In this episode, he reviews the some of the basics of patch testing.

For information about the George Washington School of Medicine Patch Testing Service, and to refer patients or schedule an appointment, please visit the GW website or call (202) 741-2610.

Dr Silverberg is an associate professor of dermatology and director of the patch testing clinic at George Washington University School of Medicine and Health Sciences in Washington, DC.

Transcript

Hello. I’m Dr Jonathan Silverberg. I’m an Associate Professor of Dermatology and Director of the Patch Testing Clinic at George Washington University School of Medicine and Health Sciences.

I run the patch testing service at GW. We test for allergic contact dermatitis, photosensitive disorders, and photoallergic contact dermatitis. We are located at 2150 Pennsylvania Avenue NW, in Washington, DC, on the second floor. To schedule a patch testing or a patch testing consult, patients can directly call (202) 741-2610. That’s (202) 741-2610.

In our first episode of the “Nailing the Patch Test” podcast series in collaboration with The Dermatologist, I will discuss the basics of patch testing. There are several different series used for patch testing throughout the United States.

The most commonly used series would be the T.R.U.E. test, which is one of the only FDA-approved testing approaches available. It tests a relatively limited number of patches. Often, it’s reported in the literature that it may not pick up the full scope or the full breadth of allergens that are relevant across different subsets of patients with contact dermatitis.

There are a number of expanded patch testing series. The two most commonly used ones would be the American Contact Dermatitis Series as well as the North American Contact Dermatitis Group series.

There are slight differences between them. The ACDS series is a little bit more in line with the standard allergens that are known to be relevant in current times, whereas the North American Contact Dermatitis Group has all of those common culprits but also includes several cutting-edge allergens in order to investigate which allergens may be emerging in the field.

I personally use the North American Contact Dermatitis Group, as I’m part of the North American Contact Dermatitis Group’s consortium. I use their series. There are a number of important allergens to keep in mind that are included within the series.

Certainly, the most common allergen that we often see would be nickel, but it’s not all about nickel. Nickel is a very common allergen that comes up, but there are many other ones that are almost as common or maybe even more common now as we’re starting to see changing trends throughout North America.

In particular, we are seeing a lot more relevance and a lot more positivity to methylisothiazolinone and methylchloroisothiazolinone. We test in those expanded series to both the combination of those two and a higher concentration of methylisothiazolinone alone. One of those would actually be missed by some of the less-expanded series, like the T.R.U.E. test.

There’s also a number of other allergen classes that are included, including fragrances, preservatives, surfactants, and a variety of other ingredient classes as well which are common culprits throughout North America.

There’s no one-size-fits-all answer as to who is the right candidate for patch testing or exactly who should be referred for patch testing, but there are a number of very important scenarios to keep in mind.

Certainly, if a patient reports that they have dermatitis that worsens with exposure to personal care products or some other occupational exposures that may come up, based on that history, we know that something is triggering the dermatitis, but we’re perhaps not sure what the specific ingredient might be or which specific trigger it might be, patch testing definitely would make sense in order to identify which ingredients are causing trouble and causing their dermatitis.

There are many classic scenarios in terms of the lesional distribution and morphology that come up, that make us think about contact dermatitis, particularly when there’s a more localized distribution. When there might be a more asymmetric or unilateral distribution, we might think about scenarios for patch testing.

Specifically within localized distributions, there are some key distributions that we think about, including eyelid dermatitis would be a very common area with relevant contact allergens. Hand dermatitis is another one that has very high rates of positive patch testing.

We see this also even in terms of things like localized scalp dermatitis. There are certainly many other things that can cause scalp dermatitis, but contact dermatitis is up there. We do have to think about that in certain select scenarios. Those are just a few of the different scenarios that come up.

Another common scenario that comes up would be that refractory atopic dermatitis patient. We certainly don’t recommend patch testing every atopic dermatitis patient.

Ones that have perhaps an atypical course, ones that have more refractory disease that’s just not responding appropriately to therapy or we might consider even stepping them up to a systemic or biologic therapy, our consensus-based guidelines recommend that patients be patched tested, if feasible, prior to stepping up therapy.

Those are just a few of the different scenarios where patients who would be good candidates for patch testing based on distribution and clinical history. There’s also one other notable subset that I see quite a bit, which is the subset of nummular eczema or nummular dermatitis.

Nummular dermatitis is a very mixed bag, but there is a subset, a large subset, of nummular dermatitis that is reported to be attributable to contact allergens. For patients with generalized and tougher-to-treat nummular eczema, patch testing should definitely be considered to examine for a variety of different potential allergen classes.

There are a number of factors that can influence the results of patch testing. There are certain key rules that we want to think about. I sort of break it down to patients into three broad categories or check marks to think about.

One is we want the back to be clear in terms of, as I explain it to patients, we want to know that we caused that little red square or circle on the back and that it wasn’t from a pre-existing dermatitis that was there.

Two is we want to make sure that if they have a generalized dermatitis, particularly if they have underlying atopic dermatitis, that it’s relatively well-controlled. The older literature has taught us that you really want to get the body surface area down to less than 10% of active lesions. Otherwise, we run the risk of getting both false positives and false negative lesions.

Third is to make sure that we don’t have any contraindicated medications that might alter or impact the success of patch testing. One, of course, would be topical steroids or calcineurin inhibitors or potentially even phosphodiesterase E4 inhibitors that we know a little less about how they might impact patch testing.

But the general rule of thumb is while patients can use topical steroids during the patch testing process on other parts of the body that might be affected with dermatitis, we do not want them to apply topical corticosteroids or calcineurin inhibitors, for at least one week, to the back or other sites where the patch test may be placed.

When it comes to systemic steroids and systemic immunosuppressants, ideally, we would like patients to be off of those systemic therapies for a solid three to four weeks, preferably even more than four weeks, before undergoing patch testing because we know these drugs can linger in patient’s system and cause false negative reactions. That is something we definitely want to think about.

Sometimes, it’s not feasible to get them clear and off of all of those therapies. These are often tougher-to-treat patients, which is why we’re thinking about patch testing them in the first place.

There are some sort of dispensations or loopholes that have been attempted to say, “Well, if you use really low doses of systemic steroids, less than 20 mg, less than 10 mg, that you might improve your chances of getting a successful positive reaction solicited,” but we still know even there there’s a possibility of getting false negatives.

Similarly, with certain immunosuppressants, like cyclosporine or methotrexate, there’s a little bit of controversy in the field. My experience has been that they do suppress positive patch test reaction and then you get false negatives, but there are some reports in the literature of successful patch testing eliciting positive reactions while on those tests.

I think for the clinician, it’s really important to try to recognize that. Oftentimes, I’ll get referrals for patch testing in a patient who’s now just starting a three-week course of systemic steroids. At that point, we’re going to have to now wait, ideally, probably somewhere around seven weeks till we get them in for patch testing.

That can sometimes be frustrating for patients because it delays their ability to understand what the underlying cause of their disease might be, but I think it’s also just making sure that when patch testing is done, it’s done under the right circumstances.

You really don’t want to slap patches on the back of someone that the back is totally inflamed. Because when that happens, you can get what’s called angry back syndrome. You get false positives. You get false negatives.

On top of that, the patient is incredibly uncomfortable during the patch testing process because they’re so itchy and painful from the adhesive and the patch tests themselves just flaring up the back. These are just a few considerations to think about to really mitigate some of these concerns around patch testing.

One of the toughest scenarios in the realm of patch testing is how to interpret weak reactions. Now, what do I mean by that? Well, we score patch testing. We read patch testing as being 1+, 2+, or 3+ reactions. The stronger the reaction, the more likely it’s going to be a true positive and, oftentimes, relevance.

The strength of the reaction doesn’t always determine relevance because patient may not actually be getting exposed. We see that very commonly for allergens like nickel, where we get strong positives all the time because it is relevant to some extent, but it may not be really relevant to their active problems.

Then we run into this other issue of what do we do when we get these weak reactions, these plus/minus reactions, where you may see more macular erythema. Or you may see a little bit of papulation that happens at the site of the patch test, but it doesn’t really fill in the full site of the patch test and wouldn’t really qualify as a full 1+ reaction.

In those scenarios, it gets a little iffy. You really have to deal with that on a case-by-case basis. One, you need to pay attention to what allergen you’re looking at. Some allergens are notoriously marginal irritants and can cause these kinds of weak reactions, for example, fragrances.

When we see weak reactions to fragrances, we don’t automatically discount them, but we also take it with a grain of salt and don’t just assume that that’s a true relevant positive reaction. On the flip side, there are other allergens which, in certain scenarios, notoriously give weaker-than-expected reactions.

One that I see very commonly is lanolin reactions. Lanolin as wool alcohol tends to be weaker than expected. You often see these very weak reactions, particularly in the atopic dermatitis patients or the other chronic dermatitis patients. Yet, it’s almost always relevant even though you don’t see those really strong reactions there.

You really need to pay attention to the context. You really need to understand the specific allergens. You need to do it often enough that you really have those patterns sort of nailed down to be comfortable recognizing when is it a true positive, when is it not, but this is something you will have to do on a sort of more individual or a case-by-case basis.

Now, why does it matter? It matters because we don’t want to give patients false expectations. We don’t want to tell them every single weak reaction, that’s a true positive. Then one, they go home. They throw out everything in their cabinets. They spend thousands of dollars buying whole new set of products.

It turns out that those were all irritant reactions that had no chance of really being positives and weren’t going to change anything. The patient is frustrated. They spent all this money. They changed all the products up. It’s done nothing.

Worse, you may have withheld stronger treatments to get them better when, in fact, chasing after allergens that weren’t really allergens. I think it’s really important to have a balanced approach and not over-call and of course not also under-calling reactions

I think one important major takeaway that I would like the audience to really think about is patch testing is as much an art as it is a science. As much as we have very standardized approaches for how to read patch tests, really, the skill comes from that fluency, from that experience of doing it over and over again, as you get when you’re running a patch testing service.

One has to be very careful about dabbling, so to speak, with patch testing. We know that there aren’t enough expert patch testers around the country to see all of the patients that need to be patch tested.

Using simplified testing like the T.R.U.E. test really can have its role in terms of improving access, but also recognize there’s major limitations. In many specific scenarios, using something like a T.R.U.E. test would completely miss perhaps the majority of relevant allergens that can come up in some of these select scenarios.

As you approach patients, don’t approach them with a one-size-fits-all solution. Really understand the literature and the nuances of the allergen profiles that go with the different clinical scenarios and use that to better select which expanded series you might use, which supplemental series you might use.

Recognize that even when you’re using something like the North America Contact Dermatitis Group series or an ACDS series, even that doesn’t pick up all positives. That might pick up maybe about 60 to 80% of positives, depending on the study.

You may still need to supplement with even more in terms of supplemental fragrance series or a cosmetic series or a rubber series, depending on the clinical context. Really, tailor the patch testing to the needs of the patient.

Thank you very much for listening. If you have any questions or comments, please submit them in the feedback box below.