What does it mean when all of a patient's patch tests come back negative?
Clinical Applications
Dr. Schwarzenberger is the former physician editor of DermWorld. She interviews the author of a recent study each month.
By Kathryn Schwarzenberger, MD, September 2, 2019
In this month’s Clinical Applications column, Physician Editor Kathryn Schwarzenberger, MD, talks with Erin Warshaw, MD, MS, about her recent JAAD article, “Patients with negative patch tests: Retrospective analysis of North American Contact Dermatitis Group (NACDG) data 2001-2016.”
Dr. Schwarzenberger: You and your colleagues in the North American Contact Dermatitis Group (NACDG) recently published an analysis of patients whom you patch tested with negative results. Can you briefly summarize your results here?
Dr. Warshaw: Much has been written and studied about patients with contact allergies. We were interested in describing the population who undergo patch testing but have no relevant positive reactions. We used the NACDG database of 34,822 patch-tested patients in the U.S. and Canada and found that almost one-third (n=10,888, 31.3%) of patients had negative results.
As compared to patients with positive patch tests, patients with negative patch tests were significantly more likely to be male, aged 40 years, non-Caucasian, and/or have dermatitis primarily involving the following body sites: scattered generalized, lips, or eyelids. The absolute differences in age, race, and site, however, were small so they may not be clinically meaningful.
Patients with negative patch tests were also significantly less likely to have occupationally related skin disease as compared to those with positive reactions. Of the 8.3% of patients with negative patch test reactions who had occupationally related skin disease, precision production workers/machine operators (28.5%), health care workers (17.0%), and mechanics/repairers (7.5%) were the most common occupations. About 23% (22.9%) of negative patch test patients had relevant irritants and 41.6% of irritants were occupationally related; cosmetics/health care products and soaps were common sources for both occupationally related and non-occupationally related irritants.
Dr. Schwarzenberger: Who is the NACDG and do you feel these results are likely to be similar to those of the “average” dermatologist who may test with only the T.R.U.E Test®?
Dr. Warshaw: The NACDG is currently comprised of 17 dermatologists (three in Canada and 14 in the U.S.). These members are considered experts in the field of contact dermatitis so they often serve as tertiary referral specialists. Many patients referred to this group have had previous patch testing with the T.R.U.E. Test or other series. Thus, these patients are not representative of the general dermatology population.
Dr. Schwarzenberger: Can you use statistics like these to know if you are testing the right patients? If all your patch test patients have a positive result, should you worry that you are not testing enough patients?
Dr. Warshaw: While not calculated on any statistical formula, it is generally thought that if you have more than 40% negative results, you are probably testing too many patients. If you have less than 20% negative results, you are probably not testing enough patients. In other words, the “sweet spot” is probably about two-thirds positive.
Dr. Schwarzenberger: If a patient with negative patch test results doesn’t have allergic contact dermatitis, what do they have?
Dr. Warshaw: Our group collects information on 12 specific diagnoses. In the negative patch test group, most had “other dermatitis” (other than atopic, stasis, nummular, seborrheic) (28.1%, exact diagnosis not collected but would include conditions such as lichen simplex chronicus, prurigo nodularis, dermatitis of senesensce/ “itchy red bump disease,” hyperkeratotic hand dermatitis, drug rash, etc.); atopic dermatitis (18.6%); irritant contact dermatitis (18.0%); psoriasis (6.0%); “other dermatoses” (8.4%, exact diagnosis not collected but could include papulosquamous conditions such as bullous pemphigoid, lichen planus, cutaneous T-cell lymphoma); seborrheic dermatitis (3.2%); nummular eczema (3.0%); photodermatitis (1.2%); pompholyx (1.2%); stasis dermatitis (0.9%); or contact urticaria (0.9%).
Could we be missing allergies by not testing with the right allergens? Absolutely. Most members of the NACDG test with additional panels (depending on exposures) as well as patients’ products. Testing with these pick up rare allergens. About 20% of individuals tested by the NACDG have reactions to allergens/products not picked up by the NACDG screening series. This underscores the importance of testing to a variety of (and many!) allergens as well as patients’ products.
Dr. Schwarzenberger: This study highlights the importance of knowing about your patient’s occupation when managing dermatitis. Based on your experience and this study, are there specific occupations that you feel are high risk for having allergic or irritant dermatitis?
Dr. Warshaw: This study focused on patients with negative patch test reactions. In my experience, the occupations at highest risk for contact dermatitis are those that involve “wet work.” These include health care workers, food service workers, cleaning occupations, and hairdressers. Many precision production workers/machine operators and mechanics/repairers are also exposed to many solvents and coolants which can contain allergens as well as act as irritants. Construction workers are also at risk.
Could we be doing more as a profession to help minimize these issues? We could always do more. In Europe, dermatologists are much more involved with regulatory processes. Canada is more proactive than the U.S. in addressing occupational concerns. If I had to pick one occupation as incredibly high risk for ACD, it would be nail salon technicians. Many do not have employer-supplied health insurance, receive minimal training in protections from acrylate monomers (disposable gloves provide little protection), and once sensitized, often have to stop working in the field. This group is hard to reach as most salons are independently owned and there is no centralized training.
Dr. Schwarzenberger: Anyone who does patch testing knows how difficult it is when you patch test a patient with suspected allergic contact dermatitis, but the results are negative. How will the results of your study help us manage these patients?
Dr. Warshaw: For so many dermatologic conditions, biopsy is our main diagnostic tool. For most forms of dermatitis, however, histopathology is similar. Patch testing is a critical tool for differentiating these conditions. Even if patch testing is negative, it is incredibly helpful in managing patients’ skin disease. The dermatologist can “move on” to other tests and consider other diagnoses. For recalcitrant dermatitis, an excellent review on systemic treatments was recently published by Sung, et al (Dermatitis. 2019; 30(1): 46-53). For patients who undergo patch testing with negative results, this study provides “reassurance” that they are not alone.
Dr. Warshaw is a professor in the Department of Dermatology at the University of Minnesota medical school and is a staff dermatologist at the Minneapolis VA Health Care System. She is also co-director of the Contact Dermatitis Clinic at Park Nicollet and runs the dermatology clinical research unit at the Minneapolis VA. She has served as president of the American Contact Dermatitis Society from 2007-2009 and is a member of the North American Contact Dermatitis Group. She is on the advisory board for Dermatitis and editorial board for Contact Dermatitis. Dr. Warshaw has no relevant financial or commercial interests to disclose. The article appeared in JAAD. doi: 10.1016/j.jaad.2018.12.062.
