The epicutaneous patch test is the definitive technique for diagnosing allergic contact dermatitis.[1] It is also the test most likely to lose information between visits: the patches go on one day, come off two days later, and the result that matters may only appear at the delayed reading. This page follows the 2015 AAAAI/ACAAI contact dermatitis parameter.[1]
Series and application
- Use a core or baseline series plus supplemental series based on the patient's exposures and the patient's own products.[1]
- Relying only on a preloaded 36-chamber kit, rather than an extended NACDG or ACDS series plus personal products, can miss clinically relevant sensitizers.[1]
- The ACDS Core Series was introduced in 2013 and updated in 2017 and 2020; the 2020 update added allergens including linalool, limonene, carmine and benzyl salicylate.[2]
- Patches traditionally stay occluded for 48 hours on the upper or middle back, with an appropriate vehicle control.[3]
The record should name the series and every allergen with its concentration and vehicle, the application date and time, and who applied it.
The reading schedule
- Remove and read at about 48 hours, and do a second reading between days 3 and 7.[1]
- Wait about 30 minutes after removal before reading, so erythema from occlusion or tape can settle.[1]
- About 30% of relevant allergens that are negative at 48 hours become positive at 96 hours, which suggests 96 hours may be the best time for the second reading.[1]
- Metals, some antibiotics and topical corticosteroids may need a late reading after day 7.[1]
- Irritant reactions seen at 48 hours tend to fade by 96 hours (the decrescendo pattern).[1]
If the patient cannot make three visits, the parameter describes removal at 48 hours by the patient or a local physician, with photos before and after, and a reading at 72 to 96 hours by the treating physician, an approach it calls "suboptimal".[1]
ICDRG grades
The 2015 parameter says to read and interpret per the ICDRG scale and to record the grade at each time point:[1]
| Grade | Meaning |
|---|---|
| − | Negative reaction |
| ?+ | Doubtful reaction, faint erythema only |
| + | Weak positive: nonvesicular erythema, infiltration, possibly papules |
| ++ | Strong positive: vesicular erythema, infiltration and papules |
| +++ | Extreme positive: intense erythema and infiltration, coalescing vesicles, bullous reaction |
| IR | Irritant reaction |
| NT | Not tested |
Most misinterpretation happens in the doubtful (?+) and weak positive (+) categories, and a weak reaction at day 7 is more likely to be clinically relevant than one at day 3.[1] The grade at each reading is the clinician's call, recorded at that visit.
Relevance and pitfalls
- Determine the relevance of each positive from the clinical and exposure history: current relevance is definite, probable or possible; past relevance applies to earlier exposures.[1]
- Pustular reactions, common with metals and in atopic patients, are frequently irritant and should not be read as positive.[1]
- Suspect "angry back" when more than 5 reactions sit close together.[1]
- Avoid topical corticosteroids, calcineurin inhibitors and UV exposure at the test site; antihistamines do not need to be withheld for patch testing.[1]
Frequently asked questions
When are patch tests read?
Per the 2015 contact dermatitis parameter: remove and read at about 48 hours, then do a second reading between days 3 and 7. About 30% of relevant allergens negative at 48 hours become positive at 96 hours.
What is the ICDRG patch test grading scale?
Negative (−), doubtful (?+), weak positive (+), strong positive (++), extreme positive (+++), irritant reaction (IR) and not tested (NT). The 2015 parameter says to read and interpret per ICDRG.
What should a patch test record include?
The series and each allergen with concentration and vehicle, the application date, the grade at each reading on the ICDRG scale, and the relevance of each positive. The 2015 parameter includes a sample form.
Related guides
Sources
Last reviewed October 21, 2026. Summaries paraphrase the cited documents; read the originals before making clinical decisions. This page is for clinicians and practice teams, not patient instruction.
- Fonacier L, Bernstein DI, Pacheco K, et al. Contact dermatitis: a practice parameter—update 2015. J Allergy Clin Immunol Pract. 2015;3(3 Suppl):S1–S39. doi.org
- Schalock PC, Dunnick CA, Nedorost S, et al. American Contact Dermatitis Society Core Allergen Series: 2020 Update. Dermatitis. 2020;31(5):279–282. pubmed.ncbi.nlm.nih.gov
- Bernstein IL, Li JT, Bernstein DI, et al. Allergy diagnostic testing: an updated practice parameter. Ann Allergy Asthma Immunol. 2008;100(3 Suppl 3):S1–S148. www.aaaai.org