Patch Testing Technique and Interpretation
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Direct answer
Patch testing is a delayed-type hypersensitivity test that identifies allergic contact dermatitis by applying suspected allergens to the back for 48 hours. The allergens are placed in petrolatum on Finn chambers, and the skin is read after removal and again at 72 to 96 hours. Reactions are graded with the ICDRG scale, and a positive result matters only when it fits the patient's exposure history.
What you must remember
- Vehicle and device: allergens in petrolatum loaded into Finn chambers mounted on Scanpor tape, applied to the upper back.
- Duration: the patches stay on for 48 hours, and the first reading follows their removal.
- Later readings: a further reading at 72 to 96 hours catches delayed reactions, and some late reactions need a day-7 reading.
- ICDRG grading: ? doubtful, + weak, ++ strong, +++ extreme; irritant reactions are recorded as IR, not as allergy.
- Standard series: a set of common screening allergens, supplemented with materials based on the patient's own exposures.
- Angry back: an exaggerated reaction across the back, often provoked by sweating, heat or heavy occlusion, which can cause false positives.
- Interfering factors: systemic corticosteroids and topical steroids on the back can suppress reactions, so timing and medications must be recorded.
A stepwise approach from history to interpretation
Patch testing is only as good as the allergen selection and the reading, so each stage matters. Work through the stages in order, and record the readings at each time point.
- Take the exposure history: list the occupational, cosmetic and drug exposures before choosing allergens.
- Select allergens: use the standard series and add materials that match the patient's exposures.
- Apply the patches: place each allergen on Finn chambers on Scanpor tape, marked for the correct side of the back.
- Remove at 48 hours: take off the patches, mark the sites and read after about 30 minutes so that pressure marks settle.
- Read again at 72 to 96 hours: look for delayed reactions and note the grade at each site.
- Grade and interpret: apply ICDRG grades, exclude irritant reactions, and relate each positive to the history.
Patch testing shows sensitisation, not necessarily the cause of the eczema, so the clinical correlation decides relevance. Record the medications and steroid use at the time of testing, since these can suppress reactions and produce false negatives.
Where patch testing catches candidates
Patch-test questions reward correct readings and correct relevance, and they test the pitfalls directly.
- Trap: calling a vesicular reaction a weak positive Correct: vesicles mark a strong reaction (++), and coalescing vesicles or bullae mark an extreme one (+++).
- Trap: labelling an irritant reaction as allergy Correct: irritant reactions are non-immunological and are recorded as IR, outside the ICDRG allergy grades.
- Trap: reading the test only at 48 hours Correct: some reactions appear late, so a reading at 72 to 96 hours is needed to avoid missing them.
- Trap: treating every positive patch test as the cause Correct: relevance depends on whether the allergen matches the patient's exposure history.
Frequently asked questions
When are patch tests removed and read in allergic contact dermatitis?
Patch tests are removed after 48 hours, and an initial reading is taken then. Further readings at 72 to 96 hours are needed to detect delayed reactions, and some late reactions require a reading on day 7.
What does a ++ reaction mean on the ICDRG scale?
A ++ reaction on the ICDRG scale is a strong positive, showing erythema, infiltration, papules and vesicles at the test site. It indicates a clinically significant sensitisation to the allergen tested, subject to the exposure history.
What is angry back syndrome in patch testing?
Angry back syndrome is an exaggerated reaction across the back during patch testing, often provoked by sweating, heat or occlusion. It can produce false positive readings, so the clinician should interpret reactions cautiously and consider repeat testing.
Which factors can suppress patch test reactions?
Systemic corticosteroids and topical steroids applied to the back can suppress patch test reactions, leading to false negatives. Exposure to ultraviolet light before testing can also alter the skin response, so these factors should be recorded.
Why is patch testing performed on the upper back?
Patch testing is performed on the upper back because the skin there is relatively uniform and does not move much, which keeps the patches in place for 48 hours. It also allows the test sites to be protected from friction and sunlight.