Patch Testing and Skin Irritation
Patch testing applies a substance under occlusion to intact skin to separate irritant reactions from true allergic sensitisation, and to substantiate tolerability claims for topical products.
Two different activities share the name. Diagnostic patch testing places standardised allergen concentrations in chambers on a patient's back, occluded for forty-eight hours, with readings at removal and again at ninety-six hours or later, because delayed type four hypersensitivity peaks after an irritant flare has faded. Safety substantiation instead uses induction and challenge on healthy volunteers: repeated occluded applications over about three weeks, a rest period, then a challenge patch on naive skin.
The numbers matter. A typical repeat insult panel enrols between fifty and roughly two hundred subjects, and cumulative irritation studies run continuous occlusion for fourteen to twenty-one days to surface slow-building irritation a single application misses. Reactions are graded on ordinal scales scoring erythema, oedema, papules and vesicles separately, so a result is a morphology judgement by a trained reader, not an instrument reading.
What such a study can support is narrow. With fifty subjects and no reactions, the rule of three puts the upper bound on sensitisation frequency near three in fifty, roughly one in seventeen: enough to exclude a common sensitiser, nowhere near enough to exclude one affecting a fraction of a percent of users. Post-market surveillance, not the panel, detects those.
The claim that misuses this is the line that a product was dermatologist-tested or hypoallergenic with no reactions observed. Neither phrase has a regulatory definition, the panel is small, occluded back skin behaves differently from facial skin used twice daily for a year, and testing an ingredient in isolation does not test the finished formulation whose preservatives and fragrance are the commoner culprits.