# Contact Dermatitis and Patch Testing

> Contact dermatitis: allergic versus irritant, patch test reading and India's parthenium menace for NEET-PG Medicine preparation.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/contact-dermatitis-patch-testing
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Contact Dermatitis and Patch Testing", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/contact-dermatitis-patch-testing

## Direct answer

Itch points to allergy, pain and stinging to irritation — that single split drives every contact dermatitis decision. Allergic contact dermatitis is a type IV, delayed reaction that appears 24–72 hours after exposure in a previously sensitised person, whereas irritant dermatitis is direct cytotoxicity that can strike anyone, especially with wet work and detergents. Patch testing on the back, applied for 48 hours and read at 48 hours and again at 72–96 hours, identifies the allergen — and in India the weed Parthenium hysterophorus heads the list as the commonest cause of airborne allergic contact dermatitis, a NEET-PG Medicine favourite.

## What you must remember

- Irritant: stinging and pain, well-demarcated erythema, scaling, fissuring; anyone exposed, no prior sensitisation; cumulative wet-work damage (housework, detergents, cement).
- Allergic: pruritus, vesiculation, poorly defined spread; delayed 24–72 hours; requires prior sensitisation of one to two weeks.
- Patch testing is the gold standard for allergic contact dermatitis: allergens in Finn chambers taped on the back, removed at 48 hours, read at 48 hours and again at 72–96 hours for delayed reactions.
- Positive reactions are graded by erythema, infiltration and vesicle formation, then assessed for current or past relevance.
- Indian standouts: Parthenium hysterophorus (airborne pattern over face, eyelids, neck and flexures), paraphenylenediamine in hair dye and black henna, nickel in jewellery and buttons, fragrance, neomycin, and rubber accelerators.
- Black henna (PPD-adulterated) temporary tattoos cause intense sensitisation and later hair-dye reactions in the young.
- Cement dermatitis: wet concrete's alkalinity plus chromate allergy, an occupational classic in construction.
- Treatment of acute exudative dermatitis: cool wet compresses, then topical corticosteroids; short oral steroid course for severe widespread reactions.
- Prevention is definitive: allergen avoidance, nitrile gloves for rubber-allergic workers, and barrier care for wet-work hands.

## How to work through a typical case

A 42-year-old farm labourer from Haryana reports six months of intensely itchy eczema over the eyelids, cheeks, neck and flexures of elbows; topical steroids partially suppress it and it rebounds fiercely. He attributes it to "heat".

Step one: read the distribution — an airborne pattern over light-exposed and exposed sites with flexural accentuation in a rural worker is parthenium dermatitis until excluded; its pollen and trichomes settle on exposed skin. Step two: distinguish from the mimics — photosensitive dermatitis spares upper lids and retroauricular folds (which airborne particles reach), and atopic dermatitis relapses from childhood rather than beginning at 42. Step three: patch test with the Indian baseline series plus plant extracts where available: chambers applied on a clear back, removed at 48 hours, read then and again at 72–96 hours; a relevant parthenium positivity with eyelid-pattern eczema completes the diagnosis. Step four: treat the acute phase — wet compresses for weeping areas, a mid-to-high potency topical steroid tapering over two to three weeks, and a short oral prednisolone course for a severe flare. Step five: the real prescription is avoidance — covering skin during farm work, bathing and changing clothes after work, gloves and barrier creams, physical removal of the weed around living spaces, and structured counselling that re-exposure means relapse. Step six: arrange follow-up for steroid-taper rebound and patch-test reading of any occupational additives.

## Where students slip

The commonest error is treating recurrent eyelid and facial eczema indefinitely as "atopic" or "seborrhoeic" while the patient keeps contacting the allergen — patch testing is the answer the examiner wants after failure of reasonable topical therapy. Second, misreading a patch test as a prick test: patch testing detects type IV delayed allergy and is read in days, whereas prick testing detects IgE-mediated immediacy in minutes — swapping them is heavily penalised. Third, forgetting black henna's PPD load when a young person with a past temporary tattoo reacts to hair dye. Fourth, applying potent steroids to the eyelids long-term without specialist review, where atrophy and glaucoma risk concentrate. Finally, cement burns in construction workers mislabelled as "allergy to gloves" rather than alkali-plus-chromate injury.

## Frequently asked questions

### How is allergic contact dermatitis distinguished from irritant?
Allergic reactions itch, vesiculate and spread beyond the contact area after 24–72 hours in sensitised individuals; irritant reactions sting, remain well demarcated and occur in anyone after sufficient exposure.

### How and when are patch tests read?
Allergens stay under occlusion on the back for 48 hours; readings are taken at removal and repeated at 72–96 hours to capture delayed reactions, graded from faint erythema to vesicular eruption and matched to relevance.

### Which plant is the leading cause of airborne contact dermatitis in India?
Parthenium hysterophorus, a ubiquitous flowering weed whose pollen and glandular trichomes produce eyelid, face and neck eczema in rural and peri-urban workers.

### Why is black henna a sensitisation concern?
Black henna is adulterated with high concentrations of paraphenylenediamine, a potent allergen that primes patients — often children — for severe reactions to later hair-dye exposure.

### How is acute exudative contact dermatitis treated?
Cool wet compresses for weeping skin followed by topical corticosteroids, antihistamines for itch, and a short oral steroid course for extensive reactions, alongside strict allergen avoidance.