Your concern
A rash where something touches
Contact dermatitis: the shape of the rash draws the cause, and testing confirms it.
What makes this picture recognisable is not the appearance of the lesions but their geometry. A red ring where the trouser button sits, a stripe under a watch strap, the exact outline of a necklace, the tops of the feet in the shape of a sandal: when a rash has a border belonging not to biology but to an object, the cause is in plain sight.
There are two different mechanisms. The irritant form depends on direct damage from a substance, appears in anyone if exposure is sufficient, and is dose-dependent. The allergic form affects only those who have become sensitised, typically appears hours or days after contact, and recurs every time, even with minimal amounts.
The difference is not academic. If it is allergic, the picture returns as long as that substance remains in the patient's life, whatever cream is used. Identifying it is the real objective, and it is done with patch tests, which apply a battery of substances to the back and are read days later. In our organisation allergy testing is performed at the Siena clinic.
Photograph the rash while it is there
This is the most practical advice on this page, and it costs ten seconds. These conditions have a bad habit of fading precisely in the days between onset and the appointment, and what arrives at the visit is often almost normal skin with an approximate account. The shape of the rash is the most valuable diagnostic clue: a sharp border tracing an object is worth more than any verbal description. Photograph it as soon as it appears, in good light, and bring it with you.
How it shows
- A rash with sharp borders reproducing the shape of an object
- Redness, itching and sometimes blisters
- Onset hours or days after contact
- Systematic recurrence at the same sites
- Revealing sites: wrists, earlobes, navel, tops of the feet, hands
- In chronic form: thickened, scaling skin
How we proceed
- 1Dermatological visit: distribution and borders of the lesions
- 2Review of contacts: work, cosmetics, jewellery, clothing, hobbies
- 3Distinction between irritant and allergic forms
- 4Patch testing where allergy is suspected (at the Siena clinic)
- 5Therapy and a strategy for avoiding the identified substance
Available treatments
- Dermatology visit
- Patch testing
- Targeted therapy
- Scheduled follow-up
Book a visit
Our specialists will evaluate your case and define the most suitable treatment plan.
Book in FlorenceBook in SienaSiena · Firenze
Frequently asked questions
How do patch tests work?+
Patches containing a standardised series of substances are applied to the back, left in place for a couple of days and then read later, usually at two separate times: allergic reactions appear with a characteristic delay, and reading them too early or only once gives incomplete results. During the test days the back must not be wetted and heavy sweating is avoided. They are performed at the Siena clinic.
Can I have become allergic to something I have used for years?+
Yes, and it is one of the most surprising points. Sensitisation requires repeated exposure over time: substances used for years are among the most likely candidates, not those to be excluded. This is why «but I have always used this» does not close the question — on the contrary, it opens it.
If I find the substance, will I get better?+
That is the route, but it must be said precisely: avoidance works if the substance can genuinely be removed from your life, and some are everywhere — nickel, fragrances, cosmetic preservatives. This is why after the test we do not simply hand over a name: we build a concrete strategy on what to read on labels, what alternatives exist and where the substance hides.
Related reading
Have a question? Let's talk.
Book a first visit: the physician will evaluate your situation and guide you to the most suitable treatment.
Book a dermatology visit