The most expensive mistake: looking for the culprit on the plate
When a dermatitis will not clear, the first reflex is almost always food. And it is almost always the wrong reflex.
Allergic contact dermatitis — what patch testing investigates — is caused by something that touches the skin, not something that is eaten. The usual culprits are depressingly everyday: nickel (costume jewellery, buckles, trouser studs, spectacle frames), fragrances and preservatives in cosmetics and creams, hair dyes, rubber accelerators (gloves, footwear, sportswear), adhesives and — very frequently — ingredients of the very products the patient is using to treat the dermatitis.
That last point matters: some patients spend months getting worse because of the cream they are using to get better.
How patch testing works
It is simple, painless and needle-free:
- Patches containing the suspected substances, at standardised concentrations, are applied to the back.
- They stay on for 48 hours. During that time the back must stay dry and heavy sweating is avoided — which means no normal showering and no intense exercise.
- They are removed and a first reading is taken. And — this is key — a second reading is taken at 96 hours, because many allergic reactions appear late. A test read only once misses diagnoses.
- The result is a concrete list: what you are allergic to and, above all, which products in your daily life contain it. That is what changes the disease: avoiding what causes it.
What is investigated, and what is not
Investigated here: contact dermatitis — what the skin touches. It is the unit’s central indication and where patch testing is the standard.
Assessed for relevance: in atopic dermatitis it is investigated when the picture is atypical, unresponsive, or an added contact allergy is suspected — which happens more often than people think. In chronic urticaria, by contrast, blind allergy panels almost never contribute anything, and saying so prevents months of useless investigation.
The rule: testing is done when the clinical history justifies it. A “just in case” panel generates irrelevant findings, anxiety and needless restrictions.
Who handles it here
Diagnosis and treatment of allergic contact dermatitis belong to the clinic’s dermatology consultation — the same one with which Dr. Francisco Arellano Ocampo founded the house in 1971. Dr. Arístides Arellano is a plastic surgeon, not a dermatologist: his contribution here is surgical and reconstructive, and the disease itself is managed in the consultation. Book at dermatology consultation.
Dr. Arístides Arellano is a plastic surgeon holding a specialty licence: he practises under a Mexican medical licence (D.G.P. 1125959) and a specialty licence in Plastic & Reconstructive Surgery (0002008), both issued by the Directorate General of Professions (DGP), which reissued his certificate of professional standing in 2025. His career began before Mexico’s present-day specialty-board system existed: he belongs to the founding generation of plastic surgery in Puebla, the second generation of a surgical tradition started in 1971 by Dr. Francisco Arellano Ocampo. Trust is verified, not promised: both licences are public and anyone can look them up in the DGP’s National Registry of Professionals.
Risks and expectations
Every medical treatment —topicals, peels and energy-based devices included— carries risk and requires a prior assessment. In darker skin (Fitzpatrick IV–VI, the most common phototypes in Mexico) a poorly indicated procedure can darken the patch instead of lightening it. Results may vary according to each patient, their phototype and their individual triggers.
Trust is verified, not promised. All 210 scanned documents —degree, licences, diplomas, publications and certificates, from 1977 to 2025— are open in Dr. Arístides Arellano’s credential archive, alongside an explanation of the difference between a specialty licence and a board certification.
Control plan
- IndicationContact dermatitis: what the skin TOUCHES, not what is eaten.
- Day 0Patches applied to the back.
- 48 hoursKeep the back dry, avoid sweating. Removed, first reading.
- 96 hoursSecond reading, mandatory. Many reactions appear late.
- ResultA concrete list: what you react to, and which products contain it.
- The treatmentAvoid what was identified. That is what clears the dermatitis.
Frequently asked questions
Do allergy tests tell me which food is harming me?
For contact dermatitis, no: that is caused by something touching the skin, not something eaten. Patch testing looks for nickel, fragrances, preservatives, dyes, rubber and adhesives, not foods. Eliminating food blindly for a dermatitis almost never works and can cause deficiencies, especially in children.
Does patch testing hurt?
No, and there are no needles. Patches with the suspected substances are applied to the back and left for 48 hours. The only real inconvenience is that during that time your back cannot get wet and you must avoid heavy sweating, so the week needs planning: no normal showers, no intense exercise.
Why do I have to come back at 96 hours?
Because many allergic contact reactions appear late: absent at 48 hours, present at 96. A test read only once misses diagnoses, and missing the diagnosis means staying in contact with the culprit. The second reading is not optional.
What if my problem is chronic urticaria?
In chronic urticaria, blind allergy panels almost never contribute anything, because in the great majority of cases there is no external cause: the immune system releases histamine by itself. Testing is done only if the clinical history justifies it. Saying so prevents months of useless investigation and needless diets.
What if the culprit is the cream I use for the dermatitis?
It happens very frequently, and it is one of the reasons the test is so useful. Some patients spend months getting worse because of the product they use to get better: fragrances, preservatives or excipients in the cream itself. Patch testing identifies it, and withdrawing that product resolves the picture.
Every case is different. Discuss yours in a personal consultation.
Book a consultation on WhatsAppEvery figure published on this site is approximate and is not a quote. Any procedure or treatment first requires a private assessment consultation with Dr. Arístides Arellano —$800 MXN, charged separately— and it is at that consultation that the definitive cost of your case is determined.
Educational content; it does not replace a medical consultation. Results may vary from person to person.