Patch testing for allergic contact dermatitis
A skin test, done over about a week, to find out whether your eczema or rash is caused or worsened by an allergy to something that touches your skin.
✓ Medically reviewed by a GMC-registered consultant · last reviewed September 2026 · next review September 2027 · how we review
In short
- Patch testing looks for delayed contact allergies, and is different from prick or blood tests for immediate allergies.
- It runs over about a week with two or three visits: patches go on, come off at 48 hours, and are read again later.
- You must keep the test area dry and avoid sweating, sun and topical steroids on the back during the test, or the results can be unreliable.
- A positive result only matters if it fits your history, so the dermatologist's interpretation is as important as the test itself.
A plain-English summary. The detail — including risks and recovery — is below.
At a glance
A general guide. Your specialist will give you advice for your situation.
Can identify the specific substance triggering or worsening a contact dermatitis
Patch testing is the wrong test for immediate allergies (such as food, pollen or anaphylaxis), which need prick or blood tests.
Keep your back dry and avoid sweating, sun and exercise. The patches may itch a little. Do not remove them yourself unless told to.
A clear final reading that interprets each positive against your history and exposures.
Keep your back dry and avoid sweating, sun and exercise. The patches may itch a little. Do not remove them...
The first reading is taken. Markings may remain on your back. Any positive areas may be itchy and will keep...
The dermatologist reads the reactions and discusses what each one means and how it fits your history and exposures.
Positive areas settle over days to a couple of weeks. Occasionally a late reaction appears, so report any new...
What is patch testing for allergic contact dermatitis?
Patch testing is a specialist skin test that looks for a delayed allergy to substances that have touched your skin, known as allergic contact dermatitis. It is different from prick testing or blood tests for immediate allergies (like hay fever or food allergy); patch testing looks for a slower, delayed-type reaction that takes a couple of days to show.
Small amounts of many common allergens, often around 50 standard substances such as metals, fragrances, preservatives, rubber chemicals and hair-dye ingredients, are applied to your back under tape. Items from your own home or workplace, like your own cosmetics, can sometimes be added. They stay on for 48 hours, and the skin is then read for reactions over two or three visits.
The aim is to find what you are allergic to so you can avoid it, which is often the key to clearing a stubborn rash. A positive test must be interpreted alongside your history, because finding an allergy does not always mean it is the cause of your current problem.
The test cannot find every cause of a rash, can sometimes irritate the skin, and occasionally misses an allergy or shows one that is not relevant. The dermatologist's interpretation is as important as the test itself.
Types, options & approaches
There may be different ways to do this. The right approach depends on the clinical question and your circumstances.
Patch testing vs prick/blood allergy tests
| Patch testing | Prick / blood tests | |
|---|---|---|
| Allergy type | Delayed contact allergy | Immediate allergy |
| Triggers found | Things touching the skin | Foods, pollen, animals, etc. |
| Time to result | Days (over a week) | Minutes (prick) or lab time |
| Used for | Contact dermatitis | Hay fever, food, anaphylaxis risk |
These tests answer different questions; your clinician chooses based on the kind of allergy suspected.
Preparing for your test
- Do not sunbathe or use a sunbed on your back for several weeks before, and avoid an artificial tan there, as this can affect the test.
- Stop applying topical steroids to your back for a few days before testing, as they can mask reactions.
- Tell the team if you take steroid tablets or other medicines that dampen the immune system, as these can reduce reactions.
- Plan for the test week: you cannot get your back wet, swim, sunbathe or do heavy exercise that makes you sweat.
- Bring details, or samples, of products and materials you use at home and work, including anything you suspect.
- Wear old, loose, dark clothing, as the markings and any leakage can stain, and the back must stay covered.
What happens
At the first visit, the chosen allergens are applied in small chambers held on your back with hypoallergenic tape, and the positions are marked, sometimes with ink, so each substance can be identified later. Applying them takes up to a couple of hours.
You go home with the patches in place and must keep your back dry and avoid sweating, sun and exercise for 48 hours. At the second visit the patches are removed and the first reading is taken. If photopatch testing is being done, some sites are exposed to ultraviolet light at this stage.
A third visit, usually a couple of days later, allows a later reading, because many allergic reactions only become clear after the patches come off. The dermatologist examines your back, notes any red, raised, itchy spots where a substance was applied, and discusses what each positive reaction means for you. Occasionally a reaction appears even later, up to about two weeks.
Is this test right for me?
A good consultation should explore whether it's the right choice for you now — including reasons to wait or consider something else.
May not be suitable if…
- Patch testing is the wrong test for immediate allergies (such as food, pollen or anaphylaxis), which need prick or blood tests.
- It may be unreliable if the back is sunburnt, tanned or has active eczema at the test site.
- It is less reliable while on moderate or high-dose steroid tablets or strong immune-suppressing medicines.
- It is not useful for a rash with no plausible contact trigger, where another cause is more likely.
Delay or rearrange if…
- You have recent sunburn, a suntan or have used a sunbed on your back within the last few weeks.
- Your back has active eczema or broken skin where patches would go.
- You are on steroid tablets or immune-suppressing treatment that should be reviewed first.
- You cannot keep the area dry and attend all the readings over the test week right now.
Alternatives to discuss
- Prick testing or blood tests if an immediate allergy is suspected instead.
- A trial of avoiding a strongly suspected product without formal testing in clear-cut cases.
- Repeat or extended patch testing if the first round is inconclusive.
- Reviewing irritant exposures and skin-protection measures, as not all contact dermatitis is allergic.
- Specialist occupational dermatology assessment for work-related rashes.
Before you decide
Use this as a shared-decision checklist. The aim is not just “can this be done?”, but whether it is right for you, now, with the risks and alternatives clearly understood.
What matters most to me?
Think about symptoms, daily life, work, caring responsibilities, sport, fertility, travel, appearance and anxiety — the right choice depends on your priorities, not just the medical facts.
What are all my options?
Ask about waiting, monitoring, medicines, rehabilitation, a smaller or larger procedure, a different test, NHS referral, or a second opinion where that would help.
What would make me pause?
Active infection, pregnancy, unstable medical problems, smoking, medicines that increase bleeding, poor support at home, or feeling pressured are all reasons to slow down and get tailored advice.
What happens if I do nothing today?
For some problems, waiting is safe; for others, delay can make treatment harder. A good consultation should explain the trade-off in plain English.
Benefits
- Can identify the specific substance triggering or worsening a contact dermatitis
- Allows targeted avoidance, which is often the key to clearing a stubborn rash
- Can pinpoint a workplace cause, which may matter for your job and any occupational claim
- Helps avoid unnecessary changes to products that are not the problem
- Provides a clear list of what to avoid, including hidden sources of the allergen
Risks & complications
- Itching and redness where positive reactions occur
- Mild irritation from the tape or chambers
- Temporary marks or colour changes where substances were applied
- The inconvenience of keeping the back dry and attending several visits
- A strong positive reaction that blisters and stays itchy for a week or more
- A flare of your underlying eczema during or after testing
- A reaction so late it is missed unless you report it
- An inconclusive result needing repeat or further testing
- Lasting change in skin colour at a test site
- Developing a new sensitivity to a tested substance
- A persistent reaction or, very rarely, a small scar
The biggest pitfalls with patch testing are practical and interpretive. If the back gets wet, sweaty or sunburnt, or if steroids are used on it, the results can be unreliable. And a positive result only matters if it fits your history and your exposures, so a careful dermatologist's interpretation is essential to avoid blaming the wrong substance or missing a relevant one. Ask what each positive means for you, where the allergen is found, and what to do if your rash continues despite avoidance.
Published figures to discuss
Patch testing is a low-risk test, but it is not perfect. Its accuracy depends on testing the right allergens, correct application and reading, and skin that has not been tanned or treated with steroids. False negatives and irrelevant positives both occur, which is why results are interpreted with your history. Because relevance varies so much between individuals, meaningful overall percentages are limited; the figures below are the small procedural risks quoted in UK patient information.
| Figure | Reported range | How to interpret it | Source / confidence |
|---|---|---|---|
| Developing a new sensitivity from testing | Around 1 in 5,000 (uncommon) | Quoted in UK dermatology patient information; an allergy can rarely be induced by the test itself. | BAD/BSCA — Patch testing guidance updatecdn.bad.org.ukPublished figure |
| Scarring at a test site | Around 1 in 10,000 (very rare) | Quoted in UK dermatology patient information; most reactions settle without any mark. | BAD/BSCA — Patch testing guidance updatecdn.bad.org.ukPublished figure |
| False-negative or missed allergen | Recognised | Patch testing only tests selected allergens under specific conditions. Steroid use, poor adhesion or the wrong allergen series can miss the cause. | Guide sourcesClinical context |
| Positive test that is not clinically relevant | Common interpretive issue | The useful result is a positive allergen that fits the exposure history and leads to realistic avoidance advice. | Guide sourcesClinical context |
These are literature figures, not a personalised prediction. Your own risks and likely benefits depend on your circumstances, your health, and how your care is carried out and followed up.
What happens afterwards
There is no physical recovery, but the test runs over about a week with restrictions on getting the back wet, and any reactions take a few days to settle.
- Itchy, red spots where you reacted, fading over days
- Ink or pen markings on the back that wash off afterwards
- Mild irritation or dryness from the tape
- A short-lived flare of eczema in some people
- Waiting for the final reading before you know the results
Aftercare
- Keep your back dry and covered until the patches and readings are complete.
- Avoid scratching positive areas; a moisturiser or prescribed cream can soothe them once testing is done.
- Report any new reactions in the marked area in the days and up to two weeks afterwards.
- Once you have your results, follow the avoidance advice and any information sheets on where the allergen is found.
- Check ingredient lists of products for the substances you react to, as they appear under many names.
- Go back if your rash continues despite avoidance, as further assessment may be needed.
- No sunbed or tanning on the back for several weeks before
- Topical steroids stopped on the back as advised
- Test week cleared of swimming, heavy exercise and sun
- Own products and workplace samples gathered to bring
- Old, dark, loose clothing for the test days
- All three appointment times noted
- Plan to report any late reactions
Scars and how they heal
Patch testing leaves no cuts. Positive reactions are red and itchy where the substance sat, and may occasionally blister. These usually settle within days to a couple of weeks, sometimes leaving a temporary patch of lighter or darker skin. Lasting colour change is uncommon and a small scar is rare.
⚠ Get urgent help if…
- A severe blistering or spreading reaction that is getting worse rather than settling
- Signs of skin infection at a test site: increasing pain, heat, swelling or pus
- A widespread flare of your eczema that you cannot manage
- A patch reaction that is still worsening well beyond two weeks
- Feeling generally unwell or feverish after testing
- Any reaction you are worried about in the marked area
Who to contact: your clinician, clinic or test provider first (keep their number to hand). For urgent advice when you can't reach them, call NHS 111. In an emergency, call 999.
General guidance — it doesn't replace the specific advice your specialist gives you.
Results & realistic expectations
A useful result identifies one or more substances you are allergic to, which, when they match your history and exposures, explains your rash and tells you what to avoid. Avoiding the trigger is often what finally clears a long-standing contact dermatitis.
Patch testing cannot find every cause of a rash. It can be negative even when an allergy is present (a false negative), or show a reaction to something that is not actually causing your current problem. It also does not test for immediate allergies. This is why results are always interpreted alongside your story, and why a normal test does not always mean there is no allergy.
A contact allergy, once identified, is usually lifelong, so the avoidance advice generally remains valid for good. However, you can develop new allergies over time, and if a new or different rash appears, repeat testing with updated panels may be worthwhile. Results stay useful as long as your exposures and symptoms fit the original findings.
Related tests, treatments or support
Patch testing is sometimes done alongside, or after, other allergy tests such as prick testing when both immediate and delayed allergies are possible. Photopatch testing may be added when sunlight seems involved. It is often combined with a careful review of your home and workplace products to track down the relevant source.
Follow-up & long-term care
Results are explained at the final reading, usually with written information on the allergens found and where they are hidden. If a workplace cause is identified, this may need discussion with occupational health or your employer. If your rash persists despite avoidance, go back, as the relevant allergen may have been missed or there may be another cause needing further assessment.
- Keeping your list of confirmed allergens and checking product labels long term
- Re-testing with updated panels if a new or different rash develops
- Reviewing workplace exposures and protective measures if relevant
- Returning for reassessment if avoidance does not clear the rash
Repeat, follow-on and what comes next
- Inconclusive results may need repeat or extended testing.
- A late reaction can appear up to about two weeks later and should be reported.
- If avoidance does not clear the rash, reassessment or testing of further allergens may be needed.
Ask what happens if the result is unclear or needs repeating, and what is included if further tests or follow-up are needed.
What good aftercare looks like
- A clear final reading that interprets each positive against your history and exposures.
- Written allergen information showing hidden sources and alternatives.
- Advice on what to do if a late reaction appears.
- A route back if the rash persists despite avoidance.
- Occupational input where a workplace cause is identified.
What affects the cost
Costs vary a great deal between people and providers, and we don't publish prices. What matters is understanding what drives the cost and making sure your quote is complete. The main things that affect it:
- The number and range of allergen panels tested
- Whether your own products or specialist series are added
- Whether photopatch testing is needed
- The dermatologist's time across the multiple reading visits
- Written allergen information and avoidance advice provided
- Any follow-up consultation to discuss results and next steps
- How many allergen panels are included
- Whether testing your own products or extra series costs more
- All the visits needed for application and readings
- The consultation to interpret and explain the results
- What happens, and the cost, if further or repeat testing is needed
- Whether written avoidance information is provided
On the NHS? Patch testing is available on the NHS through dermatology when there is a clinical reason; private testing is also available, but the same preparation and expert interpretation are needed for results to be meaningful.
You're entitled to your total cost in writing — including reports, follow-up and what happens if the result is inconclusive — before you decide.
Consent traps and marketing red flags
These are not small details. They are often where patients lose choice, time to reflect, or realistic expectations.
Consent traps
- Treating any positive result as the cause without checking it fits your history.
- Not explaining that a normal test does not rule out an allergy.
- Not warning about the dry-back, sun and steroid restrictions that affect reliability.
- No written information on where confirmed allergens are found.
- No plan for what to do if the rash continues despite avoidance.
Marketing red flags
- Claiming a single test will find the cause of any rash or any allergy.
- Offering patch testing for immediate food or environmental allergies it cannot detect.
- Selling large allergen panels without relating them to your actual exposures.
- Giving a list of positives with no interpretation of relevance.
- Promising a guaranteed cure once allergens are 'found'.
Choosing a specialist safely
- Check the specialist is on the GMC Specialist Register for this area.
- Make sure they work at a CQC-registered service, and look for membership of the relevant Royal College or professional body.
- You're entitled to time to consider and to have your questions answered before you agree — the specialist who looks after you should explain it, not a salesperson.
- Be wary of pressure: time-limited offers or deposits taken before you've had time to think are red flags, not bargains.
- You're entitled to your total cost in writing — including any follow-up — before you decide.
Questions to ask your medical professional
Take this to your consultation. A good specialist will welcome every one of these.
- Which panels will you test, and should any of my own products or work materials be included?
- Do I need to stop any creams or medicines before the test?
- What does each positive result mean for me, and where is that allergen found?
- What happens if the test is normal but my rash continues?
- Could sunlight be involved, and do I need photopatch testing?
- If this is work-related, what should I tell my employer or occupational health?
- Are you on the GMC Specialist Register for this area, and which Royal College or professional body are you a member of?
- Will you be the specialist who carries out my test, and who looks after me afterwards?
- What are the risks for someone like me, and how often do your own patients have a problem or need it repeated or redone?
- What does a realistic result look like — and what can this test not achieve?
- What are my options, including waiting, doing nothing for now, or choosing a different approach?
- Can I have written information, results and aftercare instructions in a format I can use, including any accessibility or communication support I need?
- What is the total cost in writing, including any follow-ups, and how much time do I have to decide?
Frequently asked questions
How is patch testing different from a prick or blood allergy test?
Why does it take a whole week?
Why can't I get my back wet or sunbathe?
Does a positive result mean that's definitely causing my rash?
Can the test be normal but I still have an allergy?
Is patch testing available on the NHS?
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How we made this page
Medically reviewed by a GMC-registered consultant. Written in plain English, checked against NHS, NICE, GMC and relevant Royal College / specialist-society guidance, and kept under review. No clinic paid to appear on this page, and we publish no pricing. This is general information to help you prepare — it is not a substitute for advice from your own clinician. How we review our guides →
Source hierarchy: UK regulator and NHS/NICE guidance first, then relevant Royal College or specialist-society guidance, then peer-reviewed evidence for procedure-specific figures where available.
Sources & standards: NHS — Contact dermatitis: Diagnosis (patch testing) British Association of Dermatologists — Patch testing (patient information) British Association of Dermatologists — Contact dermatitis (patient information) BAD/BSCA — Patch testing guidance update BAD — Service guidance and standards for cutaneous allergy investigations
Reviews reflect patients' experience of care, not clinical outcomes. For procedure volumes and outcome data see PHIN.
Last medically reviewed 2026-09-21. Spotted something wrong or out of date? Report an error in this guide.
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