What is patch testing?

Patch testing is a dermatology procedure to detect whether you are allergic to any substances that come into contact with your skin. This is called contact allergy and it could be causing or worsening your skin condition.

What will be tested?

You will receive a patch test for the 50 most common contact allergens, known as the British standard series. This includes fragrances, metals, preservatives, rubber, and plants. The dermatologist may also refer you to be tested for additional substances if they feel this is required. Patch testing does not test for food allergies or for anything which is inhaled (e.g., pollen).

What does patch testing involve?

Patch testing is carried out over the course of a week on a Monday, Wednesday, and Friday at Barnet hospital. You will need to attend on each of these days so that you have three appointments in total. Each appointment may take up to 30 minutes. As you will be required to keep the patch test area dry for the whole week of testing, you may wish to enjoy a bath or shower prior to attending your appointment on the Monday; this is your last chance to get your whole body wet until after you have been seen on the Friday.

Your first appointment on Monday

The dermatology specialist nurse will apply the patch tests to your skin and mark the area with a skin marker pen. The patches consist of hypoallergenic strips of tape. On each strip are usually 10 small chambers which contain a small amount of the substance that you are receiving the test for. Each strip will usually be secured to your upper back and will stay in place until you are seen again on Wednesday.

Your second appointment on Wednesday

The patch tests applied at your first appointment will be removed, and the area re-marked with a skin marker pen. The nurse will make an initial reading of the patch test. It can take two further days for 30 per cent of the results to display a reaction, so you must return for the final reading on Friday. The patch test sites may feel itchy, this is normal.

Your third appointment on Friday

You will be seen again by the clinical nurse specialist or the dermatologist. They will take the final reading of the patch tests and discuss the results with you. If you have any confirmed contact allergies, you will be given written information about these at this appointment. It is possible that your patch test results may be negative, which is also very helpful to you and your doctor, as it shows that contact allergy is less likely to be a cause of your skin condition.

What does a positive patch test look like?

A positive patch test is an itchy, raised area, which is often red or pink, where the substance was applied. This usually disappears after a few days.

Other possible side effects

Side effects are rare, but can include any of the following:

  • A strongly positive patch test can sometimes cause a local blister or an ulcer which may last several weeks.
  • A persistent reaction: some positive test reactions may remain for a few weeks before fading away.
  • A flare-up of eczema: a positive patch test may be accompanied by a temporary flare-up of your existing or previous eczema.
  • Pigment change: an increase or decrease in pigment (skin colour) may be seen at the place where the patches were, which may last for several months.
  • Scarring: very rare (1 in 10,000).
  • Allergy: very uncommon, you may become allergic to one of the substances applied during patch testing. If this happens, you will be advised how to avoid the substance.
  • Anaphylaxis: extremely rare (1 in 100,860)

What to bring with you to your first appointment

  1. A list of all your prescribed medications including any prescribed creams or ointments that you have used on your skin.
  2. Any over the counter creams or ointments that you have used.

What you should do when having a patch test

  • Do inform the clinic two weeks before the appointment if you are taking oral steroids or immunosuppressant therapy.
  • Do inform the clinic if you are pregnant or breastfeeding as patch testing cannot be performed during these circumstances.
  • Do wear old and dark coloured clothing for your visit to the clinic to prevent staining or damage to your good clothes from the tape, skin marker pen and patch test substances.
  • Do reinforce the patches with tape should they peel at the corners.
  • Do make a note of the date and time should a patch dislodge itself and fall off.
  • Do remove a patch if it causes an intolerable and severe itch or burning sensation. The likelihood of this occurring is rare.

What you should not do when having a patch test

  • Do not apply any topical steroid, protopic or elidel creams to your back one week before testing.
  • Do not get your patch test area wet during the test.
  • Do not undertake activities that will make you sweat. For example: PE, sports, physical labour, etc.
  • Do not go swimming.
  • Do not moisturise your back on the morning of your first appointment.
  • Do not attempt to re-apply any patches that may fall off.