Contact Dermatitis/ Patch Test Procedure

Contact dermatitis is inflammation of the skin provoked by contact with certain substances. This inflammation can be either irritant or allergic. Everyone who comes into contact with certain caustic substances (e.g. alkali, acids) will develop some degree of irritant contact dermatitis. On the other hand, some people are allergic to certain substances, or allergens, that come into contact with their skin. Examples of such allergens include metals, dyes, preservatives, fragrances, and rubber. Even small amounts of allergens can provoke allergic contact dermatitis (redness and itching) in sensitised individuals.

What is a patch test?

Patch testing aims to identify the allergens causing allergic contact dermatitis. Small amounts of suspected allergens are placed in small chambers and applied on your skin, usually on the back.

After determining if an allergen is relevant to your rash, you can then avoid that substance, often leading to improvement of your skin condition. After successful elimination it can take several weeks (often over a month) until improvement of your rash. Please note that patch testing does not always explain the cause of a skin reaction or rash and does not include environmental or food allergy testing (such as skin prick testing).

Are there any risks associated with patch testing?

Discomfort (burning/itching) at the test sites, blistering, flare of rashes, pigmentary marks and scarring resulting from strong allergic reactions (rare), inadvertent sensitisation to patch test substances (rare), and anaphylaxis (very rare). Pregnant or lactating women should NOT undergo patch testing

Special instructions
Before patch test

• Inform your doctor or nurse if you are taking any oral medication. Certain medications (e.g. oral steroids or immunosuppressants) can affect patch test results and need to be stopped for 1 week prior to patch testing. Antihistamines will not interfere with patch testing.
• If you are using topical medications, do not apply them to the test area for 1 week prior to patch testing.
• Avoid sun exposure to the test area for 4 weeks prior to patch testing.
• Shower and wash your hair before coming for your patch test appointment.
• Wear an old or dark coloured loose top since the sites will be marked with a blue marker (which could stain clothes).
• Bring along commonly used items that you come into contact with together with all ingredient lists, as directed by your doctor or nurse. These may include skin medicaments and personal products such as cosmetics and toiletries

During patch testing (0 – 48 hours)

• Patches containing potential allergens in individual plastic chambers will be applied on your skin and kept in place with hypoallergenic tape. They are left in place for 48 hours.
• After patches are placed it is critical to keep the area dry for 48 hours.
• You may wash or sponge the other areas of the body not covered by the patch tests.
• If the area is itchy, you can take antihistamines or use ice packs over your clothing as long as the patches remain dry.
• Avoid friction or scratching.
• Avoid sun exposure, strenuous activity or exercise that could make you sweat, as well as movements that could cause the patches to come off. Loose patches can be reinforced with additional adhesive tape.
• If the itch is unbearable with severe pain or redness, you may remove the patches but note down the date and time that the patches were removed

After 48 hours

• Patches can be removed at home.
• Please take photos of the test area at 0 and 30 minutes upon removal. Your doctor will review these photos at the final visit.
• You may wash the test area with plain water after removing the patches. Do not use soap over the test area until your final visit.

Final clinic review at 72/96/120 hours

• Wear an old or dark coloured loose top since the sites will be marked with a blue marker (which could stain clothes).
• Bring your commonly used products with their ingredient labels from home for this visit.
• The marked areas must still be visible at this appointment. Your doctor will examine the test area and look for any positive findings. If you are allergic to any of the substances, the corresponding test area will develop redness and itch.
• Further instructions will be given based on findings.
• You may resume your normal activities after result interpretation.
• Please inform your doctor or nurse if you experience delayed patch test reactions, which may occur up to 3 weeks after testing

The information above is also available for download in pdf format.