Placement of patches
The placement of patch tests will depend on several factors:
- The number of patches to be applied
- The physical size of the patient
- The condition of the skin where the patches are to be applied
- Tattoos
- Shape of the back should also be considered: Avoid bony areas of back and skin folds as patches do not stick as well
- The midline of the back should also be avoided as it is the sweatiest area, and patches may loosen
Application of patches
Here are a few examples of ways the patches can be applied. Please refer to the Patch placement information sheet for more information.
Examples of alternative patch application
The patches should be applied in an orderly fashion across the back starting at the top left-hand side, working across to the right. The middle of the back should be avoided, because this is the most common place people sweat and the patches do not stick as well.
If there is insufficient space on the back or a large number of patches to be applied (i.e. 12 or more patches), some patches can be applied to the arms.
When applying patches, be aware that some areas of the back don’t allow patches to adhere as firmly, which may cause false negative reactions (as they allergen has not been in sufficient contact for a reaction to be elicited). Examples include:
- Very bony parts of the back. Some people have a depression above or below their scapula so tests won’t stick well there.
- Bra strap areas (the horizontal strap)
- Excess skin folds (usually in overweight patients)
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Occasionally patches may have to be applied sideways on the back because it allows more patches to be applied and follows the curvature of the back. When placing patches sideways, always be consistent in the way the patches are applied and always ensure the top of the patch is always to the right (so the number on the bottom of the patch is to the left). This ensures that the numbers of the patches flow consecutively, making it easier to read results and match up with the chart. Prosthetic joint tests are treated slightly different in terms of positioning. We are now suggesting these are placed down lower on the back, ensuring the scapula is avoided and there is very good adhesion. This is important as the results of this type of testing can sometimes lead to the need for more surgery. |
Placement with tattoos
Tattoos can limit the amount of skin available for patch testing. It is important that patches are not applied on top of very dark coloured tattoos, because it may not be possible to accurately read test results. Instead place them around the tattoo – you may be required to cut them in to smaller strips so they fit.
This is a good example of how to apply patches around a tattoo –you may need to be creative! If there are no other options to apply patches to other areas of the body, patches can still be applied over tattoos, but reading patches maybe more difficult.
Mark the patches
Once patches are applied to the back, use a patch testing skin marker (or surgical marker) to place marks next to each disc. These marks indicate to us where the patches were placed, so the doctor knows where each allergen was located when reading reactions. These will need to be ‘redrawn’ on the second appointment when patches are removed.
There are 2 different markers available, one from allergEAZE® and one from Chemotechnique. The new skin markers are shown below.
Depending on the type of patch used, Micropore™ may need to be placed over the top of the patches to hold them firmly in place. This is often needed if Finn Chambers® are being used, however the allergEAZE® patch test chambers do not require extra taping, nor do the Chemotechnique Ultimate patch™ or Ultra patch™. Micropore™ is now only needed when a patch has been cut down , e.g. 3 discs remain on the patch, as these small cut patches can sometimes fall off or get stuck onto clothes.
Sunscreen UV separates
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Sunscreen “separates” refers to patches that need to be exposed to ultraviolet A (UVA) light at the second visit (day 2). The purpose of this test is to simulate photoallergy by exposing the patch test to UV light. After the patches have been removed, the area of skin where the sunscreen separate patch was located, is exposed to 5 joules of UVA light, ideally using a hand and foot machine. This reaction is then compared to the other sunscreens that should have been applied to the back but not exposed to UVA. If they react but the other tests are negative, this is indicative of photoallergy. This process is also applicable to testing the Photoallergy Series as well. |
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Sunscreen separate or photo allergy patches can be applied to the lower forearm, where possible, as this allows easy access for the UVA light exposure, as the patient can be placed into the hand and foot UVA machine. This is easier than applying to the back, which means the other patches have to be covered up so they are not exposed to the UVA. If patches are applied to the lower forearm, ensure they are positioned in a way that the UVA reaches all patches (especially those that may be wrapped around the curve of the arm). A hand-held UVA device may also be used. |
The placement of these patches also needs to be drawn on the diagram on the patient chart. Photos should also be taken.
What not to do when applying patches….
Placement of patches can be difficult at times, particularly if there is a large number of patches to be applied, or if the patient has a small or rashy back or a large tattoo. The picture below shows patches that have not been applied very well. On occasion, the doctor may not have realised how many patch tests have been ordered – ask them to reconsider if all allergens are absolutely necessary if you don’t think they will fit. However, in these images below, the patches could have been applied to arms, and not to the midline of the back.
Numbering of patch tests when applied to the patient
As mentioned earlier, the numbering system for patch testing can be confusing. Please refer to the Patch test numbering information sheet for more information.
Once applied to the patients back, the patch numbers reverse (from what they were when the patches were being prepared). The picture below shows the numbering once the patches are stuck on the patient. This is particularly important, because the dermatologist/nurse doing the reading needs to fully understand the numbering, to ensure patches are “read” correctly and the results are accurate.
Here is an example of how the numbering works once applied to the back
Each patch is numbered at the bottom. The patch should be applied to the patients’ back with this number at the bottom, as shown above. On each patch, there are 10 discs, each with a different substance on each disc.
On patch one = allergens 1-10 (top left-hand corner is number one and bottom right-hand corner is disc 10).
Patch 2 = 11-20 (top left-hand corner is disc 11 and bottom right-hand corner is 20)
Patch 3 = 21-30 (top left-hand corner is 21 and the bottom right-hand corner is 30)
And this continues for the number of patches applied.
The image below demonstrates how the numbering location on each disc, matches up with the chart so each substance can be identified. For example, on the image below, numbers on each patch are as follows: number 1 allergen matches with number one on the chart, which is nickel, no. 2 is Balsam of Peru etc.
Make sure the location of patches and their numbers are documented on the chart accurately, so the nurse and dermatologist both know where each patch and allergen are located.
Instructions to give patients after patches are applied
Once patients have had patches applied to their back it is important to give them advice about how to care for their patches. It is also useful to give these instructions as a printed information sheet (Care of Patches – Patient Instruction Sheet) so they can read the important instructions once they return home. We recommend adding contact details of clinic (phone number or email address) in case the patient experiences any serious reactions.
This advice should include:
- Don’t get the back wet for the period of testing. Have a shallow bath, wash from the hand basin or use a hand-held shower, sparing the back
- Avoid physical activity (including work) that will cause heat and sweating, as this may cause patches to loosen and can fall off. Some workers who have physical jobs may need time off work or modified duties. Consider supplying a medical certificate if necessary
- Keep an eye on patches – place additional tape such as Micropore™ over the top of patches if they look like they are loosening
- Avoid scratching the back while patches are on, and after patches are removed. Scratching can cause false results and make it impossible to read the reactions accurately
- Ask patients to bring back their own samples to all appointments- you may need to retest, read labels etc. (even if product was not tested)
- If patches become unbearable, the patient can phone to talk to the nurses or doctor. If concerned ask the patient to take a photo and discuss with the patch testing doctor, who may consider a face to face review of the patient
- Antihistamines may help ease any discomfort and do not interfere with patch test results
- Using a cold ice pack wrapped in a tea towel might help settle the itch. It might also help to lean again a cold metal fridge door or cool glass window
Videos to watch:
