Patch test results
Patches should always be removed after 48 hours of application (day 2). Traditionally the reading of patch test results occurs on two occasions, on day 2 and day 4, which allows for clinics to occur over one week, e.g. Monday/Wednesday/Friday. Some clinics in Europe run slightly differently, with tests being removed on day 2, then readings being performed on day 3 and day 7. A day 7 reading means that any late reactions may be observed. The most important day for reading results is at day 4.
The reading should be performed at least 10-15 minutes after patches have been removed. Once patches are removed, the purple marks are re-touched, and the patches are numbered with thin pieces of tape, as seen in the image below. Any additional tests that may have been applied on the second appointment should also be removed before the patient is seen by the doctor.
Examples of positive reactions, ranging from weak/doubtful to strong are shown below. At the third and final appointment, the patient will spend the majority of the time with the dermatologist talking about the patch test reactions, the final diagnosis and management.
Examples of reactions and strengths:
Irritant reaction (IR):
Weak positive (+):
Strong positive (++):
Along with the strength of the reaction being recorded, the relevance of the reaction is also determined. This is recorded on the patients’ patch testing chart. There are 3 categories of relevance:
Unknown (U) relevance
Relevant (R) reaction
Old or past (O) relevant reaction
If this testing is negative, it may indicate that the patient may have a different condition such as irritant contact dermatitis, eczema or contact urticarial. See the section “Making a diagnosis” for more information about negative patch testing.
Determining relevance requires taking a comprehensive history of exposures. It is equally important to consider exposures at home as well as work. Pay particular attention to use of hand washes and gloves, as well as substances handled.
As mentioned above, reactions need to be “labelled” and should be classified as of current relevance, of old or past relevance or of unknown relevance.
Often there is a tendency for inexperienced patch testers to classify reactions as relevant, when they are in fact NOT the cause of the patient’s dermatitis. For a positive patch test reaction to be relevant to the patient’s dermatitis there must be a history of exposure to the allergen, and a clinical history of dermatitis which is compatible with this exposure. Often this means reading product labels on work and home products and checking Safety Data Sheets (SDS) to see if the allergen is contained in any of the products. For example, if a patient has a positive patch test reaction to coconut diethanolamide, it is contained in the hand wash that is currently being used, and the clinical course of the dermatitis would fit with exposure to this hand wash, we would class this as a relevant reaction. However, if after checking all the product labels and SDS where possible, we are unable to locate coconut diethanolamide in any of the products currently being used, we would then class the reaction as being of unknown relevance. If however, it had been present in a product that the patient had used previously but is no longer using, we would class the reaction as being of old or past relevance.
A common cause of past relevant reactions is nickel. Nickel is the most common positive patch test reaction seen in most patch testing clinics. A patient may describe problems when wearing cheap jewelry some years ago, but which is no longer worn. In this situation the patient is allergic to nickel, but it is not contributing to their current presentation (old/past relevance). If the patient has no recollection of a history of skin problems with nickel, the reaction is classed as of unknown relevance. On the other hand, if a middle-aged man presents with dermatitis involving his lower abdomen caused by nickel in his belt buckle, then his reaction to nickel is classed as relevant.
Determining the relevance of patch test reactions and giving the patient appropriate advice can be a time-consuming task. There is no “quick fix!” It is important to read labels, ask patients to obtain the SDS, talk to product manufacturers, talk to workplace personnel and over time you will acquire experience regarding common sources of exposures to particular allergens. We have found it particularly advantageous to provide patients with information on positive allergens at the 48-hour reading, so that they can also assist with the hunt to find sources of allergen exposure. This also aids in the education process of the patient.
Once you have diagnosed a relevant reaction, you will need to assist the patient in finding products they can safely use, such as particular gloves that do not include certain rubber chemicals or a hand wash without a certain preservative or a hand cream that does not include a fragrance. This is not always easy: there are no readily available one-size-fits-all databases in Australia for this.
Reactions to “mix” ingredients
It is not uncommon for people to react to mix allergens, particularly fragrance mix or thiuram mix. These “mix” allergens are made up of several allergens, and sometimes identifying the exact allergen is necessary and makes label reading easier for the patient. For example, if a person reacts to Fragrance mix 1, you could test the individual ingredients which make up fragrance mix 1:
- Amyl cinnamal
- Cinnamyl alcohol
- Oakmoss absolute
This may be useful if the patient only reacts to one or two of these allergens, and if individual fragrances are listed on product ingredient labels. If a product only lists ‘fragrance’ or ‘parfum’ in their ingredients however, there may be little value knowing exactly which fragrance(s) the patient is allergic to.
For Contact Allergen Bank Australia users, there is a series in PatchCams called ‘Frag Mix 1 individual ingredients’ and these can be selected if further patch testing a patient. This further testing could be applicable to fragrance mix 2, mercapto mix, carba mix, thiuram mix and textile dye mix, to name a few.
What if all tests are negative?
If testing is negative, it generally indicates that either the patient does not have allergic contact dermatitis, or that there have been false negative results (more detail below). Common differential diagnoses of eczematous rashes include irritant contact dermatitis, endogenous eczema of varied types including atopic eczema, or protein contact dermatitis. It is important to be aware of the following reasons for negative patch tests.
Contact urticaria/protein contact dermatitis
Some patients have negative tests for delayed hypersensitivity on patch testing, but have in fact developed immediate hypersensitivity. Common causes include natural rubber latex, ammonium persulphate (hairdressing bleach) and foodstuffs in chefs with hand dermatitis. Bakers may develop immediate reactions to wheat and rye, causing both skin problems and baker’s asthma. Contact urticaria will cause transient erythema and itching, but recurrent episodes will cause actual dermatitis (protein contact dermatitis). Testing is performed with blood serum tests (formerly known as RAST) or prick tests.
At SHI Melbourne, we routinely perform a blood serum test on ALL patients using disposable or reusable rubber latex gloves. While latex allergy is less common now, we still see it regularly, especially in the non-acute healthcare setting or in people who buy powdered disposable latex gloves from the supermarket. Of course, these people should also be patch tested to the rubber series which contains rubber accelerators, which may cause allergic contact dermatitis.
Most blood tests are not relevant to making a diagnosis in a patient with dermatitis. The exceptions are suspected allergies to latex and certain foods (as mentioned above), and in the occasional cases where patients present with aggravation due to house dust mite or animal allergy. In these situations a blood test (serum specific IgE), or referral to an allergist for prick testing, is useful.
It is important to be aware that late patch test reactions are possible. They may occur up to 10 days after testing. It is important to mention this to patients at their final appointment, and to give instructions about what to do if a late reaction occurs. The patient should be advised to phone the clinic and if possible, email a photo of their back. They may need to return to the clinic for the dermatologist to review their reactions. At SHI, photos are routinely taken with patches on the back and then a photo of the patches removed, to help identify actual allergen locations if there is a late reaction.
Insert photo as example
Allergens which are more likely to cause late reactions include antibiotics, corticosteroids and some metals, including nickel and gold.
False negative reactions
It is important to be aware of the possibility of false negative results in patch testing. Causes of false negative results include:
- Insufficient penetration of the allergen e.g. due to insufficient amount of allergen applied, patch not applied properly etc.
- Ophthalmic allergens. Eyelid skin is very thin, which makes this area prone to develop contact dermatitis. Ophthalmic allergens (such as beta blockers used for glaucoma) are weak allergens, and sometimes testing these on the thicker skin of the back is negative. Tape stripping the area of the back where the patch tests are to be applied will reduce the skin barrier and increase the penetration of weak allergens. Tape stripping involves placing tape, such as Micropore™, on the skin and removing it repeatedly. The skin should be stripped some 20-30 times. For more information refer to attachment ?
- Topical corticosteroid application in the area of patch testing
- Ultraviolet (UV) light exposure on the back in the days leading up to patch testing. UV exposure can decrease Langerhans cell function which may theoretically diminish delayed hypersensitivity reactions. We advise our patients to avoid UV for a week before testing, but there is little formal evidence as to whether this advice is appropriate. Some authors suggest waiting at least 4 weeks after heavy sun exposure before patch testing10.
On occasion, an allergen may be missed (not applied) during patch testing. This may have occurred because the history taking was incomplete, or the patient forgot to mention particular exposures. The concept of a standard or baseline series was developed to include the most common and important allergens, so that this situation was avoided. This was also the basis of the Australian Baseline Series, which we proposed using evidence from our Australian database of thousands of patch test reactions over many years. If the patient history is suspicious of allergy but patch testing has been negative, it is important to re-visit the history to see if anything has been missed. For more information about the Australian Baseline Series, see attachment ?
Inadequate patch test application
Where patches are applied on the skin can also influence the strength of the patch test reactions. For good adhesion, bony places on the back, such as the scapula, should be avoided. We have seen cases (see below) where the strength of the reaction has varied quite markedly depending on where the tests are applied. In this case a nickel reaction was suspected, but the nickel allergen was re-applied in a different area when the 48-hour reading was negative. Compare the strength of the reaction on the scapula (photo 1) compared to the reaction where the test was applied to the lower back (photo 2).
A similar example occurred with potassium dichromate.
Not enough allergen applied
There have been some studies from Malmö, Sweden which have measured the quantity of allergens dispensed by different patch test nurses/technicians. This is relevant when you are making up your own patch tests, rather than using pre-prepared tests. There will be variation in the strength of patch test reactions if too little or too much allergen is applied. The correct amount is 20 micrograms, and the amount of allergen should look like this on the chamber (thin line from one side of the disc to the other). See page ? for example of this. For liquid allergens, 20 micrograms is approximately 2 drops.
If testing is negative but you are still suspicious of allergy:
The following should be considered if a patient did not react upon patch testing but the dermatologist is still highly suspicious of contact allergy.
- Was an allergen missed- did you test with everything that the patient was exposed to? Did you test patients own products?
- Always record your suspected diagnosis at the start, so you know whether there might be the possibility of missing an allergen. Consider reviewing the history again in more detail.
- Consider duplicate testing of critical allergens e.g. for prosthetic joint allergy
- Involve the patient in the diagnostic process: go through the diagnostic algorithm/flow chart on the patients’ second appointment and provide them with relevant information sheets to engage them in the search for relevant exposure to an allergen whether this be at work or home.
Words of Caution
Be aware of the “Angry back” syndrome
An “Angry back” occurs when all areas where the tests were applied become red and itchy and look like patch test reactions: however they are not true reactions but in fact false positives. They are a result of the skin being very excited and irritable and this is also called the “Excited skin syndrome.” When this occurs, true results are extremely hard to read. This is more likely to occur when patches are applied to active areas of rash or eczema, so it is very important that tests are not applied to areas of active skin inflammation.
Sometimes the false reactions will settle over 2-3 days, leaving the true reactions visible. A late reading such as day 7 may be required for patients that have an angry back.
Sometimes this occurs however because there are in fact many true positive reactions underlying the inflammation, causing a reaction akin to an id or auto-eczematisation reaction.
Example of angry back syndrome
People with pre-existing eczema on their back
If patches are applied to areas of active eczema, the eczema may flare and become quite inflamed. This makes interpreting results very difficult. In our experience, we have also found that in some atopic patients who have eczematous or very dry skin, some brands of patch tests tend to adhere “too well,” meaning that they are very difficult to remove. Remnants of the tape and the adhesive remain on the back until the person can shower at the end of the testing process. If the tape has stuck too well (and won’t come off), olive oil can be used to soak the tapes off. Tape adhesive remover solutions may be used, however we have found these do not remove the patches. Olive oil is best.
Has there been a tape reaction?
On rare occasions there can be reactions to the patch testing tape used. An obvious clue is that the area where the allergen is applied is spared (the actual chamber area), but there is redness and inflammation where the tape was applies. We have found very few adverse reactions to patch tapes in recent years. If patients are concerned about the possibility of tape reactions, albeit rare, we often forward them some practice tapes to apply to their inner arm and leave in place for 48 hours and watch for any reactions, prior to testing.
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