Patch testing is the ‘gold standard’ in the diagnostic practice
of contact dermatitis. This clinical procedure is the mainstay in identifying
and differentiating allergens or irritants that cause contact dermatitis.
It is generally combined with a detailed medical history and skin examination
to attain a comprehensive diagnostic methodology for contact dermatitis.
The patch testing technique was pioneered by a Dr Jadassohn over 100
years ago. It was developed upon as a diagnostic method in contact dermatitis
by Drs Sulzberger and Wise in the United States in 1931. Over the years
since, it has been standardized and further modernized.
The patch testing procedure
Patch testing is an easy, clear-cut process. However, skill and training
comes into play regarding when and what to patch test. The TRUE® Test
is approved by the US Food and Drug Administration (FDA), though various
other tests are available and other countries have their own test standards.
The advantage of the TRUE patch test are:
- It is a time saving process
- It is a preimpregnated test
that is easy to implement
Its disadvantages are:
- It only examines for 23 common allergens. Though screening for the
23 irritants is often sufficiently beneficial, greater testing is always
better. Less common allergens not included in the test could be the
- Extensive testing beyond the 23 allergens have reported 37-76%
results. In fact in one study, 47.3% of patients screened more extensively
showed positive response exclusively to those allergens not included
in the original 23. These other allergens are available in multipurpose
Hence, patch testing that limits the allergens to 23 does have an effect
on the treatment, detection and avoidance of the allergens.
In fact, many professional patch test centers regularly include more
than 50 allergens in their patch tests, following certain extended panels
like the North American Contact Dermatitis Group Screening Series and
the European Standard Series. These series are large panels of allergens
(a few hundred) that doctors can choose from in creating standard and
special patch tests unique for individual patients.
Preparatory steps of a patch test
Professionals conducting a patch test should undertake the following
measures before starting the procedure:
- Find out about the patient’s exposures to allergens at home
and at work.
- Comprehend the home and work environment of the patient.
- Find out
about the details of any trips taken during a vacation and possible
- Information of the patients’ habits, daily
routine, hobbies and pastimes should be gathered. One very important
aspect of this
is to find
out about the personal care products used by the patients and
The TRUE® Test method
The various steps in the process are:
- Knowing about the chemical allergens in various products
the products for the test
- Preparing the patient for the test
- Application of the patches
- Reading the patches
- Interpreting the patch test results
Knowing about the chemical allergens in various products
Chemicals and products that patients suspect may be a cause of their
dermatitis can bring them to their doctors or the patch testing clinic.
Then the doctors can actually test the product on the patient’s
skin and examine the skin reaction. However the doctor is at a disadvanatage
as they probably don’t have a thorough knowledge of the product
constituents and all their effects – there are just too many products
and chemicals out there to know about them all. As a rule, no unfamiliar
product or chemical should be patch tested without some prior research.
A doctor can refer to the Materials Safety Data sheets (MSDS) for all
ingredients to be tested. MSDS sheets should be available for all products
and chemicals available in North America and Europe by law. However, this
list may exclude chemicals that have a low representation in the product.
Because of this, the direct contact of the company that manufactures the
product is the best means to get full details of the composition of products
and all the possible chemicals that might be causing a skin reaction.
Preparing the products for the test
Personal care product tests must follow certain guidelines according
to their usage pattern. They are:
- Leave-on products: The products that are applied to the skin and left
on it, for instance creams and make-ups, should be tested in its present
- Rinse-off products: Products that are washed off with
water, for instance soaps and shampoos, must be diluted before a patch
There tips are vital in ensuring the correct patch test potency of the
various chemicals, in order to get the best results.
Preparing the patient for the test
After the proper screening of product or chemical to be tested, a patient
should be suitably prepared for the test. Here are a few things to consider
when going for patch testing:
- The upper back is the ideal site for the test and this is usually
the area of skin that is chosen for most tests.
- There should be
no sunburn at the site to be tested.
- The location of the patch should
not have been subjected to topical corticosteroids for a week before
- The individual should also have been free of systemic
corticosteroids for a month.
- If indispensable, then a token dose
of 20 mg qd of corticosteroids is acceptable during the patch test.
Application of the patches
Only trained professionals should apply the patches. There are two main
ways of applying the patches:
- Prepackaged allergens placed on the back, as in the TRUE® test.
poured into Finn chambers (little plastic cups) that are attached
to tape and then placed on the back. Thereafter, more tape
is used to further secure the patches.
Once any of the two procedures are over, the patient is discharged
until the next visit after 48 hours, usually with the following
- Keep the back dry.
- Keep the patches secure.
- Avoid extreme sweating to prevent
the patches from being dislodged from their position.
lifting anything heavy to prevent the patches from coming
Antihistamines may be permitted without affecting the results of the
test. This apart, the diagram of the location of allergens must be created
for later reference.
In the second visit after 48 hours, the patches must be checked to determine
whether the testing technique was correct. An initial visual impression
of whether the patches are in place is an indicator. However, the best
evidence is discernible skin imprints in places where the Finn chambers
Reading the patches
Once the patches are removed, their reading begins. The first step to
the process is the demarcation of the spot of application of each allergen.
There are two types of marking pens for this process.
- A permanent surgical marker
- Fluorescent highlighter
The latter is preferred since it is less messy and more permanent in
nature. On the other hand, the permanent marking often spoils clothing
and hinders second readings.
Positive reactions are read in conformity with the International Grading
System. The second reading is conducted 72 hours to a week after the first
use of patches. During the second reading, the allergen diagram is utilized
to trace positive reactions. In case of the fluorescent marker, a Wood’s
lamp (UV light) is often required to locate the markings. Once again,
the positive readings are marked in accordance with the standard grading.
Late readings are also required as some irritants, for example gold and
disperse blue dyes, often show a late response.
Thereafter, the actual products the patient uses are screened and their
constituents equated with the positive reactions. The process should involve
the following categorization:
- Products that are devoid of the alleged allergen(s) and are safe to
- Products that consist of the causative chemical(s) and should
Interpreting the patches
Interpreting the exposures and products is a two-phased process.
Past relevance: For instance, the case of a patient who has a history
of nickel allergy, to jewelry for example, shows a positive outcome on
patch testing. The nickel may have a link with the history of the patient’s
irritation caused by jewelry but no current significance so far as a patient’s
chronic hand dermatitis is concerned.
Current relevance: For instance, a florist suffering from hand dermatitis
and showing a positive response to tuliposidase A and exposure to alstroemeria
while handling flowers, is a case of present relevance.
Unknown relevance: Here we can take the instance of a patient with eyelid
dermatitis and a positive result to thimerosal, but showing no previous
history of any problems with contact lens solutions and no particular
exposure to thimerosal.
By considering this possible options in explaining a response to a patch
test, a doctor can decide if the response is relevant or not and how
important it is in explaining a case of contact dermatitis. So there
is a certain
amount of interpretation required when evaluating a patch test. The
interpretation will decide what advice and treatment a patient would