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 problem.
• Extensive testing beyond the 23 allergens have reported 37-76% more positive 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 syringes. 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 exposures therein.
• 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 their families.

The TRUE® Test method

The various steps in the process are:

• Knowing about the chemical allergens in various products
• Preparing 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 condition.
• Rinse-off products: Products that are washed off with water, for instance soaps and shampoos, must be diluted before a patch test.

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 test.
• 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.
• Allergens 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 directives:

• Keep the back dry.
• Keep the patches secure.
• Avoid extreme sweating to prevent the patches from being dislodged from their position.
• Avoid lifting anything heavy to prevent the patches from coming loose.

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 were attached.

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 use
• Products that consist of the causative chemical(s) and should be avoided

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 get.