Patch test (medicine)
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|Patch test (medicine)|
A patch test is a method used to determine if a specific substance causes allergic inflammation of the skin. Any individual with eczema suspected of having allergic contact dermatitis and/or atopic dermatitis needs patch testing.
Patch testing helps identify which substances may be causing a reaction in a patient. It is intended to produce a local allergic reaction on a small area of your back where the diluted chemicals were planted. The chemicals included in the patch test kit are the offenders in approximately 85-90 percent of contact allergic eczema and include chemicals present in metals (e.g. nickel), rubber, leather, hair dyes, formaldehyde, lanolin, fragrance, preservative and other additives.
A patch test relies on the principle of a type IV hypersensitivity reaction.
The first step in becoming allergic is sensitization. When the skin is exposed to an allergen, the antigen presenting cells (APCs) - also known as Langerhans cell or Dermal Dendritic Cell - eat up substance (phagocytoze) and break it into smaller pieces. This is where a substance is recognized by immune cells in the skin. They then put parts of the substance onto their surface (technically holds the part of the molecule on the surface in the major histocompatibility complex type two (MHC-II). Once this is done the APC moves down the lymphatic system to a lymph node where it presents this part of the substance (what we now call an antigen) to a particular immune cell called a CD4+ T-cell or T-helper cell. The T-cell, if it recognizes the substance as dangerous, expands in number and sends out more of itself to the skin, at the site of antigen exposure. When the skin is again exposed to the antigen, the memory t-cells in the skin recognize the antigen and produce cytokines (chemical signals) which cause more T-cells to migrate from blood vessels. This starts a complex immune cascade leading to skin inflammation, itching and the typical rash of contact dermatitis. In general, it takes 2 to 4 days for a response in patch testing to develop. The patch test is really just induction of a contact dermatitis in a small area.
Interestingly, the size of the molecule necessary to be picked up and recognized is ten times the size of the largest molecule that can pass through the skin. Therefore, it is likely that an antigen (like nickel) when it has passed through the skin, combines with something else before it is recognized.
Setting up a service 
Setting up a patch testing service is a time consuming process (see below) and takes up three clinic spaces a week for each patient. It is useful to know how many patients are likely to be seen and how useful the testing will be to pick up positive results. Studies have shown that the optimum number of patients to be tested is 1 in 700 of a population. So in a population of 70,000, 100 patients should be tested annually to detect contact allergy.
Coupled with this, differences exist between the rates of positive reactions to the patients tested in different centers. Effectively, the greater the proportion of the population tested for contact allergy, the lower the proportion of positive outcomes identified. When setting up a service, it is important to know what impact it will have on the population.
The British Association of Dermatology has set guidelines for patch testing within the UK which state that a patch test service needs:
- A named lead dermatologist for the unit who has received training for at least 6 months at a recognized contact dermatitis investigation unit or who can demonstrate comparable experience.
- That the contact dermatitis investigation unit conforms to best practice guidelines. The unit and the staff should
- Have a dedicated investigation clinic which should include an area for storage (refrigerator) and preparation of allergens.
- Record investigation results on an electronic database with a minimum data set: site of onset of dermatitis and duration; gender, occupational, atopy, hand dermatitis, leg dermatitis, face dermatitis and age index; details of occupation and leisure activities; patch test results including type (allergic/irritant) and severity of reaction; relevance of positive tests, occupational or otherwise; and final diagnosis.
- Participate in regular audit of data and ‘benchmarks’ results with nationally pooled data. This is evolving and will be reviewed periodically.
- The lead dermatologist demonstrates regular attendance at CME-approved update meetings on contact dermatitis (at least every 2 years).
- The unit should have up-to-date reference textbooks on contact dermatitis including occupational dermatitis and relevant journals.
Prior to testing, avoid taking oral prednisone or other immunosuppressive medications for at least a week prior to testing. Steroid inhalers are OK to use. Avoid sunlight/sunburn for at least a week on the back as this may suppress positive reactions. Antihistamines such as diphenhydramine (Benadryl) or cetirizine (Zyrtec) are permissible prior to and during testing.
Application of the patch tests will take about half an hour, though many times the overall appointment time will be longer as your provider will take an extensive history. Tiny quantities of 25 to ~150 materials (allergens) in individual square plastic or round aluminium chambers are applied to the upper back. They are kept in place with special hypoallergenic adhesive tape. The patches stay in place undisturbed for at least 48 hours. Getting the back wet during patch testing should be avoided (no shower). Vigorous exercise or stretching may disrupt the tests.
At the second appointment, usually 48 hours later, the patches will be removed. Sometimes further patches are applied. The back is marked with an indelible black felt tip pen or other suitable marker to identify the test sites and a preliminary reading is done. These marks must be visible at the third appointment, usually 24–48 hours later (72–96 hours after application). The back should be checked and if necessary re-marked on several occasions between the 2nd and 3rd appointments. In some cases, a reading at 7 days may be requested, especially if a special metal series is tested.
Interpretation of the results 
The dermatologist or allergist will complete a record form at the second and third appointments (usually 48 and 72/96 hour readings). The result for each test site is recorded. One system used is as follows:
- Negative (-)
- Irritant reaction (IR)
- Equivocal / uncertain (+/-)
- Extreme reaction (+++)
Irritant reactions include miliaria (sweat rash), follicular pustules and burn-like reactions. Uncertain reactions refer to a pink area under the test chamber. Weak positives are slightly elevated pink or red plaques, usually with mild vesiculation. Strong positives are ‘papulovesicles’ and extreme reactions have spreading redness, severe itching and blisters or ulcers.
Relevance is determined by exposure to the positive allergen(s) and is rated as definite, probable, possible, past or unknown. For an allergen to have definite relevance, the product the patient is exposed to must be tested and also be positive in addition to the test allergen. Probable would be used to describe a positive allergen ingredient which is in a product the patient uses (i.e. quaternium-15 listed in a moisturizing cream used on the sites of dermatitis). The interpretation of the results requires considerable experience and training. A positive patch test(s), might not explain the present skin problem since the test only indicates that the individual became allergic during the encounters with that chemical(s) at some point in their life. Relevance, therefore, has to be established by determining the casual relationship between the positive test(s) and the eczema. The confirmation of relevance will occur after the patient has avoided exposure to the chemical(s) and after they have noticed that the improvement or clearance of your dermatitis is directly related to this avoidance. This outcome usually occurs within four to six weeks after stopping the exposure to the chemical(s).
If all patch tests are negative, the eczema is probably not due to an allergic reaction to a contactant. It is possible, however, that you were not tested to other chemical(s) that can produce allergic reactions on the rare occasions. If the suspicion is high in spite of negative patch testing, further investigation might be required. This can be discussed during the final evaluation of the patch test procedure.
Common allergens 
The most frequent allergen that was recorded in many research studies all around the world is Nickel. Nickel allergy is more prevalent in young women and it is especially associated with ear piercing or any nickel-containing watch, belt, zipper or jewelry. Other common allergens are surveyed in North America by the North American Contact Dermatitis Group (NACDG).
The latest update of top allergens from 2005-2006 were: Nickel sulfate (19.0%), Myroxylon pereirae (balsam of Peru, 11.9%), fragrance mix I (11.5%), quaternium-15 (10.3%), neomycin (10.0%), bacitracin (9.2%), formaldehyde (9.0%), cobalt chloride (8.4%), methyldibromoglutaronitrile/phenoxyethanol (5.8%), p-phenylenediamine (5.0%), potassium dichromate (4.8%), carba mix (3.9%), thiuram mix (3.9%), diazolidinyl urea (3.7%), and 2-bromo-2-nitropropane-1,3-diol (3.4%)
Food allergy 
There is often an assumption that certain foods can cause or worsen skin complaints like eczema. However, there is very little evidence that cutting out foods such as milk and eggs actually improves eczema.
Dermatologists may refer patients with suspected food allergies for patch testing. Sometimes this is justified as certain food additives and flavorings can cause dermatitis around the mouth, around the anus and vagina as food allergens pass out of the body or cause a widespread rash on the skin. While this is controversial, allergens such as nickel, Balsam of Peru, parabens, sodium benzoate or cinnamic aldehyde may worsen or cause skin rashes.
However, the foods that cause urticaria (hives) or anaphylaxis (such as peanuts) cause a type I hypersensitivity reaction whereby the part of the food molecule is directly recognized by cells close to the skin called mast cells. Mast cells have antibodies on their surface called immunoglobulin E (IgE). These act as receptors and if they recognize the allergen, they release their contents, causing an immediate allergic reaction. Type I reactions like anaphylaxis are immediate and do not take 2 to 4 days to appear. In a recent study of patients with chronic hives who were patch tested, those who were found allergic and avoided all contact with their allergen, including dietary intake, stopped having hives. Those who started eating their allergen again had recurrence of their hives. Often, patch testing for food allergies is not necessary, but in selected individuals it may be helpful.
See also 
- Bhushan M. Beck, MH. An audit to identify the optimum referral rate to a contact dermatitis investigation unit. British Journal of Dermatology, 1999 141(3) P570-572
- Statham B, Mughal A, Bodger O. The relationship between the proportion of the population tested annually and the rate of patients with positive patch test reactions. Contact Dermatitis. 2011 64(1) P54-57
- J. Bourke, I. Coulson and J. English Guidelines for the management of contact dermatitis: British Journal of Dermatology 2009 160 P946-954.
- Zug KA, Warshaw EM, Fowler JF Jr, Maibach HI, Belsito DL, Pratt MD, Sasseville D, Storrs FJ, Taylor JS, Mathias CG, Deleo VA, Rietschel RL, Marks J. Patch-test results of the North American Contact Dermatitis Group 2005-2006. Dermatitis. 2009 May-Jun;20(3):149-60.
- Guerra L, Rogkakou A, Massacane P, Gamalero C, Compalati E, Zanella C, Scordamaglia A, Canonica WG, Passalacqua G. Role of contact sensitization in chronic urticaria. J Am Acad Dermatol 2007; 56:88-90.
- Patch tests at the US National Library of Medicine Medical Subject Headings (MeSH)
- DermNet NZ: Patch tests (contact allergy testing)
- Netdoctor: Patch testing for skin allergies
- American Contact Dermatitis Society