Dr Catherine Drummond :: Dermatologist

Services

 

PATCH TESTING

Patch testing is a process used to detect whether someone has allergic contact dermatitis. During patch testing, small amounts of chemicals or products that are used at work or home are diluted and placed onto discs mounted on hypoallergenic tape and then placed on the back.

Services

Patch testing sample

The number of patches applied is specific to each individual. There are 10 discs on each patch, and up to 10 patches are placed on the back. 


Taking a history about your exposure to potential allergens at home and at work helps the doctor decide what the most likely problem substances are. It is not possible to test you against all available allergens, only the most likely.


Chemicals from the “Australian Baseline Series”  are almost always used. There are 60 allergens in this series. If you have specific exposures to other chemicals, allergens from other series may be chosen. These are ordered from the Contact Allergen Bank Australia, in Melbourne.

Patients are also tested to samples of their own products. This helps to improved diagnosis because commercial allergens will not detect all cases of allergic contact dermatitis. Also, sometimes a patient may not react to individual ingredients, but may react to a complete product.


Patches are applied on a Monday and then removed on a Wednesday. Dr Drummond or one of the practice nurses will look for any reactions at that time, and then again on the Friday, to check for late reactions.


Showers must be avoided for the length of the testing (Monday through to Friday after the final appointment). This is to allow reliable assessment of reactions.

During testing, sweating must be avoided, as sweating can sometimes cause patches to fall off or loosen.


There is often confusion about different methods of diagnosing allergy. Skin prick testing is often used to diagnose food allergies or airborne allergies to grasses, pollens, dust mite and pet fur. These reactions are called immediate hypersensitivity reactions and most commonly manifests as urticaria (hives), hayfever, asthma or even anaphylaxis. Skin prick testing is performed by immunologists rather than dermatologists. Patch testing is looking for delayed hypersensitivity reactions which manifest as dermatitis.


Patch testing is arranged after an initial consultation for assessment of a rash, to decide if it is an appropriate investigation. Other diagnoses have to be excluded e.g. psoriasis or tinea.


Patch testing is performed in those people with severe, persistent dermatitis who have not improved after advice about appropriate skin care, reducing irritation, and treatment with topical corticosteroid ointments (adequate potency, amount and duration). Most dermatitis is endogenous, (atopic dermatitis) or due to irritation (irritant contact dermatitis), rather than due to allergy.


There is a Medicare item number for patch testing, depending on number of allergens applied. There will be out of pocket costs. You will be given a quotation before patch testing is arranged.


Contact Dermatitis

Dermatitis (or eczema) is classified as being either endogenous or exogenous. The most common type of endogenous dermatitis is atopic dermatitis, which occurs in people who have inherited a tendency to a faulty skin barrier, meaning they have sensitive skin. These people often have a personal or family history of eczema, asthma or hayfever. They are more prone to develop both irritant and allergic contact dermatitis.


Contact dermatitis is caused by exogenous or external factors. There are two main types of contact dermatitis: allergic contact dermatitis and irritant contact dermatitis.


Irritant contact dermatitis is the more common, but there is no specific test for it. Instead it is diagnosed based on history of exposure to different substances, and negative patch tests. Common skin irritants are soaps and detergents, water, heat, sweating and friction.

Allergic contact dermatitis is diagnosed by patch testing. This is a delayed type hypersensitivity or allergy.


People are not born allergic to substances, but they can develop contact allergies at any time during their life. To become allergic to something, you must have been exposed to it before. The allergy lasts for life. Your body has learnt to react to this substance and each time that you are exposed to it, it will produce an allergic reaction (dermatitis).


The dermatitis can be treated, but there is no cure that will stop you being allergic to that substance. The best treatment is to avoid exposure to the substance in the future. If the particular substance can be avoided, then the rash should settle within a few months. This will take longer if there is continued exposure, or if the initial rash was very severe or had been present for a long time. If irritant factors are not addressed, dermatitis will also persist. The skin will remain quite easily irritated for some time after it appears to be better.


Appointments

Most patients have to attend 3 appointments over a 5 day period.

Appointment 1 (Monday afternoon):
Patients will have a consultation with Dr Drummond who will decide what needs to be tested. The clinic nurse will then make the patches up and stick them on to the patient’s back. This appointment may take up to an hour.

Appointment 2 (Wednesday):
The patches are removed and the patient will then wait at least 15 minutes before their back is looked at to see if there have been any allergic reactions. This appointment may be conducted by the practice nurse. This appointment lasts about ½ hour.

Appointment 3 (Friday):
Dr Drummond will again look at the patient’s back to see if there has been any other reactions appearing where the patches were. Dr Drummond will then discuss the results, management of your condition and skincare. The duration of this appointment depends on the results and complexity of the problem and its management.


Once the patches are taken off, the dermatologist looks for any red raised spots that may have appeared. These are normally smaller than a 5 cent coin and appear where the discs were placed. Sometimes people find these to be itchy.

What if the patch testing is negative?

If this testing is negative, it may suggest that you have a different skin condition such as irritant contact dermatitis, eczema or contact urticaria (other forms of testing known as prick testing or a blood test is used to diagnose contact urticaria).

Reactions to patch testing

Patients are often disappointed if patch testing is negative but really this is very helpful as it means none of the products you are using are the direct cause of your dermatitis.

Dr Catherine Drummond copyright 2016

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Australian National University

Dr Catherine Drummond  is a dermatologist with subspecialty interests in paediatric and vulval dermatology. She is also an Accredited NSW Workcover Assessor of Permanent Impairment.

The Australasian College of Dermatologists

ACT Dermatology

Suite 6, 5 McKay Gardens Professional Centre, Turner ACT 2612. Phone: 02 6247 5429

The Skin Hospital

121 Crown St Darlinghurst NSW 2010

Phone: 02 8651 2000

ASSESS Medical Group

Level 5, 179 Elizabeth Street, Sydney NSW 2000

Phone: 02 9261 56990

Example of positive patch test reactions

Example of positive patch testing