Two things with one name
Clinical patch testing is a diagnostic investigation. A clinician applies a standardised series of allergens to the skin under occlusion, leaves them in place for a defined period, and reads the reactions at set intervals afterwards. It is used to identify allergic contact dermatitis and to work out what specifically is responsible. It requires trained interpretation, and readings taken at the wrong time or by an untrained eye are not informative.
Self testing is applying a small amount of something new to a discreet area of your own skin for a few days before using it properly. It is sensible, free and worth doing. It is not diagnostic and does not become diagnostic by being done carefully.
The distinction matters because people who have done the second sometimes believe they have ruled out the first, and then apply something over a large area of already compromised skin.
Why the self version is worth doing anyway
Two reasons.
The first is irritation, which is different from allergy. Irritant reactions are dose related and occur in anyone given enough exposure, and they are far more common than allergy. Something that stings on a small area will sting more on a large one, and finding that out on a patch of forearm is better than finding it out on a whole face.
The second is that it slows you down. The single most common mistake people make with bad winter skin is to change four things at once, so that when something improves or worsens they cannot tell which change did it. Introducing one thing at a time, in one place, is a discipline that produces information.
How to do the self version usefully
The principles are ordinary.
Use a site that is comparable to where you intend to use the product, because skin from different regions behaves differently. Testing a facial product on a palm tells you very little.
Use it in a place you can see and that is not going to be rubbed by kit, since a mark under a strap will be uninterpretable.
Apply it more than once. A single application detects strong irritation and little else. Repeated application over several days is more informative, and this is one respect in which the self version resembles the clinical one, since allergic reactions typically develop over a period rather than immediately.
Change one thing at a time, and keep a note. Memory across a bad winter is not reliable.
And stop if it stings, burns or becomes red. That is the test producing a result.
| Clinical patch testing | Trying something on a small area | |
|---|---|---|
| Who does it | A healthcare service, with trained reading | You |
| What is applied | A standardised series of allergens | The product you are considering |
| What it identifies | What you are allergic to | Whether this product obviously irritates you |
| What it cannot do | It is not a test of irritancy for a specific product | It cannot rule out or identify an allergy |
| How it is read | At defined intervals by a clinician | By looking, which is not the same thing |
| How you get it | Usually via a GP referral | At home, for nothing |
Ordering framework written by this publication from general skin physiology. It is not a measurement, it is not taken from any study, and no number in it is a reading.
What it cannot do
Being clear here matters more than being encouraging.
It cannot rule out an allergy. Allergic contact dermatitis can develop after repeated exposure over months or years to something previously tolerated, and a negative result on a small area for a few days does not exclude that.
It cannot identify what you are allergic to. If you react to a product with twenty ingredients, you have learned that the product does not suit you, which is worth knowing, and nothing about which ingredient is responsible.
It cannot be interpreted with confidence on skin that is already inflamed, because you cannot separate the reaction from the background.
And it does not substitute for proper investigation when a pattern suggests contact allergy. If a rash appears at a fixed site, recurs with a particular product, garment or task, and settles when the exposure stops, that is a pattern worth taking to a GP, who can refer for clinical patch testing where appropriate.
The version relevant to this readership
For people in cold water sport, the exposures worth testing are not only the products.
New kit that will sit against skin for hours. A different wetsuit lining. A cleaning product used on the kit. A base layer washed in a different detergent. A new pair of gloves. All of these are prolonged contacts against skin that occlusion has softened, which is a far more demanding exposure than a cream on a forearm.
The same discipline applies: introduce one at a time, note what you introduced, and give it a few sessions before deciding.
When to hand it over
A pattern of recurring rash at a fixed site, particularly if it settles away from the exposure and returns with it, is exactly the kind of thing clinical patch testing exists to investigate. So is a rash that spreads beyond a contact area, that is severe, or that keeps returning through a season.
The route in Britain is a GP first, who can refer to dermatology where it is warranted. Occupational cases have an additional dimension, discussed in the article on occupational skin.