Contact dermatitis
	Patch (contact) testing
This is caused by an allergic reaction to something with which the patient is in contact.  A very wide range of substances is known to be capable of causing this problem.

For example, builders working with cement are at risk from developing cement dermatitis because of sensitivity to potassium dichromate (picture above).  As the same substance is used in the tanning industry, patients who experience problems with cement, may also experience problems on their feet from leather shoes.  Nurses who wear rubber gloves are at risk from contact eczema to chemical substances present in the gloves.

So to establish if eczema is being caused by contact, the physician first needs to take a full history to establish the kinds of substance with which the patient is in regular contact.  Quite often, the original cause may have gone, and it turns out to be chemical substances such as lanolin in the creams the patient is applying that are causing the problem.  Cosmetics and hair and clothing dyes can also turn out to be the cause.

Patch (contact) testing

To establish if contact sensitivity is causing eczema, it is necessary to have patch testing performed.

In this test, a standard battery of thirty commons skin sensitisers, each prepared in petroleum jelly, is placed on the back of the patient.  Three adhesive strips, each containing ten substances each contained in an individual tiny aluminium chamber, form the complete test.  The patient then has these strips removed after 48 hours, and the reactions are read after 96 hours. 

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