Eczema diagnosis is generally based on the appearance of inflamed itchy skin .

However, given the many possible reasons for eczema flare ups, a doctor is likely to ascertain a number of other things before making a judgment:

An insight to family history
Dietary habits
Lifestyle habits
Allergic tendencies
Any prescribed drug intake
Any chemical or material exposure at home or workplace

To determine whether an eczema flare is the result of an allergen, a doctor may test the blood for the levels of antibodies and the numbers of certain types of cells. In eczema, the blood may show a raised IgE or an eosinophilia.

Radioallergosorbent Test (RAST)

The blood can also be sent for a specific test called Radioallergosorbent Test (RAST) or a Paper Radioimmunosorbent Test (PRIST). In the test, blood is mixed separately with many different allergens and the antibody levels measured. High levels of antibodies in the blood signify an allergy to that substance.

Another test for eczema is skin patch testing. The suspected irritant is applied to the skin and held in place with an adhesive patch. Another patch with nothing is also applied as a control. After 24 to 48 hours, the patch is removed. If the skin under the suspect patch is red and swollen, the result is positive and the person is probably allergic to that substance.

Skin lesion biopsy.

Occasionally, the diagnosis may also involve a skin lesion biopsy: removal of a small piece of skin for microscopic examination in a laboratory.

Blood tests and biopsies are not always necessary for eczema diagnosis. However, doctors will at times require them if the symptoms are unusual, severe or in order to identify particular triggers.