Patch testing
The mainstay of diagnosis in allergic contact dermatitis is
the patch test. This test has a sensitivity and specificity of
between 70% and 80%15 (Quality of evidence II.ii) (Strength of
recommendation A).
Patch testing involves the reproduction under the patch tests
of allergic contact dermatitis in an individual sensitized to a
particular antigen(s). The standard method involves the application
of antigen to the skin at standardized concentrations in
an appropriate vehicle and under occlusion. The back is most
commonly used principally for convenience because of the
area available, although the limbs, in particular the outer
upper arms, are also used. Various application systems are
available of which the most commonly used are Finn chambers.
With this system, the investigator adds the individual
allergens to test discs that are loaded on to adhesive tape. Two
preprepared series of patch tests are available – the TRUE
(Pharmacia, Milton Keynes, U.K.) and the Epiquick (Hermal,
Reinbek, Germany) tests. There are few comparative studies
between the different systems. Preprepared tests are significantly
more reliable than operator-prepared tests16–20 (Quality
of evidence I). There is also some evidence that larger chambers
may give more reproducible tests,21 but this may only apply to some allergens22 (Quality of evidence II.ii), and can be used to
obtain a more definite positive reaction when a smaller chamber
has previously given a doubtful one. The International
Contact Dermatitis Research Group has laid down the standardization
of gradings, methods and nomenclature for patch
testing.23