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Patch testing is a measure of delayed type (type IV) hypersensitivity and is used to investigate a suspected allergic contact dermatitis (see Chapter 22). It is of no value in the investigation of urticarial (type I) reactions.

Small amounts of allergens in suitable concentration are placed on aluminium discs or small plastic trays (Figure 7.3) and applied to the back of the patient (Figure 7.4), and then securely attached with hypoallergenic tape. These are removed and read at 48 hours, and then read again at 96 hours after application. A positive test results in an eczematous reaction at the site of application (Figure 7.5).

Table 7.1 Uses of Wood’s light in vulval disease.

Disease	Fluorescence colour
Vitiligo	Bright white
Infections
Pseudomonas	Green
Erythrasma	Coral‐red
Tinea capitis – Microsporum species	Bright green
Tinea capitis – Trichophyton schoenleinii (Note: Other trichophyton species do not fluoresce)	Blue
Pityriasis versicolor	Copper‐orange

Figure 7.3 Allergens for testing placed in plastic trays.

There are standard series that are used in all patients undergoing patch testing, but it is important to extend these for investigation of an anogenital dermatosis. Patients should be referred to a contact dermatitis clinic so that multiple series to include preservatives, fragrances, medicaments, and topical steroids are tested, otherwise relevant allergens may be missed. It is also helpful to test the patient’s own products that have been used, but these may require dilution to avoid severe irritant reactions. Education on sources of any relevant positive allergens is then provided so that they can be avoided in future.