Patch tests are in vivo diagnostic tests indicated in case of suspicion of contact allergy. They are performed by placing suspected allergens on normal skin in the correct way and in a suitable concentration. Contraindication applies in acute or widespread dermatitis, when the patient is undergoing systemic corticosteroid therapy or immunosuppressive drugs, during pregnancy, and when the patient is exposed to UVB-UVA rays. Application of the patch tests to the skin shall be done on the back and the outer side of the upper part of the arms. The test apparatus is removed 48 h after application. A first reading is made 30–60 min after the removal of the patches; a second reading is made at 72 or 96 h.