The diagnosis in all these cases can be made based on patch testing. Chambers loaded with specific concentrations and amounts of known allergens are applied to the back of the individual. The patches are left on for 48 hours and then removed. After an hour, the first reading is made, based on the reaction seen under the chamber. Elevation of the skin or vesiculation is considered to be a positive reaction. The presence of only macular erythema needs to be interpreted cautiously but can be considered a positive result in certain situations. Pustular reactions are considered to be irritant reactions and not relevant. The patient must come back for a final reading 3 to 7 days after application of the patches. This is the most critical reading and gives the most valuable information.