Patch tests can detect a wide range of sensitivities to inorganic and organic chemicals, drugs, biologic molecules, inhalants acting as contactants, food allergens, allergens that have not been commercially extracted, and solid allergens. Because patch tests detect the full range of immunologic reactions, Gell and Coombs type I to IV, they may be uniquely reactive when other allergy tests are negative. Because of the large number of published studies that utilize patch testing, clinicians often can use a literature search of prior studies of similar problems, or for particular allergens, to choose technical details that make successful patch testing more likely.