title: Interference of the position of substances in an epicutaneous patch test battery with the occurrence of false-positive results.
abstract: BACKGROUND
  Epicutaneous patch tests represent a practical and objective method that help in the etiologic diagnosis of allergic contact dermatitis. The technique of patch test application is an important factor in obtaining good results.
  
  
  OBJECTIVE
  The aims of this study were (1) to determine whether the substances that form the test battery interfere with the patch test result and (2) to establish a rule for positioning the substances during patch test application.
  
  
  METHODS
  Two hundred patients were studied. The standard patch test battery was applied in 3 versions. The original, with the substances applied in alphabetical order was called version 1 (V1) and tested on the left back in all patients In 100 patients, on the right side, the same substances were tested but applied at different positions, avoiding the proximity of elements with a tendency to cross reaction and/or cosensitizing. This version of the battery was named version 2 (V2). Another 100 patients had V1 applied to the left back, and, on the right side, the version 3 of the battery (V3) was applied, consisting of the same allergens but placed close to those with a tendency for cross reaction and/or cosensitization.
  
  
  RESULTS
  In the group V1-V2, 163 results were positive in V1 and 124 in V2, with the difference being statistically significant (P <.05). In the group V1-V3, 134 results were positive in V1 and 207 in V3, with the difference also being significant (P <.005). The substances with the largest number of positive results, when tested close to other elements with a tendency for chemical affinity, were parabens, fragrance-mix, thimerosal, balsam of Peru, potassium dichromate, cobalt chloride, mercapto mix, and propylene glycol.
  
  
  CONCLUSIONS
  (1) The substances of a patch test battery eliciting positive responses may interfere with test positivity to neighboring substances. (2) In addition to the already established techniques, the position of the substances forming the test battery needs to be determined; substances with a tendency to cross reaction or cosensitizing substances should be tested distant from one another, thus preventing the occurrence of false-positive results.
authors.affiliation: Dermatology Clinic, Department of Medicine of Santa Casa de São Paulo, São Paulo, SP, Brazil.
authors.forename: Ida
authors.initials: I
authors.lastname: Duarte
authors.forename: Rosana
authors.initials: R
authors.lastname: Lazzarini
authors.forename: Roberta
authors.initials: R
authors.lastname: Buense
mesh_terms: D000485:Allergens / Q000008:administration & dosage; D017449:Dermatitis, Allergic Contact / Q000175:diagnosis* / Q000209:etiology; D005189:False Positive Reactions; D006801:Humans; D010328:Patch Tests / Q000379:methods*
chemical_list: D000485:Allergens