The reproducibility of patch tests

There is conflicting evidence regarding the reproducibility of patch testin g. Discordant results have been reported in up to 44% of cases. The clinica l relevance of these discordant patch tests has not been previously assesse d. We studied 383 consecutive patients receiving simultaneous duplicate pat ch testing on opposite sides of the upper back with 10 allergens from the E uropean standard series. Completely discordant patch tests-a negative test on one side with a positive test on the opposite side-were recorded in 30 ( 8%) patients. Two patients had discordant tests to two of the allergens; 28 had discordant reactions to one allergen, Completely discordant tests were recorded for nickel in 10 (3%) patients, balsam of Peru in two (0.5%), thi omersal in one (0.3%), cobalt in four (1%), paraphenylenediamine in three ( 0.8%), fragrance mix in two (0.5%), formaldehyde in four (1%), potassium di chromate in two (0.5%), lanolin in three (0.8%) and Kathon CG in one (0.3%) . Of those patients with completely discordant patch tests, the allergen wa s deemed to be a true positive in 11 (3% of total) cases and of possible re levance in a further three, The allergen was felt to be relevant to the pre senting complaint in seven (2% of total) patients.