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.