The "strip" patch test:: results of a multicentre study towards a standardization

Background The "strip" patch test (SPT) is a variant of patch testing which is used for substances with a poor percutaneous penetration. Penetration of the substances is enhanced by repeated applications of adhesive tape prior to their application to the skin. However, no guidelines exist for standardized performance of the SPT. Objectives The aim of this multicentre study was to obtain a first practical approach towards a standardized SPT procedure. Methods Intact noninflamed skin of the upper back of 83 healthy volunteers was tape-stripped. For sequential strips, a 25-mm diameter 3M Blenderm surgical tape was vertically applied and gently pressed downward using the fingertips for about 2 s. The tape was removed in one quick movement at an angle of 45degrees in the direction of adherence. Each strip was performed with a new piece of tape on exactly the same skin area. Results In each subject, we first determined the number of strips (A) until the skin surface started to glisten and calculated the median number of strips ((A) over bar) in the sample ((A) over bar =26 strips). We then ascertained the median number of strips ((a) over bar) in the sample that was necessary to achieve a statistically significant and twofold increase in TEWL ((a) over bar =11 strips), revealing a "critical" stratum corneum strip depth. The unknown number of strips (a) for each subject was finally calculated from the formula a/A=(a) over bar/(A) over bar, i.e. the individual number of strips (A) until the skin surface started to glisten was multiplied by a derived tape-specific correction factor (cf=(a) over bar/(A) over bar =11/26=0.4). The increase in percutaneous penetration in strip patch testing by performing "a" strips versus conventional patch testing was shown by scoring of clinical and subjective SLS irritant reactions. Conclusions The present multicentre study outlines an experimentally derived approach for a uniform SPT procedure, which does not require the use of complex technical equipment. This first approach now requires validation by a study involving the application of allergens to obtain evidence of enhancement in the sensitivity of patch testing.