The outcome of an additional patch-test reading on days 6 or 7.

In a retrospective study (period 1997-1998), the usefulness of an additional patch-test reading on day (D) 6 or 7 was investigated. In 62 out of 760 patients (8.2%), 77 late-positive reactions, those manifest after D3, were seen. 4 late reactions of allergens of which a related allergen reacted earlier (at D2 or D3) in the same patient were not incorporated in the study as late-positive. Allergens most involved in producing late-positive reactions were nickel sulfate (20 reactions), neomycin sulfate (7), tixocortol-21-pivalate (5), p.t. butylphenol formaldehyde resin (5) and Cl+ Me isothiazolinone (5). Special attention is paid to ?+ reactions at D6 or D7, because these reactions could add extra information to the outcome of patch-test readings, considering that this might be a group of allergens that has been positive between D3 and D6/7 or become positive later on. When this group is included, the total number of late-positive reactions is 105 in 88 out of 760 patients (11.6%). By doing D3 readings in combination with D6 or D7 readings only, one would miss 24 positive reactions. However, we concluded that it was preferable to have a reading on D2 and D3. An extra reading on D6 or D7 is very useful as it gives additional information in 8.2% of patch-tested patients.