 intraderm test cobalt chlorid select group patient cobalt allergi nickel allergi well normal control test intraderm cobalt chlorid dilut 10 10 reaction wheal flare type appear hour 24-hour read mislead papular respons cobalt chlorid 10 read 48 96 hour inject appear reliabl test cobalt allergi patient combin nickel-cobalt allergi exclud studi question group reaction discuss nickel allerg stronger reaction control impur cobalt chlorid prefer degre puriti cobalt chlorid ten time greater 