I havent had a positive PPD, but I've had the nursescreening me take me to the head of employee health. She took one look at me, saw I was a fair skinned, freckled redhead and said it was an allergy. Of course, I've had 3-4 PPDs in the past year and the "reaction" keeps getting bigger and bigger. Actually, my last one was SORE. After about 24 hours, I did have a reddened area about the size of a dime, but it wasnt indurated.
Here's some pics: http://images.google.com/images?q=positive+mantoux&um=1&hl=en&sa=2