I had that rash a couple of weeks ago, and got very swollen and painful joints. The Doc said it was "just a virus" but took bloods anyway becasue he wanted to check that the pain and swelling was caused by my Fibro and not the rash!
Anyway got a letter back today saying they need to be done again, so i called health centre to ask why and the receptionist said it was becasue my rheumatiod factor was very high. I've had tests taken for that before, and it's always been normal