I think they should do the aspiration.
Let me tell you this - when I got pain in my left knee, my surgeon first did bloods for infection which were clear, then he did a bone scan which, like you, showed flares around one of the implants. On a bone scan, this just indicates some cellular activity but not what's causing it. It could be a loosening or an infection. The only way to be thorough about this is to get fluid from the joint which my chap did. In fact, he aspirated both knees on a 'better be safe than sorry' basis. I was lucky, the synovial fluid was clear of infection so then we knew it was a loosening and I'm waiting to have a revision.
I cannot emphasise this strongly enough - you need to have your knee aspirated to confirm or refute the negative result of the blood test which only shows, as it were, one side of the coin.