^For me, this would happen when I would be having so much difficulty hitting that I would give up, or else I would prepare a fresh shot, and then the next day when I was out of drugs I'd be like, "hmm... I still have this rig full of drugs with a little blood in it..." Also, sometimes it would take me like 5 hours to hit a vein properly, but during that process I'd have blood enter the syringe, not enough for a proper register, like that trickle that meant I knew I would miss if I actually injected, or it would register a bit and then magically stop working, and so on. Damaged veins. 5-10 seconds, I envy you man.
I think the best idea would be to throw it out, if that's not going to happen then filter it and put it in a fresh syringe.