That's a hard thing to find. You can look at ATDSRs fact sheets on Co and Cr, and any other heavy metals for that matter. The problem with that is that it's from ingestion or inhalation of the contaminant and not from within a fairly benign environment, like a joint, within your body. I've been trying to find a study that looks at the level of Co and Cr in the blood and how that relates to an impact, both acute and chronic on human health. Not easy to find. You'll probably find more on Cr because of the connection to hexavalent Cr and all the hubbabb about it. However, I don't think that's the form of Cr from the CoCr alloy. As far as Co? A study from the '60's on Quebec beer head retention and Co salts? That might be as good as it gets.
Long story short, you can use the Co and Cr action levels as a guide to consider addtional assessment. 5-7 ppb is not horrible. I think 7 ppb for Co is the level at which additional assessment is considered (FDA?). Don't quote me on that, double check it. Additional assessment would be talking to an experienced HR surgeon, maybe an x-ray, a few more blood tests. If there's no pain or discomfort, then maybe annual monitoiring of blood is all that is required.
Good luck.