I think I would re-test in 6 weeks (maybe more for peace of mind, it's $80-100 for bloodwork plus 4dx around here, maybe $120 with tax and stuff). I haven't used tick stuff (for a variety of reasons but looks like I will seriously hvae to consider it with my next dog; at 13 with health issues Rocky isn't really walking much or outside enough for me to use it at this point). I found 1-2 ticks on each dog over the years (one per year); they were removed engorged and we just re-tested in the fall to be safe (er).
Edited: actually I may not test right at 6 weeks, but test in the fall after tick season.