Dawn, probably testing is more reliable today than when I had to deal with it, particularly if your vet combines the newer SNAP ELISA test with 1 or 2 of the other methods. Back then, testing was notoriously unreliable, with numerous false negatives occurring. Here is some more info from CAPC, including what you can to to prevent any further contamination, and also help on deciding whether to give your other dogs precautionary preventative doses of panacur.
http://www.capcvet.org/capc-recommendations/giardia/
