We have two dogs just getting over a bout of bloody diarrhea and during our search for possible causes, another member mentioned hemorrhagic diarrhea (HGE) to me. My connection is so slow that I can't search out a link for you, but if you browse for 'hemorrhagic diarrhea' you'll find a lot of info. It's characterized by 'raspberry jam' diarrhea and lethargy...and if I'm remembering right, it's diagnosed primarily by ruling out other causes...
Our two dogs did
not have HGE (no lethargy)...but they, too, have tested negative for everything. We're beginning to think that they may have picked up giardia (it's very rainy here with lots of puddles standing in the yard to drink from
). Giardia can be difficult to detect in some cases.
K, so now I'm out of ideas.
I hope your dog is on the road to recovery soon.
It does sound like your vet is doing a good job trying to get your dog healthy again.