I'm so incredibly sorry. =0( Not being able to figure out what's going on is the WORST. Especially combined with rapid decline. I hope the transfusion helps and he perks back up.
mofongo, I don't normally read ML, but saw this post on the app. My dog had hemolytic anemia last summer, and it was the same thing - she was fine 2-3 days before, and then was suddenly critically ill. In the end, she had 4 transfusions, with 2 being pretty close together in the beginning, before they figured out what it was. That diagnostic period was excruciating.
I just wanted to offer my encouragement. My dog is perfectly healthy now! If your dog doesn't perk up enough from the first transfusion, I just wanted to give you a success story from keeping going with the treatment. I'm sure you have wonderful vet care, but if you're interested in what our vet (we went to an emergency hospital) did, please feel free to PM.
Love of my life baby boy born 11/11. One and done not by choice; 3 years of TTC yielded 4 MMC and 2 CPs, through 4 IUIs and 2 IVFs. Focusing on making the world a better place instead...and running.