NickD, I am wondering if your past dog had a seizure type disorder. I was working at a shelter that was housing a gorgeous mastiff X. Was aprox 6 month old. Owners gave it up because of sudden onset of agression for no reason. We couldn't figure out why this dog would turn on you out of nowhere.... until she had a gran mal seziure infront of us. The shelter got her on anti seziure meds and she didn't have another episode. Although, I guess it wouldn't explain the blood test you got back from the U of Penn. But, its a theory.