Did your cat have the diarrhea before the vet ran these tests? There are a few things you could try to help. You can withhold food for a day just to give the stomach a chance to heal itself and then give small meals every few hours when you begin feeding again to make sure you don't overwhelm the stomach. But you should consult with your vet about it. Maybe call them and ask if this would be okay. Also, there are fibre diets out there like Medi-Cal Fibre that can help cats if they need to add bulk to their stools. It also helps if they have the opposite problem of needing softer stools. Not sure where you live but this is a Canadian diet. Again, a quick call to the vet would help. You could also ask them about pumpkin.