I am pretty sure that you must have the heartworm test before giving meds. I know my DIL is a vet tech and I use the vet where she works. I have had rehome dogs in the past that we didn't know the history and my vet always did blood work to test for heartworm before giving meds. It is my understanding if they are positive for heartworm you can't give the heartworm meds. I would talk to a different vet if it were me.
