When and how often pets should be tested for heartworm infection is also a matter of debate. In making a decision on when to test, we must consider how common heartworm disease is where the pet lives, what heartworm preventive the pet is receiving, and how long the mosquito season lasts.
Cats should be tested before they are started on a heartworm preventive. Experts do not agree on how often a cat that is taking a preventative should be tested, however, it would be based on risk of exposure and consistency of administering preventatives. Consult with your veterinarian to determine what is best for your cat.
My kitties have also been tested before I started the medication - Revolution - that also included protection against Heartworm ( together with Ear Mites, Fleas and Ticks - isn't Revolution amazing?) Now they get the medicine regularly - once a month.
Indoor cats don't usually need heartworm protection unless you get a lot of mosquitos in the house!
I prefer not to overmedicate cats by giving them drugs they don't need. All medications, including Revolution, can have side effects, some of them serious. Best to discuss with your vet what each animal really needs and don't go overboard!