The answer is an absolute YES.
It is very important that your dog has a NEGATIVE test every year before starting prevention.
A lot of the medications given to prevent Heartworm Disease can harm your dog if they have the infection, therefore a NEGATIVE test prior to giving the prevention is very important. Also, there is no reason to prevent Heartworm Disease with a once-a-month preventative if your dog already has the infection. If your dog has Heartworm Disease then they need to be treated, not given a prevention.