Resuming heartworm preventative without a test ?

He's on Heartguard. He has had no problem with it in the past. I will probably call my vet and run him out there for a test just to be on the safe side, and hope they get the test results soon - we're leaving Wednesday and I'd like to give him the dose by Monday.
Thanks Dr. Doug - I've learned my lesson to keep him on preventative all 12 months, even when it zero outside!

Rita