Prior to administration, dogs should be tested for existing heartworm infection. (trifexis.com)
Therefore, it is recommended that a blood test for existing heartworm infection is conducted to assess the pet's current heartworm status, before beginning a prevention program. (primarycarevet.com)
Precautions: All dogs should be tested for existing heartworm infection before starting treatment with HEARTGARD which is not effective against adult D. immitis. (medi-vet.com)