My pup is nine monthes old and has been on heartworm preventive since 8 weeks, she has missed two doses, does she really need to be retested before starting again? She has also been on K-9 Advantix and has not missed any, this repels mesquitios.

I have never re-tested, but it's really up to you and your vet. Chances are the medication is still in his blood stream and the chances of him being infected are slim.

While I do agree that she is probably safe as far heartworms go, it may be a simple matter of hospital policy. In areas where the mosquito population thrives year round it is common to retest 6 months after a missed dose, because at that point IF a dog has been infected that is when he/she will have a positive test result. And while K-9 Advantix has been thoroughly tested it is not 100% for the entire month.
You can check out their testing results here.
http://www.animalhealth.bayerhealthcare.com/5614.0.html

The heartworm (HW) life cycle involves infective larval stages that are deposited on the dog by infected mosquitoes. These organisms circulate and develop more while in the dog's tissues before locating in the vena cava and right atrium of the heart. This takes more than a month, and generally less than two months.

It is during this migratory period that the preventative medicine does it's work. It kills any such circulating stages that may be in the body. However, the medicine does not remain circulating in the system more than a few days, and is not necessary to do so, since another lethal dose will be given before any newly deposited larvae may develop into juvenile heartworms.

While it is not mandatory that subsequent preventative doses be given exactly monthly, but it is important that they be given more frequently than bimonthly... some larval stages, deposited after the previous dose and thus expected to be killed with the next scheduled dose, may be allowed to develop normally into juvenile heartworms. The likelihood of this happening is proportional to the severity of the heartworm problem in a dog's environment.

Heartworm populations in a dog develop only by repeated infective mosquito encounters, not by reproduction of the parasite in the dog. Reproduction also requires a male and female heartworm 'interaction' in the dog to produce circulating microfiliaria which must then develop into infective larvae in a mosquito.

The next problem... these 'occult' heartworms which develop by a missing monthly dose, may also escape detection if it depends on finding microfiliaria circulating in the blood. The preventative also kills these organisms as well.

Bottom line: If you live in a serious HW area, consider getting back on track, however your vet recommends.

My Bad! I planned to leave a link to more on HWs, but got sidetracked.

Here it is http://www.marvistavet.com/html/heartworm_prevention.html

Sorry... it runs in the family, I guess.