There is no specific blood test for cancer in dogs. There are some abnormal parameters that can indicate that there is something occuring that may be cancer, but only biopsy and other tests can give a definitive answer. Stated simply, elevated calcium could indictate cancer, some elevated liver enzymes could mean hepatitis, liver disease, or liver cancer, elevated white blood cells may indicate lymphoma, or an infectious or inflammatory process, for instance.
If your dog is a candidate for a dentistry with no risk, than it probably is a good idea, especially, since the vet was unable to give a full oral exam due to biting. An abscessed tooth, and/or peridontal disease can be very painful, and can cause problems in other organs due to the seeding or spread of infection. The tooth pain alone may make your dog not interested in eating. While under anesthesia or sedation the vet will be able to perform a more complete examination in other areas that had been limited when the patient was awake, and the vet may also be able to take X-Rays to see if there is another cause for the inappetance.
here is a bit more info that i wrote on another board:
i need some help - advice! not regarding my diabetic dog, but my other non diabetic dog who is a 13 year old dacshund. over the past month or so he has lost interest in food. he would barely eat for the last week and when he would he would over chew his food and it would end up on the floor instead of in his stomach for the most part. yesterday he only ate 2 tablespoons of canned dog food. even his favorite people food he will not show interest in. he use to be very food agressive. i took him to the vet last tuesday and they did a complete blood panel and listened to his heart and lungs (all which were normal). the vet could not look in his mouth as he is a biter (always has been) especially when he is nervous. we scheduled him for a dental which was today. he had 13 teeth pulled out and had a couple of abscesses. has anyone had anything similar happen to them with their dogs? if so, was it a dental problem and your dog started eatting again after getting it taken care of? he has gotten so skinny from not eatting (he always was a slender dog his whole life). over the past 4 months or so he has had occasional diarrhea as well, which in the beginning had some blood in it. we took him to the vet for that and they did a rectal exam and feces test both which were normal. he was given an antibiotic and some digestive enzymes. the blood has stopped but he still has diarrhea sometimes, or maybe just soft stool (not every time though). if this were something more serious then a dental problem (such as cancer) would something have been "suspicous" on the blood panel??? the blood panel he got was called an "IDEXX Total Health" blood test. i could only talk to the nurse today when i picked him up because the vet was taking care of an emergency... he was sent home with liquid clindamyacin (antibiotic) and was given nothing for pain. the nurse said having the pressure (teeth) removed should relieve his pain. i tryed giving him some food about 1/2 hour ago and he just walked away. he is now hiding under the bed.
update to future people who may read this:
this was all dental related! my dog is now eatting with a strong appetite!! also, it seems his diarrhea has subsided so maybe that could have been related to his dental pain as well. it took about 2 days for him to get his appetite back and he is once again "food agressive" which makes me so happy!!