Recently a client was mildly upset about some lab tests. Rather, she was bothered by was wasn't found. Her dog was sick and she wanted us to test for cancer. When we ran basic blood tests everything was normal and she wanted to know about the "cancer test". One of my assistants ended up talking to her and eventually explained that there was no such test and we had to look for cancer by other methods.
This is an example of where a little information and a lot of extrapolation can get people into trouble. In human medicine there are blood tests that can help identify cancer, such as the PSA test for prostate cancer. However, these are always specific tests and they are only available for a very small number of cancers. There is no such thing as a general cancer test. Cancer is detected by looking at its affects on the body (decreased kidney function, liver enzyme elevation, etc.), feeling masses, or discovering abnormal lumps on x-rays and ultrasound.
In veterinary medicine we run into the same problems of being able to find cancer. We still have to look for its influence on the body, but we don't have any of the tests available for human doctors. For example, prostate cancer is normally detected through a rectal exam and ultrasound, as we don't have the PSA test for dogs and cats. We often have to do more investigation and educated guessing than in human medicine.
Sometimes cancer must be inferred rather than definitively diagnosed. We're currently treating an elderly cat who is losing weight. All of the blood chemistries are normal, ruling out kidney disease, liver disease, and diabetes. The thyroid level was normal, ruling out hyperthyroidism. The only blood panel abnormality was a slightly elevated globulin level, indicating immune stimulation. The white blood cell count was elevated, and remained so after two weeks on antibiotics. The cat is continuing to lose weight and doesn't want to eat, but there really aren't significant clues in his lab tests. Elevated globulin levels and a white cell count suggests either a viral infection or cancer. Since a virus would normally improve in a couple of weeks and it hasn't, that leaves cancer.
This is where a "cancer test" would be very helpful. In human medicine we'd be delving further and likely be going to imaging such as x-rays and CT scans. But that's not realistic for most clients. So by process of elimination we're suspecting the cat has cancer, though we can't identify where or what kind. Currently we're trying appetite stimulants and trying to buy some time, but the prognosis is poor.
Cancer is a horrible disease and it's not always easy to find. And there isn't a test for it. Have some patience with your veterinary team as they try to help figure out what's wrong with your pet.