Recently we had a dog come to our clinic to evaluate a mass next to her rectum. It was pretty large and felt very invasive. We sedated her to do a needle aspirate, which is the most common first-line diagostic for masses. It's pretty simple and minimally invasive, and typically done without sedation. We did sedate this dog due to the potential discomfort at that location. We use a large-gauge needle, poke it into the mass, pull back to aspirate material, then send it off to our pathology lab for analysis. This "fine needle aspirate" (FNA) is one of the most common and easy diagnostic tests that veterinarians do, and is much less expensive and less invasive than a surgical biopsy.
Sometimes, however, we don't get good answers. In this particular case the mass was very fibrous, and the pathologist didn't see any significant cells, making the sample non-diagnostic. One of my associates talked to the client, who was understandably confused and even frustrated that the test didn't give us any sort of answer. However, that's one of the realities of medicine. Diagnostic tests don't always tell us what we need to know.
"Well, then why am I paying for you to do them if you can't tell anything?"
No matter what test we are running, there are limitations. A full chemistry panel may come back completely normal in a sick pet, leaving us with almost as many questions as when we started. The FNA may come back without appreciable cells, meaning that the next step is cutting into the mass with a scalpel. Every test has some degree of false negatives or false positives, as well as limits to what it can detect. Yes, sometimes we do get a clear answer, and it's easy to support the decision to run the test. I wish that was always the case.
But even "normal", "negative", or "non-diagnostic" tests can help some of the decision process. If a chemistry panel is normal, I may not be able to tell exactly what the problem is, but I can definitely make some assessment of what it is not. For example, if the everything on the panel is within normal range, we can rule out liver or kidney failure and start looking at other parts of the body. The non-diagnostic biopsy that we performed actually gives us some clues, as fibrous tumors don't shed cells very well and we likely can narrow down the possibilities to that category of cancer. We may still need to do further follow-up, but we have a little more of a direction to go.
That discussion can sometimes be difficult to have with clients. We need to celebrate the normal results and help them understand that this is valuable information that helps the diagnostic process. Yes, we may need to do more tests, do a more invasive biopsy, or otherwise go to the next step, but we at least have a little more direction than we did before. Ruling something out is just as important as ruling something in.
Diagnosing a disease is a process. Rarely is it "run this test and then we'll have the answer and can start X treament." It's more along the line of "start with this test, then based on the results we may have to go down diagnostic pathway Z rather than pathway W." Yes, this can be frustrating, and it does cost money. But it really is the only way to get to the bottom of what is going on. Be patient with your vet, ask lots of questions, but understand the challenges and limits of these tests. Your vet is doing the best that they can to figure out what is wrong with your pet, and it is not always straightforward to do so.