How Vets Think

We get into vets heads to understand how they make diagnoses when presented with symptoms that could be indicative of any one of a number of diseases. A diagnosis could be differential, presumptive, or definitive.


A dog is brought to the clinic because she is both urinating more than ever and also drinking more than ever. The condition is referred to by veterinarians as PU/PD, which stands for polyuria-polydipsia — the medical term for over-urinating/over-drinking.

What could it be? Diabetes? Something else?

“We start with a list of differential diagnoses, or ‘differentials,'” says Your Dog editor-in-chief John Berg, DVM, DACVS. Differentials, he explains, are simply a list of possible diagnoses based pretty much on a history given by the owner and a physical exam. In many cases, a dog can’t be treated based on the list of differential diagnoses that the vet comes up with — it’s not narrowed down enough — but it “drives the selection of tests that are chosen” to work toward nailing the diagnoses.

“A common synonym for ‘differentials’ is ‘rule-outs,'” Dr. Berg says. You have to keep ruling out differentials until you get to the right one.

“Often,” he comments, “there’s a differential that’s most likely to be the culprit of the dog’s symptoms, and the initial test or tests might be chosen to quickly and definitely rule that most likely possibility ‘in’ or ‘out’ in order to avoid unnecessary testing and spare the pet owner the costs of more screenings. But that’s not the case for a dog with PU/PD. There are four possible differentials, with the first three having “roughly the same likelihood of being the culprit,” the doctor says:

diabetes too much sugar in the bloodstream.

Cushing’s disease the secretion of too much of the hormone cortisol by the adrenal glands.

kidney disease a failure of the kidneys to properly filter waste and other toxins from the blood.

psychogenic polydipsia a drive to drink too much based on a problem in the central nervous system.

Because the chances are relatively even that it could be any one of the first three conditions causing the problem with drinking and urination, the first tests a veterinarian would probably order in this particular case are a biochemical profile and a CBC, or complete blood count. These two tests, both conducted by analyzing a blood draw, can tell whether there’s too much sugar in the blood (diabetes); whether there are high levels of urea nitrogen and creatinine (both indicators that point to kidney problems); or whether there’s a high blood level of alkaline phosphatase, which suggests Cushing’s disease.

Let’s say the test comes back normal for sugar and urea nitrogen and creatinine but high for alkaline phosphatase. At that point, the vet can rule out diabetes and kidney disease with a reasonable degree of confidence and can make a presumptive diagnosis of Cushing’s disease, which over time can make a dog sluggish and, if left untreated long enough, can lead to life-threatening complications like blood clots in the lungs or an inability by the body to fight even minor infections. A presumptive diagnosis means the veterinarian believes he knows what the problem is but can’t guarantee it.

At this juncture, Dr. Berg says, “the doctor is reasonably sure but not positive. You’ve ruled out everything else and by playing the odds, a presumptive diagnosis is what you come up with.” (Physicians treating people work the same way, and sometimes, with a difficult case in which a physician or a veterinarian is stumped, one doctor may ask another, “What are your differentials for that patient with a lump on the head” or “unusual lethargy,” etc.)

This is where further testing comes in to arrive at a definitive diagnosis. The additional testing in this case is called a dexamethasone suppression test. In a normal dog, if you intravenously inject a synthetic version of the hormone cortisol called dexamethasone, her body’s production of cortisol will decrease. But in a dog with Cushing’s disease, injection of a cortisol-like substance will not diminish her secretion of her body’s own cortisol.

The way it works is that the vet draws blood from the dog and measures her cortisol levels. Then he injects the dexamethasone and, a few hours later, conducts another blood draw. If the dog’s own cortisol level has not decreased in response, he can definitively diagnose Cushing’s disease and prescribe appropriate treatment — drugs, and sometimes, surgery.

Two steps forward, one step backward
The route from differential to presumptive to definitive diagnosis is not always neat and clean, as in the case described here. Getting to the root of clinical symptoms can be messy, with a lot of equivocal results along the way that require further testing. That’s why testing can be expensive and the bills can run up (a good reason to have pet health insurance).

Consider, for instance, that even with the dexamethasone response test, it might not be clear what the cause of the Cushing’s disease is and whether it requires medication or surgery. For that reason, the doctor may want to do an abdominal ultrasound. That will determine which treatment protocol is in order.

Hitting a wall
Sometimes the vet has narrowed down the possibilities of what’s making the dog sick, but the final test doesn’t show a response. “Now you’re back to zero,” Dr. Berg says. “That does happen.” It may not be that the presumptive diagnosis was off the mark. Sometimes the test fails to pick up the problem. “Most tests are not 100 percent sensitive or specific,” he explains. “They have their limits in their ability to diagnose.” (Think of the rare case in which a pregnancy test fails to detect a pregnancy.)

What do you do then? This is where the art of medicine intersects with the science because it depends on the situation and how the doctor chooses to handle it. For instance, Dr. Berg says, if the veterinarian still presumes the problem is Cushing’s disease, “maybe he does an ultrasound. If the adrenal glands are enlarged, that’s suggestive of Cushing’s disease.”

But do you treat even without certainty? Sometimes it does come down to that. “In some cases,” the veterinarian relates, “you could wind up with two possibilities instead of just one. That can be a situation where we’ll treat for whatever we think the problem is based on the dog’s history, age, breed, and other factors. In such a case, the therapeutic trial — the effort to heal the dog — is itself the diagnostic test. It’s a common step taken in everyday practice, frequently driven by the client’s pocketbook.”

Not all clients can afford to give the go-ahead for every test possible in order to try to make a definitive diagnosis. In such cases, the veterinarian will discuss the situation with the client, and together they will decide on a course of treatment based on the presumptive diagnosis. “If the patient gets better,” Dr. Berg says, “the presumptive diagnosis may have been correct. I say ‘may’ because sometimes the patient would have gotten better, anyway. That possibility can’t always be ruled out” (although it can for something like Cushing’s disease — that condition doesn’t go away on its own).

It’s not always tricky
“Sometimes, the first test you do gives you the answer,” Dr. Berg says. “If it’s an old greyhound with recent lameness who has a bony swelling above her wrist, our first differential is osteosarcoma. It could potentially be a sprain, but that’s not where a veterinarian’s mind is going to go. We’re thinking mainly osteosarcoma [bone cancer], which is a large breed disease; small dogs don’t get it. We’ll also suspect osteosarcoma because mast cell tumors and soft tissue sarcomas — other masses that can occur in the same area — usually do not cause lameness.

“So we’ll start with an x-ray, which is non-invasive and relatively inexpensive, and if what we see is suggestive of osteosarcoma, we’ll go to a biopsy for a definitive diagnosis. That’s a fairly straight pathway to a diagnosis.”

Getting a definitive diagnosis for something like a broken bone is fairly easy, too. A dog has an accident, hits her leg against something, and starts limping. She comes in and gets an x-ray, which shows right away that she has a fracture.

“You never know up front whether you’re going to know instantly what you’re dealing with or whether there are going to be many shades of grey that have to be sorted through,” Dr. Berg says. “But going from differentials to a presumptive and then to a definitive diagnosis is the most efficient way to go,” even in simple cases. It allows a veterinarian to rule out the less likely causes of a problem and get more quickly to the tests that will yield answers and, thereby, treatment plans — getting your dog healed as quickly as possible with the least damage to your wallet.


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