Veterinary Q&A: Why blood work is necessary

Seattle Times
Tails of Seattle

Posted by Neena Pellegrini

Dr. Tessa King, a veterinarian at Pilchuck Veterinary Hospital, answers this week's questions.

Question: If my dog is healthy, why does he need expensive blood work?

Answer: Blood and urine tests are extremely useful tools for looking at the overall health of your pet. From them, a vet can learn if your pet is a diabetic, has kidney or liver disease, or an infection. Many of these problems may not be obvious on a simple physical exam, but if caught early can be successfully treated or controlled, giving your pet a longer and better quality of life.

It's also important to have lab tests performed when a pet is healthy, so the veterinarian has a baseline to compare results when/if your pet does become ill. It's important to note that lab tests must be coupled with a complete physical examination and patient history from the owner to be of the most use.

Sometimes we can come to a definitive diagnosis with routine blood work, but oftentimes we find changes in blood work that lead us to do more diagnostic testing to come to a definitive diagnosis. When we don't get an absolute answer from blood work, it helps guide us as to what the next diagnostic steps should be (radiographs, ultrasound or more specific blood testing).

Question: What kinds of things can a complete blood count (CBC) tell a vet about the health of my dog? What should a vet be looking for?

Answer: Usually a CBC and either a full or partial chemistry panel is done, sometimes with pancreatic enzymes and a T4 (thyroid level).

A CBC evaluates the number of red blood cells, white blood cells and platelets, along with the concentration of red blood cells (hematocrit or PCV) and hemoglobin levels.
Abnormal changes on a CBC could include anemia (low red blood-cell count), leukocytosis (high white blood-cell count) or thrombocytopenia (low platelets). Anemia has a wide range of causes, and finding a low red blood-cell count, depending on the severity, usually warrants searching for a cause.

There are five different types of white blood cells, and elevated or low counts in each can mean different things.

For example, neutrophils are the most common elevated white blood cells because they are one of the first responders to any type of inflammation, including infection. Eosinophils, another type of white blood cells, are typically elevated with parasitic or allergic diseases.

Platelets are very important in normal clotting. A significant decrease in platelets can lead to life-threatening, spontaneous bleeding. There are many causes of decreased platelets, including infectious diseases and immune-mediated diseases, such as ehrlichiosis, a tick-borne bacterial infection, and immune-mediated thrombocytopenia.

Much more information can be gained from a CBC. The ones I just mentioned are only a few examples of some of the more common changes that we find.

A chemistry panel usually includes testing of liver enzymes, kidney values, glucose, electrolytes and protein levels.

Elevations in liver enzymes can be caused by a number of different issues and may warrant either monitoring or further testing, depending on which enzymes are elevated and how much. Inflammatory disease of the liver, endocrine diseases such as Cushing's, toxin ingestion and cancer are just a few things that could cause elevations in liver values.

The two most common reasons for elevations in both kidney values (BUN and creatinine) are dehydration and damage to the kidneys. The most common cause of damage to the kidneys in older pets - cats in particular - is chronic renal failure. This is where a urine sample becomes very important.

The concentration of the urine, or urine specific gravity, helps us to differentiate between dehydration and kidney failure. A dehydrated animal will have very concentrated urine, while an animal in kidney failure will have dilute urine.

Probably the most common change we see in glucose is elevation from diabetes, though there are a few other diseases where we find a low blood glucose (which can be life-threatening).