The Results are In - 1st Post

I've had trouble thinking of good ideas to blog about. So I've decided to spend a couple of posts talking about "lab results."

How many times have you heard, "We're going to run some tests," or, "the blood work all came back normal." WHAT DOES THAT MEAN?

Let's break it down and over the next few posts talk about some of the common tests that are run and what kind of information we can get. I will discuss the complete blood count, basic blood chemistries (including electrolytes), endocrine chemistries, urinalysis, stool analysis (see previous blog on this one), biopsy (histopathology) and cytology, and bacterial or fungal cultures.

For the first installment, I thought we would start with the Complete Blood Count (CBC). It is the most straight forward to discuss.

A complete blood count is just what it sounds like. We are trying to measure all of the types of blood cells and provide information about their age, distribution, and health. There are three main numbers we look at. Total white cell count, total red cell count, and platelets.

White cells got their name because when you get a bunch of them together, they look white (or kind of white - think pus which is a few billion white cells all together). There are 5 types of white cells we generally measure and the total white count (WBC) is simply all of these types added up.

• Neutrophils - these are the fast attackers. They are shortest lived of the white cells. They are the "first responders" to inflammation or infection and it's the neutrophils that are sacrificed in huge numbers (again, think pus). Typically, the neutrophils are the most numerous of the white cells - typically 60-70% of the total count. If the neutrophils appear young or have "toxic" changes, it helps tell us more about the type of response the body is mounting. Young, toxic neutrophils are usually in response to severe, systemic bacterial infections.
• Lymphocytes - The smaller, rounder white cell. They make up the second largest population. usually 20-40%. There are multiple "flavors" of lymphocytes. T-cells, B-cells, plasma cells. Basically all of the same family tree. Lymphocytes are responsible for various roles in managing the immune system. They don't eat stuff, they tell other cells to go work, go eat, or to make certain chemicals. They take control by using cellular contact chemicals (known as cytokines). For instance, Killer T-cells will tag a cancer cell and trigger it to die. The T-cell then tells the macrophages to eat the dying cell. B-cells are largely responsible for making antibodies.
• Monocytes - These are the biggest white cell and are responsible for much of the clean-up work. They can eat all kinds of stuff. If it needs to be cleaned up or eaten, monocytes are usually involved. They are usually about 5% of the total population. When monocytes leave the blood stream and enter a tissue that are re-named a macrophage. There are some highly specialized macrophages located in certain places in the body (like in the liver) that play a major role in regulating the immune system (kind of like the lymphocytes).
• Eosinophils - Similar to a neutrophil. These guys have large, reddish granules in their cytoplasm. Usually 1% or so of the total. Eosinophils have similar roles to the neutrophils, but they are primarily "turned on" when parasites are involved or in the case of allergic reactions/chronic allergies. Cats have a very interesting ability to mobilize eosinophils and the eosinophils can cause all kinds of problems when you get too many together all at once.
• Basophils - Very few are seen in the blood. It's actually kind of exciting to find one because they are typically fairly hard to find. They are in many ways just like an eosinophil.

If white cells are increased, we look at which ones. Neutrophils usually increase with infection of some kind (but any kind of stimulation to the immune system can cause increased numbers). Too few indicate life-threatening infection or that the bone marrow has been hit (as with parvovirus or chemotherapy). Increased monocytes can be seen in fungal infections or in cancer. increased eosinophils are usually associated with parasitic or allergic causes.

There are even some blood "parasites" that can be seen inside the white cells. Ehrlichia is one such infection.

Red blood cell counts:

The most common way to describe how many red blood cells an individual has is to use the packed cell volume (PCV) or hematocrit (HCT). PCV and HCT are basically percentages. If you take whole blood and spin it down (pack it) then you can compare the volume of red cells to the total volume of blood. Most blood is around 40-50% cells and the rest is all plasma.

Too few red cells = anemia.

PCV 30-40% - mild anemia

PCV 20-30% - significant anemia. Probably seeing "tired" pet.

PCV 15-20% - severe anemia. Something needs to be done.

PCV 10-15% - life threatening anemia. Something HAS to be done

PCV less than 10% - incompatible with organ function. Rapidly fatal.

Red blood cells are first and foremost an oxygen delivery system. Too few means not enough oxygen getting to tissues. Too many (pretty rare) and the blood is too thick and can't flow.

Baby red blood cells are darker and larger. Really immature red cells still have a nucleus (something that the adult cells have lost). Baby cells aren't very good at transport of oxygen. They're not the right shape yet and don't have all of the hemoglobin they will have as an adult. If you see baby reds in any significant number, you know the bone marrow has been told to make more. Usually this means the body is responding to an anemia.

Platelets - these are actually little cell fragments of a cell called a megakaryocyte (found in the bone marrow). Platelets are responsible for making a clot. You are always leaking. Always. Platelets go around plugging up that leak. Without them, you bleed to death. Honestly. There are usually 200 thousand or so in each milliliter of whole blood.

I could continue discussing all of the interesting things about blood cells, but I've gone on long enough. At least you have an idea of what they are looking at when a CBC is done. Complete discussions on interpretation are entire books in and of their own.

Next post - The basic blood chemistry.

AMH