An Insider’s Guide To Understanding Blood Work: The CBC

If you’ve ever heard the phrase, “all the (standard) ‘blood work’ was normal,” you’ve maybe wondered what that is supposed to mean? While there are innumerable blood tests that can be done/ordered, there are a few tests that are done much more commonly than others. Among these is the CBC, or complete blood count. In my practice (and I would venture to guess that I’m not unique), the CBC is the one blood test that gets ordered more than any other test. It is the one blood test that, as a parent, is worth knowing.

What is a CBC?

This simple blood test measures many components of the blood. It tells a lot about the kinds and numbers of cells in the blood. The CBC gives huge insight into a person’s overall health and is a relatively easy screen for many diseases.

What exactly does a CBC include?

A CBC (with the differential—which is the expanded, non-short-cut version) includes:

1. WBC or White Blood Cell count: White blood cells are responsible for infections. When a person has an infection, the white blood cell count will typically rise. The white cells are further separated into different types. These types can give doctors all sorts of insight into the kinds of infection. There are neutrophils, lymphocytes, monocytes, eosinophils, basophils, and immature cells. All of these different subtypes of white cells exist in a balanced percentage of the total. If any of these subtypes of white cells are off percentage-wise, it can give clues to what is going wrong (e.g., bacterial infections, viral infections, allergic reactions, or even different blood cancers, like leukemia).
2. RBC or Red Blood Cell count: Red blood cells carry oxygen from the lungs to the rest of the body. Again, if the numbers are off it can indicate problems. A low RBC will indicate a condition called anemia. A high RBC can also be problematic and indicate a condition called polycythemia.
3. HCT or hematocrit: A hematocrit measures how much volume or space the red blood cells take up in the blood. The result always represents a percentage. For example, a HCT of 35 means that 35% of the blood volume is made up of red cells.
4. Hgb or Hemoglobin: The red cell is made up of hemoglobin. It carries oxygen and makes the red cell red color. You will hear the phrase: “H and H.” This refers to the results of the hemoglobin and hematocrit. These two tests are typically used to determine anemia.
5. Red blood cell indices: Just like the white blood cells have different kinds to help distinguish various problems, the red cells also have supporting information. There are 3 added pieces of information to the red cells that clue doctors to the various types of anemia.
   1. MCV or mean corpuscular volume: Tells the size of the red cell. This is helpful if your doc understands the different kinds of anemia and cell size differences in them (for example, in iron deficiency anemia, the cells are small).
   2. MCH or mean corpuscular hemoglobin: Tells the amount of hemoglobin in the red cell.
   3. MCHC or mean corpuscular hemoglobin concentration: Tells the concentration of the hemoglobin in the red cell (a very subtle difference).
6. Platelet (or thrombocyte) count: These are the cells responsible for clotting. Platelets prevent you from bleeding to death from a small cut or skinned knee. If there aren’t enough (thrombocytopenia), you can have uncontrolled/life threatening bleeding. If there are too many, you can get blood clots in places you shouldn’t.

How are CBCs taken?

The CBC is a blood draw. Typically the blood is drawn from the vein that runs on the inside of the arm (called the antecubital area), but it can be drawn from just about any blood vessel.

Do I have to fast for the CBC test?

No. It can be done anytime.

Is the CBC test expensive?

Everything in medicine is expensive. As far as tests and how much information it can give, a CBC is relatively cheap.  Depending on where it is drawn (office, hospital, lab facility, etc.) it can range from $15-65. The average CBC drawn in the facilities around me are in the $30-35 range.

What diseases does a normal CBC rule out?

While there will always be the one in a million exception (perhaps the disease is so early in the course that the cell lines haven’t  been affected yet), the test generally rules out tons of diseases including, but not limited to: anemias of all sorts,  many cancers and leukemias, thrombocytopenia, polycythemia, infections, effects of chemotherapy,  bone marrow problems, some autoimmune disorders, and some medication effects.

Why would my pediatrician order a CBC?

If your child has symptoms that make your pediatrician concerned for any of the previously mentioned diseases, your pediatrician will order a CBC. Common concerning symptoms may include:

1. Looking pale
2. Being tired
3. Sleeping a lot
4. Bruising/bleeding
5. Swelling
6. Weight loss (unintentional)
7. Unexplained fever
8. Unexplained symptoms
9. Concerns for poor nutrition

How long does it take to get CBC results?

Depending on whether the test is run “in house” (meaning the machine to run the test is the facility that draws the blood), the results can come back the same day or will take 1-2 days (if sent out). A CBC is such a common test, that most places have nearby facilities that run the test.

So the next time you are at the office or hospital worried about your sick little one, take significant reassurance (most of the big bad things have been ruled out) when the doctor says, “the CBC is normal.”