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Why your physician tests your blood

If you (or someone close to you) have been diagnosed with a blood disorder, blood tests may become a frequent part of your life. Your body may not be making the right amount of blood cells, or the cells it makes may not work properly. For this reason, your doctor will check your blood regularly with a test called a complete blood count, or CBC for short. For a closer look at blood counts, your doctor may use a test called a “CBC with differential.”

The results of blood tests help your doctor monitor your condition. Combined with other tests, blood tests help to determine when treatment is needed, what treatment is best for you, and how well your treatment is working. This brochure is designed to help you understand some basics about your blood and the blood tests you may have.

What’s in healthy blood?

Plasma is the fluid that carries blood cells and other nutrients throughout the body. The plasma usually makes up 50% to 55% of blood. Red blood cells (RBCs) are another 40% to 45% of the blood, and the rest is made up of white blood cells (WBCs) and platelets.

There are 3 main types of blood cells

  • RBCs give blood its red color and are filled with hemoglobin, a protein in the blood that carries oxygen. Your doctor will use blood tests to monitor your RBCs and hemoglobin, along with your hematocrit (percent of total blood volume that is made up of RBCs) and the number of immature RBCs (reticulocytes) in your blood.
  • WBCs are part of your body’s defense system and they help prevent and fight infection. There are different kinds of WBCs. Neutrophils are the most common WBC in the blood. They make up over half of all WBCs and are critical to fighting infections and disease.
  • Platelets help stop or prevent bleeding by forming blood clots.

“Blood counts” offer one way to measure your condition
There is a “normal range” for each type of blood cell—a level necessary for those cells to do their job and keep you healthy. The normal range varies for each type of cell and from person-to-person, depending on your age, sex, and many other factors.

This is just a guide. Never try to guess what your blood count means. Always discuss this or any other test results with your doctor or nurse.

What does it mean when blood levels fall below the “normal range”?

If your body has lower-than-normal levels of blood cells, this condition is called cytopenia. Anemia is a type of cytopenia that can occur when your body does not make enough RBCs. Anemia is common in many disorders and is diagnosed by measuring hemoglobin and hematocrit. A person with anemia may feel tired, weak, and short of breath.

The phrase “low white count” means lower-than-normal levels of WBCs. Medical terms for this are neutropenia, leukopenia, and granulocytopenia, depending on which type of WBC is affected. A person with too few WBCs is less able to prevent or fight infections.

Patients with lower-than-normal levels of platelets have thrombocytopenia. This condition may cause a person to bruise or bleed more easily.

You may have more than 1 cytopenia at the same time.

These ranges and the units used can vary. Only your doctor or nurse can determine what an adequate blood count is for you.

Looking at the bone marrow

Most blood cells are made in the bone marrow, the soft, sponge-like tissue in the center of bones. Your bone marrow provides important information about how your body makes blood cells and what might be the main cause of lower-than-normal blood counts. Therefore, your doctor may also check your bone marrow by performing a test called a bone marrow biopsy where a needle is used to pull some of the marrow from your bone (often from the back of the pelvis or hip). The area used from the testing is numbed by using medicine, but it still can be uncomfortable. You will usually feel a dull ache at the needle entry site.