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A complete blood count, or CBC, measures the physical characteristics of blood cells. CBCs also help monitor existing blood disorders and let doctors know how patients respond to treatment.
- They are usually part of a routine checkup
- A health care professional will draw blood from your hand or arm, while children and newborns get a finger stick or heel stick
A CBC Checks Your Overall Health
Blood contains white blood cells (WBC), red blood cells (RBC), and platelets (PLT) suspended in a fluid called plasma. Blood cells are produced and mature mostly in bone marrow, and they are usually released into the bloodstream as needed. Most medical problems impact CBC results, so doctors use the test to diagnose anemia, infections, blood loss, and other disorders. They can count the numbers of each type of blood cell to find problems.
White blood cells, or leukocytes, fight infections, and there are five different types, including neutrophils, eosinophils, and lymphocytes. A high number of neutrophils indicates a bacterial infection, allergies can increase the number of eosinophils, and viral infections increase lymphocytes. Diseases like leukemia can cause abnormal white blood cells to multiply rapidly.
Red blood cells, or erythrocytes, contain hemoglobin, a protein that transports oxygen. They live for about 120 days, so your bone marrow constantly replaces the cells that age and die. Platelets, also called thrombocytes, are cell fragments that help blood clot. If you don’t have enough platelets, you could bleed or bruise easily, and too many platelets could cause excessive clotting.
Your blood cells can paint a detailed picture of what is happening in your body by identifying al of these factors and more.
What a CBC Includes
A CBC includes a white blood cell count and a WBC differential that measures the percentage of each type of white blood cell. It also includes a red blood cell count and a hematocrit. A hematocrit (also called an HCT, packed cell volume, or PCV), measures the amount of space red blood cells use as a percent of blood volume.
A hemoglobin test measures the amount of hemoglobin in blood and your body’s ability to transport oxygen. The mean corpuscular hemoglobin (MCH) is the amount of hemoglobin in an average red blood cell, and the mean corpuscular hemoglobin concentration (MCHC) measures the concentration of hemoglobin in an average red blood cell. A mean corpuscular volume (MCV) test measures the sizes of red blood cells. Mean platelet volume (MPV) measures the average amount or volume of platelets. MPV can be too high or low, even if your platelet count is normal.
Taking the Test
You don’t need to do anything to prepare for a CBC, and the only side effect is usually a small bruise from giving a blood sample. You can reduce bruising by putting pressure on the site. Tell your doctor before the test if you have bleeding or clotting problems, or if you are taking any medications. Common drugs like aspirin can change CBC results or cause excessive clotting or bleeding. Normal CBC values change with patients’ age, weight, and other health factors, but most tests contain healthy measurement ranges to help your doctor interpret the results.