Red Blood Cells (RBCs) contain hemoglobin, a molecule which binds oxygen in order to transport and deliver it to all our body’s cells. The Complete Blood Count (CBC) lab test measures the amount of hemoglobin, and also the percentage of our blood that are RBCs compared to amount that’s liquid (“hematocrit”). If hemoglobin & hematocrit are low, the person has Anemia (see that topic for an in-depth discussion). There is also an uncommon condition Polycythemia, in which the hemoglobin and hematocrit are way too high.
The CBC also estimates whether the RBCs are too small, too big, or just right (normal size). The measurement is called the “MCV;” it’s described under Anemia. Small RBCs occur with iron deficiency, but also with the hereditary condition Thalassemia. Large RBCs occur with deficiencies of Vitamin B12 or Folic Acid, with thyroid disease, and liver disease. A high MCV may make us wonder that a patient is an alcoholic if they don’t admit it. So it’s important to realize that not everyone with “Anemia” needs to take iron, only those whose anemia is caused by iron-deficiency.
One brief comment — if a person is acutely bleeding, all their RBC tests may be completely normal, Imagine rapidly removing half of a person’s blood, and then one more test tube’s worth, which would still be normal. That’s because it takes hours for the body to attempt to replace lost blood with fluid, wherever it can recruit it from, to keep the bloodstream flowing. Then the CBC can reveal the amount of blood loss diluted down. Before that occurs, we can only tell if we infuse lots of IV fluid (the initial treatment for acute bleeding), then repeat the CBC. See Acute Anemia.