CBC Explained

CBC Explained

Recently I was thrilled to report to the parent of a kiddo who we’ve been treating for iron deficiency is no longer anemic! Mom was excited and also wanted to know more about the CBC (complete blood count) and what we’re looking at when we’re looking for anemia.

Though you can diagnose anemia with one value (hemoglobin), you get a lot more information by considering more factors, like the size, shape, and color of the red blood cells.

Without further ado, the part of the CBC I’m typically looking at in assessing for anemia:

  1. Hgb = hemoglobin. Hemoglobin are the iron-containing proteins inside of red blood cells that bind and carry oxygen.
  2. Hct = hematocrit, the percentage of the blood that is red blood cells. Blood has red blood cells, white blood cells, and fluid. If you’re dehydrated and have less fluid in your blood, then your hematocrit will look higher, even if there aren’t more red blood cells than otherwise.
  3. RBC = red blood cell count. Literally how many red blood cells.
  4. MCV = mean corpuscular volume. How big are those red blood cells? Very big red blood cells often lack the ability to get divided, so there’s often a folate or B12 deficiency. Very small red blood cells are often the result of not having enough iron to make new red blood cells.
  5. RDW = red blood cell distribution width. How many different sizes of red blood cells are present in the blood? ‘Baby’ red blood cells are big, ‘old’ red blood cells are little. If there’s a huge variety in sizes, that suggests that the bone marrow is trying to keep up with production, and kicking out cells that aren’t quite mature yet.
  6. MCH = mean corpuscular hemoglobin, which is the average hemoglobin content (weight or mass) in the cell. When there is not much hemoglobin in the red blood cells, they appear pale (hypochromia), which is suggestive of iron deficiency or thalassemia (which is a disorder of hemoglobin production).
  7. MCHC = mean corpuscular hemoglobin concentration, which is a ratio of the average weight of hemoglobin per unit of red blood cell. This is more like the density of hemoglobin relative to the whole cell, or how big the cell is relative to the amount of hemoglobin there.

Simplified illustration of different types of anemia. 1a) Normochromic normocytic anemia: normal MCV (normal RBC size = normal tube size), normal MCH (normal hemoglobin in each RBC = normal amount of air in each river tube), normal MCHC (normal average concentration of hemoglobin in a given volume of packed RBCs = normal density of air, i.e., normal inflation); 1b) Microcytic hypochromic anemia: low MCV (small RBC size = small tube size), low MCH (less hemoglobin in each RBC = less air in each river tube), low MCHC (low average concentration of hemoglobin in a given volume of packed RBC = low density of air, i.e., underinflation); 1c) Macrocytic normochromic anemia: high MCV (large RBC size = large tube size), high MCH (more hemoglobin in each RBC = more air in each river tube), normal MCHC (normal average concentration of hemoglobin in a given volume of packed RBC = normal density of air, i.e., normal inflation). (Medscape)

Normal MCH and elevated MCHC in hereditary spherocytosis (ball-shaped RBCs): Imagine the river tubes were bitten by alligators (loss of surface) and reconstructed into balls with the same amount of air (normal MCH); therefore, the balls will be overinflated (high MCHC) due to the reduced surface/volume ratio. (Medscape)

 

Other useful measures include:

  • ferritin, which can give a sense of the body’s iron reserves (but can also be elevated in acute infection)
  • reticulocyte count, which gives a measure of the bone marrow’s ability to respond to a deficiency of red blood cells
  • peripheral blood smear, which lets a pathologist look directly at the appearance of the red blood cells, and can note particular abnormalities associated with particular condition (like lead poisoning, for example)

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