Macrocytic Anemia: Definition and Causes

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  • 0:01 Really Big Things
  • 0:32 MCV and RDW
  • 1:24 Macrocytic Anemia with…
  • 2:15 Macrocytic Anemia with…
  • 5:13 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will go over the different types and causes of macrocytic anemia. This will include a discussion of something known as megaloblastic anemia as well.

Really Big Things

Things that are really big grab people's attention. Big signs with big writing, big cars and big diamond rings are just a few of them.

Big red blood cells are also something that grab a clinician's and a pathologist's attention. They are an indicator of something going awry in the body, and this lesson will seek to explain why red blood cells may be overly large.


On a CBC, a complete blood count, your doctor has several values with which to help classify the cause of anemia. Anemia is a decreased function of or number of red blood cells.

One of the values is mean corpuscular (cell) volume, or MCV. In essence, this value tells you how big or small a cell is. It can be macrocytic, an abnormally large red blood cell; microcytic, an abnormally small red blood cell; or normocytic, a normally sized red blood cell.

The other value on your CBC that can help classify anemia is red cell distribution width, or RDW. This value measures the variation in red blood cell volume.

Macrocytic Anemia with Normal RDW

One type of anemia is called macrocytic anemia. I think you won't be surprised when I tell you this is when red blood cells are abnormally large. A subtype of macrocytic anemia occurs with a normal RDW.

One of the reasons for this is something known as aplastic anemia. Aplastic anemia refers to a disease where the bone marrow doesn't make enough red blood cells. The prefix 'a-' refers to being without something and 'plasia' refers to growth. So we are without the growth of red blood cells in the bone marrow.

There are a ton of reasons for aplastic anemia, including:

  • Infections with HIV
  • Toxins, such as benzene
  • Radiation exposure
  • Genetic disorders

Macrocytic Anemia with Increased RDW

The other subtype of macrocytic anemia occurs with an increased RDW. When there is a high RDW, we expect anisocytosis. This is a fancy word for red blood cells that have unequal sizes.

Macrocytosis with increased RDW can occur as a result of autoimmune hemolytic anemia. This is when your own body's (auto) antibodies attack your red blood cells and destroy them. The antibodies are proteins created by white blood cells. These proteins are actually supposed to attack foreign invaders, like bacteria, viruses and fungi. However, sometimes, due to improper genetics, a cross reaction between an invader, or unknown reasons, the antibodies target your red blood cells instead.

I sometimes think these autoantibodies are like boomerangs. They're created by you, they're thrown by you for your own needs, but if you're not careful they can comically fly back and hit you in the head, hurting you in the process. Of course, autoimmune hemolytic anemia isn't comical as it can result in a person's death.

Macrocytosis with increased RDW can also occur due to chronic liver disease. Many times this is as a result of alcohol abuse over a long period of time.

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