Anisocytosis is the medical term for having red blood cells (RBCs) that are unequal in size. It can be caused by anemia, certain blood diseases, or by some cancer drugs.

Normally, a person’s RBCs should all be roughly the same size. For this reason, the presence of anisocytosis is often helpful in diagnosing blood disorders like anemia.

Treatment for anisocytosis depends on the cause. The condition isn’t dangerous on its own, but it does indicate an underlying problem with the RBCs.

Depending on what is causing anisocytosis, the RBCs might be:

The main symptoms of anisocytosis are those of anemia and other blood disorders:

  • weakness
  • fatigue
  • pale skin
  • shortness of breath

Many of the symptoms are a result of a decrease in oxygen delivery to the body’s tissues and organs.

Anisocytosis in turn is considered a symptom of many blood disorders.

Anisocytosis is most commonly a result of another condition called anemia. In anemia, the RBCs are unable to carry enough oxygen to your body’s tissues. There might be too few RBCs, the cells might be irregular in shape, or they may not have enough of an important compound known as hemoglobin.

There are several different types of anemia that can lead to unequally sized RBCs, including:

  • Iron deficiency anemia: This is the most common form of anemia. It occurs when the body doesn’t have enough iron, either due to blood loss or a dietary deficiency. It usually results in microcytic anisocytosis.
  • Sickle cell anemia: This genetic disease results in RBCs with an abnormal crescent shape.
  • Thalassemia: This is an inherited blood disorder in which the body makes abnormal hemoglobin. It usually results in microcytic anisocytosis.
  • Autoimmune hemolytic anemias: This group of disorders occur when the immune system mistakenly destroys RBCs.
  • Megaloblastic anemia: When there are fewer than normal RBCs and the RBCs are larger than normal (macrocytic anisocytosis), this anemia results. It’s typically caused by a deficiency in folate or vitamin B-12.
  • Pernicious anemia: This is a type of macrocytic anemia caused by the body not being able to absorb vitamin B-12. Pernicious anemia is an autoimmune disorder.

Other disorders that can cause anisocytosis include:

In addition, certain drugs used to treat cancer, known as cytotoxic chemotherapy drugs, can result in anisocytosis.

Anisocytosis may also be seen in those with cardiovascular disease and some cancers.

Anisocytosis is typically diagnosed during a blood smear. During this test, a doctor spreads a thin layer of blood on a microscope slide. The blood is stained to help differentiate the cells and then viewed under a microscope. This way the doctor will be able to see the size and shape of your RBCs.

If the blood smear shows that you have anisocytosis, your doctor will likely want to run more diagnostic tests to find out what is causing your RBCs to be unequal in size. They will likely ask you questions about your family’s medical history as well as your own. Be sure to tell your doctor if you have any other symptoms or if you’re taking any medications. The doctor may also ask you questions about your diet.

Other diagnostic tests may include:

The treatment for anisocytosis depends on what is causing the condition. For example, anisocytosis caused by an anemia related to a diet low in vitamin B-12, folate, or iron will likely be treated by taking supplements and increasing the amount of these vitamins in your diet.

People with other types of anemia, like sickle cell anemia or thalassemia, may require blood transfusions to treat their condition. People with myelodysplastic syndrome may need a bone marrow transplant.

Anisocytosis during pregnancy is most commonly caused by iron deficiency anemia. Pregnant women are at a higher risk of this because they need more iron to make RBCs for their growing baby.

Research shows that testing for anisocytosis can be a way to detect iron deficiency early during pregnancy.

If you’re pregnant and have anisocytosis, your doctor will likely want to run other tests to see if you have anemia and begin treating it right away. Anemia can be dangerous for the fetus for these reasons:

  • The fetus might not be getting enough oxygen.
  • You can become excessively tired.
  • The risk of preterm labor and other complications is increased.

If left untreated, anisocytosis — or its underlying cause — can lead to:

  • low levels of white blood cells and platelets
  • nervous system damage
  • rapid heart rate
  • pregnancy complications, including serious birth defects in the spinal cord and brain of a developing fetus (neural tube defects)

The long-term outlook for anisocytosis depends on its cause and how quickly you’re treated. Anemia, for example, is often curable, but it can be dangerous if left untreated. Anemia caused by a genetic disorder (like sickle cell anemia) will require life-long treatment.

Pregnant women with anisocytosis should take the condition seriously, because anemia can cause pregnancy complications.