Anisocytosis is the medical term for having red blood cells (RBCs) that are unequal in size. Anemia, certain blood diseases, or some cancer drugs can cause it.

Typically, a person’s RBCs are 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.

Keep reading to learn more about the causes and symptoms of anisocystosis and how doctors treat it.

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

The main symptoms of anisocytosis are also symptoms of anemia and other blood disorders. They include:

  • weakness
  • fatigue
  • pale skin
  • shortness of breath

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

Anisocytosis is a laboratory finding associated with many blood disorders.

Anisocytosis commonly results from another health condition called anemia.

In anemia, the RBCs cannot carry enough oxygen to the body’s tissues. This may be because there are:

  • too few RBCs
  • RBCs that are an irregular shape
  • RBCs may not have enough hemoglobin, the component of blood that carries oxygen

Several different types of anemia can lead to unequally sized RBCs, including:

  • Iron deficiency anemia: Anemia from iron deficiency is the most common form. 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 disease: Sickle cell disease is a genetic disease that causes RBCs to have an abnormal crescent or sickle shape.
  • Thalassemia: Thalassemia 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 occurs when the immune system mistakenly destroys RBCs.
  • Megaloblastic anemia: You develop this type of anemia when there are fewer than normal RBCs and the RBCs are larger than usual (known as macrocytic anisocytosis). A deficiency in folate or vitamin B-12 typically causes it.
  • Pernicious anemia: This is a type of macrocytic anemia caused by the body’s inability to absorb vitamin B-12 from food. 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.

Doctors typically diagnose anisocytosis during a blood test known as a blood smear.

After your blood is collected, a laboratory technician 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 allows the doctor to see the size and shape of your RBCs.

If the blood smear shows that you have anisocytosis, your doctor will likely order more tests to determine what is causing your RBCs to be unequal in size.

They will likely ask you questions about your family’s medical history and your own. Be sure to tell your doctor if you have any other symptoms or are taking any medications. The doctor may also ask you questions about your diet.

Other tests may include:

The treatment for anisocytosis depends on the cause.

Nutrient deficiencies

For anisocytosis caused by anemia due to nutrient deficiencies, such as vitamin B12, folate, or iron, doctors may recommend:

  • oral supplements of these nutrients
  • dietary changes, such as eating more foods high in vitamin B-12, folate, or iron
  • intravenous (IV) administration of certain nutrients, if you cannot absorb them from food

If you have an iron deficiency, a doctor or dietitian may recommend eating certain foods along with iron to increase how much iron your body absorbs.

Other types of anemia

Other types of anemia, like sickle cell disease or thalassemia, may require blood transfusions and additional treatments. During a blood transfusion, you receive donated blood through an IV. Other treatment options can include:

  • gene therapy for sickle cell disease
  • blood and bone marrow transplant
  • oral or infused medications to prevent blood cells from sticking together, or clotting
  • antibiotics to lower the risk of infection or treat an infection

Myelodysplastic syndrome

Doctors treat myelodysplastic syndrome depending on the severity and type. Treatment may include:

  • blood transfusion
  • erythropoiesis-stimulating agents (ESAs), which are medications that help your bone marrow produce more red blood cells
  • chemotherapy
  • immunotherapy
  • stem cell transplant

Anisocytosis during pregnancy usually results from iron deficiency anemia. People are at a higher risk of iron deficiency anemia during pregnancy because pregnancy requires an increased iron intake to support the fetus’s development.

Research from 2011 suggests 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 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:

  • It can cause excessive fatigue, or tiredness.
  • The fetus may not get enough oxygen.
  • It increases the risk of preterm labor and other complications.

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

  • low levels of white blood cells and platelets
  • nervous system damage
  • a rapid heart rate
  • pregnancy complications in people who are pregnant while they have anisocytosis, including issues with 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 it’s treated.

Anemia, for example, is often curable, but it can be dangerous if left untreated.

Anemia caused by a genetic disorder (like sickle cell disease) usually requires lifelong treatment.

Pregnant people with anisocytosis need to receive prompt treatment because anemia can cause pregnancy complications.

Doctors diagnose anisocytosis if your red blood cells (RBCs) are not all the same size.

This condition can be caused by anemia, certain blood diseases, or some cancer drugs and usually requires treatment.

Treatment and outlook can depend on the underlying cause.