Anisocytosis: Definition, Types, Causes, Symptoms, Diagnosis and Treatments

The variation in red blood cell diameters in stained blood laboratory tests is called anisocytosis.

In medicine, the word is used to describe a condition in which red blood cells are of unequal sizes .

In this uneven variation in cell size, normal erythrocytes can vary (larger or smaller) up to a third of their size.

The importance of anisocytosis depends on the number of red blood cells of variable size and the specific morphology of the red blood cells that constitute anisocytosis (that is, spherocytes, polychromatophils, among others).

Effects of anisocytosis

Anisocytosis increases when there are different populations of cells, as can occur after a transfusion.

Anisocytosis can occur when substantial numbers of smaller-than-normal cells are produced, as is the case with iron deficiency, or when substantial numbers of larger-than-normal cells are produced, as occurs when a greater number of reticulocytes are produced.

Consequently, increased anisocytosis is usually present in regenerative anemia, but may be present in some cases of non-regenerative anemia resulting from dyserythropoiesis.

The degree of variation is measured in the laboratory by the distribution width of red blood cells. If this value is high, that means that the difference in the size of the red blood cells is greater.

It is important to remember that “Width” in the RDW term does not mean the width or diameter of the red blood cells, but rather the width of the volume curve.

Anisocytosis is usually seen in various anemias. To correctly diagnose the cause of anemia, we use RDW in conjunction with the mean corpuscular volume.

The mean corpuscular volume is not the volume of a single RBC, but it is the average volume of RBC in the given blood sample. The reference value of the mean corpuscular volume is 80 to 96 fL / RBC

Normal red blood cells

A normal RBC is disc-shaped with a concave (biconcave) surface and no nucleus.

In a peripheral smear (a glass slide with a blood sample for examination under a microscope), the red blood cells have a slightly pale central area compared to the surrounding area.

This area is about a third of the total area of ​​the red blood cells. Diameter is approximately 6 to 8 microns

Types and causes of anisocytosis

Based on RDW measurements and mean corpuscular volume, anisocytosis can be:


The cell size decreases compared to a normal cell.
This can be seen in iron deficiency anemia, thalassemia, sideroblastic anemia, lead poisoning, pyridoxine deficiency, and sickle cell anemia.


The size of the cell increases more than normal.
This occurs due to vitamin B12 deficiency (megaloblastic anemia), autoimmune hemolytic anemia, chronic liver disease, myelodysplastic syndrome, chemotherapy, hypothyroidism, posterior splenectomy, among others.


The cell size is within the normal range.

There is early iron deficiency, early vitamin B12 deficiency, early folic acid deficiency, chronic liver disease, sickle cell disease, myelodysplastic syndrome, dimorphic anemia.

When red blood cells are of different sizes, the oxygen carrying capacity of these cells varies. This in turn affects the supply of oxygen to different tissues.

Anisocytosis in Pregnancy

One of the common observations during pregnancy is anemia, mainly iron deficiency anemia or folic acid deficiency anemia. These conditions can present as mild anisocytosis, especially in the early stages of anemia.

Symptoms of anisocytosis

The symptoms of anisocytosis are mainly due to the inefficient oxygen carrying capacity of the red blood cells, resulting in decreased oxygen supply to the tissues and organs.

Pale skin, weakness, fatigue, dyspnea, and increased heart rate may be seen in people with anisocytosis.

Diagnosis of anisocytosis

The diagnosis of anisocytosis can be made by examining the blood. The estimation of RDW and mean corpuscular volume is the main stay for a correct diagnosis. More investigations may be required to find the underlying cause of anisocytosis.

Anisocytosis itself is not a disease, but an expression of an underlying abnormal condition.

The classification of anisocytosis is based on the degree of variation from normal size. The grades awarded are + (Slight / 1+), ++ (Moderate / 2 +) or +++ (Severe / 3 +).

Increased anisocytosis occurs when a significant number of small diameter red blood cells, large diameter red blood cells, or a combination of both are present along with normal diameter red blood cells.

Anisocytosis is recognized more frequently with a significant number of spherocytes in IMHA and a large number of macrocytic polychromatophils in regenerative anemias.

Treatment of anisocytosis

Treatment of anisocytosis can only be done by correcting the underlying cause. If it is due to anemia, the correct cause must first be diagnosed. Nutritional supplementation with diet modification can help anemia due to deficiencies.

But in thalassemia, a blood transfusion may be what is needed, while myelodysplastic syndrome may require a bone marrow transplant.