Red blood cell distribution width (RDW)

Synonyms: Red cell distribution width, RDW-SD (standard deviation), RDW-CV (coefficient of variation), Erythrocyte Distribution Width

Providing all tissues and organs with oxygen is one of the most important processes in the human body. When you inhale, air enters the lungs where the work of red blood cells (erythrocytes) begins. The red blood cells carry oxygen to every cell of our body, and also take part in the reverse process – they carry carbon dioxide to the lungs so it can be removed from the body. Failures in the work of red blood cells often lead to the development of serious pathologies.

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In order to evaluate the morphology (appearance) of red blood cells, red blood cell indices are used:

Red cell distribution width (RDW) test measures the variation in red blood cell size and volume and not the actual size of individual cells. In other words, the difference between the largest and smallest red blood cells is determined.

Why is the RDW test done?

RDW test is usually done to diagnose anemia, a condition in which red blood cells cannot carry enough oxygen throughout the body.

RDW is also used to diagnose thalassemia, diabetes, liver disease, heart disease, and cancer.

Your doctor may also order complete blood count (CBC) with RDW and other RBC indices if you have:

  • an iron and vitamins deficiency
  • anemia symptoms (weakness, pale skin, dizziness, etc.)
  • significant blood loss
  • diseases affecting red blood cells

In recent years, many clinical studies have shown that changes in RDW levels may be associated with many cardiovascular and cerebrovascular diseases (CVDs). RDW plays a significant role in assessing the severity and progression of CVDs. However, the relationship between the RDW and the prognosis of CVDs remains unclear.

RDW Measurement Methods and Units of Measurement

Depending on the laboratory equipment, the red cell distribution width can be calculated as the coefficient of variation (RDW-CV) or as standard deviation (RDW-SD).

One important factor should be taken into account. The red cell distribution width is not the actual width or diameter of individual red blood cells, but it is the width of the red blood cell distribution curve.

RDW-CV shows how much your red blood cells vary in size and volume.

RDW-CV is calculated based on the width of the distribution curve, as well as the mean cell volume. The formula is as follows:

RDW-CV = (1 SD / MCV) * 100

SD – standard deviation of the mean cell size

MCVmean corpuscular volume

RDW-CV is expressed as a percentage (%). It is directly dependent on MCV, so it may not always reflect the actual variation in the size of red blood cells. For example, if most of the red blood cells are small, then RDW-CV will remain within normal limits.

RDW-SD is an actual measurement of the red blood cell distribution curve in femtoliters (fL), which is measured by calculating the histogram width (in fL) at the 20% height level.

1 fL = 10-15 L

Since RDW-SD is the actual difference between the maximum and minimum volume of red blood cells and is not dependent on MCV, it more accurately reflects changes in the red blood cells size.

Red cell distribution width (RDW). Red blood cell distribution curve.

What is RDW normal range?

Reference values may vary depending on the laboratory equipment, gender, and age.

The normal range for RDW-SD is 39-46 fL. According to the Mayo Clinic, the normal level of RDW-CV is:

0-14 days:14.8-17.0%14.6-17.3%
15 days-4 weeks14.3-16.8%14.4-16.2%
5 weeks-7 weeks13.8-16.1%13.6-15.8%
8 weeks-5 months12.4-15.3%12.2-14.3%
6-23 months12.9-15.6%12.7-15.1%
24-35 months12.5-14.9%12.4-14.9%
3-5 years11.3-13.4%11.3-13.4%
6-17 years11.4-13.5%11.4-13.5%

What is anisocytosis?

The normal size of a red blood cell is approximately 7-8 μm. Most red blood cells in the blood are of the normal size but there also smaller and bigger red cells. If the red blood cells size varies greatly (high RDW level) this is called anisocytosis. Anisocytosis is most often caused by anemia.

What are the symptoms of high RDW (anisocytosis)?

If the red blood cells in the blood are irregular in size or shape, oxygen cannot be carried through the body efficiently. Therefore, the main symptoms of anisocytosis are those of anemia:

  • weakness, fatigue
  • dizziness
  • pale skin
  • shortness of breath
  • fast or irregular heartbeat

What does high RDW mean?

High RDW is often associated with the deficiency of iron, vitamin B-12, or folate. Increased RDW levels also may be caused by liver diseases, some types of anemia (hemolytic, megaloblastic), Alzheimer's disease, cardiovascular disease, and cancer. Red cell distribution width (RDW) is usually evaluated together with mean corpuscular volume (MCV).

What does high RDW mean in pregnancy?

The most common cause of an elevated RDW during pregnancy is iron deficiency anemia, as the need for iron increases during pregnancy.

According to the study, red blood cells distribution width (RDW) is a reliable and useful parameter for detection of iron deficiency during pregnancy.

What does a low RDW mean?

Low RDW means that red blood cells are almost the same in size. This condition is not consistently associated with any hematological disorder and usually has no diagnostic value.

What does normal RDW mean?

A normal level of RDW does not mean the absence of disease. It indicates that most of the red blood cells are about the same size, but they can be either all small or all large.

Typically, RDW is evaluated together with MCV. Normal RDW and low MCV may be associated with chronic disease or thalassemia.

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