What does rdw mean in a blood test

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Last updated: April 4, 2026

Quick Answer: RDW stands for Red Blood Cell Distribution Width. It measures the variation in the size of your red blood cells. An abnormal RDW can indicate various underlying conditions, such as anemia or nutritional deficiencies.

Key Facts

RDW measures the variation in red blood cell size.
A normal RDW is typically between 11.5% and 14.5%.
High RDW can suggest iron deficiency anemia, vitamin B12 deficiency, or folate deficiency.
Low RDW is less common and usually not clinically significant on its own.
RDW is often interpreted alongside other blood count parameters, like MCV (Mean Corpuscular Volume).

What is RDW in a Blood Test?

The Red Blood Cell Distribution Width (RDW) is a common measurement included in a complete blood count (CBC). It quantifies the variation in the size of your red blood cells. Red blood cells, also known as erythrocytes, are vital for transporting oxygen from your lungs to the rest of your body. Normally, red blood cells are relatively uniform in size. However, in certain conditions, the body may produce red blood cells of different sizes, leading to an increased RDW.

Understanding Red Blood Cell Distribution Width (RDW)

The RDW test measures the range of red blood cell volumes, essentially how much the sizes of your red blood cells vary from the average. A low RDW means your red blood cells are mostly of similar size, which is generally considered normal. A high RDW indicates that there is a significant variation in the size of your red blood cells, meaning you have a mix of large and small red blood cells. This variation can be a clue that something is affecting red blood cell production or survival.

Normal RDW Ranges

The typical reference range for RDW is usually between 11.5% and 14.5%. However, these ranges can vary slightly depending on the laboratory performing the test. Your doctor will interpret your RDW result in conjunction with other blood test results, such as the Mean Corpuscular Volume (MCV), which measures the average size of your red blood cells.

What Does a High RDW Mean?

An elevated RDW can be an early indicator of several health conditions, often related to different types of anemia. Some common causes of a high RDW include:

Iron Deficiency Anemia: This is one of the most common reasons for an increased RDW. As the body's iron stores deplete, it starts producing smaller red blood cells, while older, normal-sized cells may still be present, leading to a wider distribution of sizes.
Vitamin B12 Deficiency Anemia (Megaloblastic Anemia): A lack of vitamin B12 can lead to the production of abnormally large red blood cells (macrocytes). If the body also has a concurrent iron deficiency, you might see a mix of small and large cells, elevating the RDW.
Folate Deficiency Anemia (Megaloblastic Anemia): Similar to B12 deficiency, a lack of folate can also cause the production of large red blood cells, contributing to a high RDW.
Hemolytic Anemia: In this condition, red blood cells are destroyed faster than the bone marrow can produce them. The body may respond by releasing immature, often smaller red blood cells, alongside normal ones.
Recent Blood Transfusion: If you've recently received a blood transfusion, the donor's red blood cells might have different sizes than your own, temporarily affecting the RDW.
Liver Disease: Certain liver conditions can affect red blood cell production and lifespan.
Myelodysplastic Syndromes (MDS): These are a group of disorders where the bone marrow doesn't produce enough healthy blood cells.

What Does a Low RDW Mean?

A low RDW is generally considered less significant than a high RDW. It typically indicates that your red blood cells are very uniform in size. In most cases, a low RDW is not indicative of a specific medical problem and is often not followed up on unless other blood test results are abnormal.

RDW and MCV Correlation

Interpreting the RDW is most effective when considered alongside the MCV. The MCV indicates the average size of your red blood cells. The combination of RDW and MCV can help doctors differentiate between various types of anemia:

High RDW, Low MCV: Suggests iron deficiency anemia or thalassemia trait.
High RDW, High MCV: Suggests vitamin B12 or folate deficiency anemia, or potentially liver disease.
Normal RDW, Low MCV: May indicate anemia of chronic disease or early iron deficiency.
Normal RDW, High MCV: Can be seen in aplastic anemia or hypothyroidism.

When to Consult a Doctor

If your RDW result is abnormal, it doesn't automatically mean you have a serious condition. However, it is an important indicator that warrants further investigation by your healthcare provider. They will consider your RDW result along with your symptoms, medical history, and other blood test results to determine the underlying cause and recommend appropriate treatment if necessary. Early detection and diagnosis are key to managing many conditions that can affect red blood cell production.