RDW Blood Test Meaning and Normal Range on Your CBC

What an RDW Blood Test Measures

A red cell distribution width (RDW) test measures variation in the size and volume of your red blood cells, which are also called erythrocytes. In laboratory reports, this marker is sometimes referred to as an erythrocyte distribution width test or an RDW-SD (standard deviation) test.

Red blood cells are produced in the bone marrow, the spongy tissue found inside large bones. These cells contain hemoglobin, an iron-rich protein that transports oxygen from the lungs to every cell in the body. Healthy red blood cells are typically about the same size, and marked differences in volume can alter how well they deliver oxygen.

Laboratory instruments assess cell sizes using a histogram. When your red blood cells are relatively equal in volume, their measurements cluster together on the graph, resulting in a lower distribution width. When cell sizes vary widely, the measurements spread out across the histogram, producing an elevated RDW value that may signal an underlying health issue.

Why an RDW Test Is Part of a CBC

An RDW is often part of a complete blood count (CBC) and is commonly used to help diagnose or determine what is causing anemia. Anemia develops when the body does not produce enough red blood cells to meet its needs.

Healthcare providers order a CBC with RDW during routine physical checkups or when evaluating symptoms of anemia. Common symptoms of anemia include:

• Shortness of breath
• Weakness or fatigue
• Headaches
• Dizziness
• Arrhythmia (irregular heartbeat)

Providers also check RDW when an individual has a family history of inherited blood conditions such as sickle cell anemia or thalassemia, an inherited disease that can cause severe anemia. Other clinical reasons include chronic conditions like Crohn's disease, diabetes, or human immunodeficiency virus (HIV), long-term infections, significant blood loss from surgery or trauma, or an eating pattern low in iron and other minerals.

RDW Blood Test Normal Range on Your Report

A normal RDW result means that your red blood cell sizes fall within the standard reference range used by the testing facility. Laboratories generally report RDW as a percentage.

Reference ranges may vary depending on the laboratory used. Always check your specific lab report to see the exact reference interval for your sample.

A normal RDW does not completely rule out a blood disorder. Even when red cells are uniform in size, they may still be uniformly larger or smaller than typical healthy cells, and an individual may still have anemia or another condition that requires clinical evaluation.

High RDW Causes and Health Conditions

A high RDW means your red blood cells show greater variation in size than what is considered normal.

An elevated RDW can be a sign of several chronic medical conditions. These include chronic liver disease, heart disease, diabetes, kidney disease, and cancer, especially colorectal cancer. A provider reads the result with other blood tests.

Low RDW Meaning and Red Cell Uniformity

A low RDW means that your red blood cells are very uniform in volume, clustering tightly below the laboratory reference threshold. This result indicates that red cells are similar in size and close together.

A low RDW is not a sign of anemia and is not usually something to worry about.

How RDW Is Read With Other Blood Tests

Healthcare providers interpret RDW in context with other blood indices rather than evaluating it alone. Combined results show a more complete picture of the red blood cells.

Evaluating RDW alongside other hematology values helps clinicians identify iron deficiency and separate different types of anemia. Combined results can also help diagnose thalassemia and sickle cell anemia.

Getting the Blood Drawn

An RDW test requires a simple blood draw collected from a vein in your arm with a small needle. The entire collection procedure generally takes less than five minutes.

No special preparation is necessary. The procedure involves very little risk, though you might notice mild soreness or slight bruising where the needle entered your vein, which usually clears up quickly.

Reviewing Your RDW Blood Test Result

If your laboratory results indicate a chronic blood condition such as anemia, a treatment plan may be used. The primary objective is to increase the amount of oxygen your red blood cells can carry to your tissues.

Depending on the condition, a provider may recommend iron supplements, medicines, and/or diet changes. You should always speak with a healthcare provider before starting supplements or making substantial changes to your diet.

You can compare your complete blood count markers using our blood test results guide before discussing your RDW blood test with your doctor.

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