Guide to RDW

WRITTEN BY

The Levels Team

Updated: 05/22/2025|12 min read

Summary

Red cell distribution width (RDW) measures the variation in the size of your red blood cells. 

Why It Matters

Under normal circumstances, red blood cells produced by the bone marrow are typically fairly uniform in size. However, various health conditions can disrupt this process, releasing red cells of different sizes into circulation. 

Unlike other blood tests that measure the average size or number of cells, RDW specifically tells you how uniform or diverse your red blood cells are in terms of their volume. It essentially measures whether your red blood cells are all similar in size or if they vary significantly.

RDW has emerged as a useful metric because it often changes earlier than some other blood parameters during the development of certain conditions. For example, in early iron deficiency, RDW may increase before hemoglobin levels fall noticeably. Similarly, vitamin B12 or folate deficiencies typically cause increased RDW before anemia develops.

Beyond blood disorders, elevated RDW has been linked to a surprising range of conditions, including heart disease, liver problems, and certain cancers. Research has even identified it as a predictor of overall mortality risk in various populations, though the exact mechanisms behind these associations aren't fully understood.

It's important to remember that an abnormal RDW result, especially if it's only mildly elevated, is rarely cause for immediate concern on its own. It's often a subtle early indicator that warrants further investigation rather than a definitive diagnosis of a serious condition. Your healthcare provider will interpret this value in the context of your overall health, symptoms, and other laboratory findings.

Associated Symptoms

RDW levels themselves are laboratory findings rather than medical conditions. However, abnormal levels may be associated with various health conditions, each with its own symptoms.

Common symptoms that may indicate conditions associated with high RDW:

• Fatigue and weakness: Reduced oxygen delivery to tissues can be due to red blood cell abnormalities affecting energy production
• Pale skin: Lower hemoglobin content or irregular red blood cell formation potentially resulting in reduced color in tissues
• Shortness of breath: Especially during physical activity, can reflect reduced oxygen-carrying capacity
• Cold hands and feet: Poor circulation and oxygen delivery to extremities can be due to inefficient red blood cell function
• Brittle or spoon-shaped nails: May be associated with iron deficiency, which commonly causes elevated RDW
• Hair loss: May occur with nutritional deficiencies that also cause abnormal red blood cell production

Normal or low RDW values are generally not associated with symptoms, as they indicate uniform red blood cell sizes, which is typically a sign of healthy blood cell production.

It's important to understand that many factors can cause these symptoms, and RDW is just one piece of diagnostic information. The presence of symptoms alongside abnormal RDW provides context for further investigation, but additional testing is typically needed to determine the underlying cause. Many people with mildly elevated RDW may not experience any noticeable symptoms at all.

Clinical Ranges

Lab Reference Ranges: 11.0-15.0 %

Lifestyle Factors That Can Impact It

Activities that may increase RDW include the following:

• Poor nutrition, particularly diets low in iron, vitamin B12, or folate
• Excessive alcohol consumption
• Smoking
• Severe caloric restriction or crash dieting
• Vegetarian or vegan diet without proper supplementation

Activities that may help normalize elevated RDW include the following:

• Vitamin C consumption with meals (enhances iron absorption)
• Regular moderate exercise
• Adequate hydration
• Getting enough sleep

Other Factors That Can Impact It

Medical Conditions

• Hemoglobinopathies such as sickle cell disease or thalassemia: Elevate RDW through production of abnormally shaped and sized cells
• Liver disease: Elevates RDW through various effects on red cell membrane composition and metabolism
• Kidney disease: Increases RDW through effects on red cell production and lifespan
• Inflammatory conditions such as rheumatoid arthritis or Crohn's disease: Raise RDW through complex effects on iron use and red cell production
• Myelodysplastic syndromes: Dramatically increase RDW due to ineffective and abnormal red cell production
• Certain cancers, such as colon cancer: Elevate RDW through effects on bone marrow function and nutritional status

Medications

• Chemotherapy drugs: Increase RDW through disruption of normal blood cell production cycles
• Anticonvulsants, such as phenytoin: Raise RDW through effects on folate metabolism
• Hydroxyurea: Raises RDW by altering red cell production patterns
• Erythropoiesis-stimulating agents: Initially increase RDW as they stimulate production of new, larger red cells

Testing Accuracy and Stability

RDW testing is generally reliable. However, understanding several factors that can affect your results can help put the results in the proper context.

Factors That Can Affect the Accuracy of Your Test

• Very high white blood cell counts can interfere with accurate RDW measurement in some older analyzers.
• The presence of fragmented red cells can artificially increase RDW as fragments are counted as very small red cells.
• Recent blood transfusions reflect donor blood characteristics rather than your own.

How It Relates to Other Markers

Other tests can provide insights about health status when they're viewed alongside RDW results. These tests may include:

• Mean corpuscular volume (MCV): The combination of RDW and MCV is particularly powerful for classifying anemias:
• Normal MCV with high RDW: may indicate early iron deficiency or early B12/folate deficiency or mixed anemias
• Low MCV with high RDW: May indicate iron deficiency anemia
• High MCV with high RDW: Can suggest B12/folate deficiency
• Low MCV with normal or high RDW: May suggest thalassemia trait
• Normal MCV with normal or high RDW: May indicate anemia of chronic disease or acute blood loss
• Hemoglobin and hematocrit: These tests help determine if abnormal RDW is associated with anemia.
• Red blood cell count: Your total count provides context for understanding whether RDW abnormalities reflect changes in cell production or destruction.
• Reticulocyte count: This marker measures young red cells. A high count with elevated RDW may suggest active bone marrow response to blood loss or hemolysis.
• Iron studies, B12, and folate levels: These tests help determine specific nutritional causes of abnormal RDW.

What Results May Mean in the Context of Other Markers

• High RDW with normal hemoglobin: May represent early iron deficiency before anemia develops or recovery from recent anemia.
• High RDW with low hemoglobin and low MCV: May suggest iron deficiency anemia---a mix of small, pale cells with some normal or larger cells.
• High RDW with low hemoglobin and high MCV: May suggest vitamin B12 or folate deficiency---a mix of normal cells with abnormally large cells.
• High RDW with abnormal white blood cells or platelets: May indicate bone marrow disorder affecting multiple cell lines.
• Normal RDW with low hemoglobin and low MCV: May suggest thalassemia trait---cells are uniformly small.
• Increasing RDW over time: May indicate developing nutritional deficiency or progressive bone marrow condition.

Follow-up Considerations

If your RDW values are abnormal, your provider may make some of the following recommendations. You should always speak to your doctor if you have medical questions or before making medical decisions.

When Re-Testing May be Appropriate

• Will vary based on underlying condition and provider's opinion

Additional Testing or Monitoring Your Doctor May Consider

• Complete blood count with peripheral blood smear review
• Hemoglobin electrophoresis (if hemoglobinopathy suspected)
• Kidney and liver function tests
• Inflammatory markers (if chronic disease suspected)
• Celiac disease screening (if unexplained iron or B vitamin deficiencies)
• Bone marrow examination (in severe, complex, or unexplained cases)

When Additional Care May be Warranted

• RDW >20 percent without clear cause
• Rapidly increasing RDW values
• Abnormal RDW with moderate to severe anemia (hemoglobin < 10 g/dL)
• Abnormal RDW with symptoms of anemia
• RDW abnormalities with other abnormal blood cell lines
• Persistent unexplained elevation despite initial treatments
• New-onset elevated RDW in older adults (to rule out serious underlying conditions)
• RDW abnormalities during pregnancy

Bibliography