Red Cell Distribution Width (RDW) is a common laboratory test reported as part of a Complete Blood Count (CBC). It measures how much size variation exists among your red blood cells (RBCs). Red blood cells are responsible for carrying oxygen throughout the body, and they are normally fairly uniform in size. RDW is usually reported as a percentage (RDW-CV) or occasionally as a value in femtoliters (RDW-SD), depending on the laboratory.

In plain language, RDW answers the question: “Are my red blood cells all about the same size, or is there a wide mix of small and large cells?” A higher RDW means there is more variation in red blood cell size (a pattern called anisocytosis). This can happen when your body is producing new red blood cells of a different size than older ones, or when nutrition-related problems affect red blood cell production.

RDW is not a standalone diagnosis. Doctors interpret RDW together with other CBC results—especially hemoglobin, hematocrit, mean corpuscular volume (MCV) (average RBC size), and sometimes a peripheral blood smear—to understand your blood health and to help narrow down causes of anemia.

What results can mean: A normal RDW generally suggests your red blood cells are fairly uniform in size. A high RDW often points toward anemia related to nutritional deficiency (such as iron deficiency, vitamin B12 deficiency, or folate deficiency), mixed types of anemia, or recovery after blood loss or treatment. A low RDW is uncommon and usually not clinically significant on its own.

RDW is typically checked during a routine blood test (CBC) as part of a general health screening or when a clinician is evaluating symptoms that may be related to anemia or other blood disorders. Because RDW reflects red blood cell size differences, it can help identify patterns that suggest specific causes of anemia and guide follow-up testing (such as iron studies, vitamin levels, or tests for inflammation).

Your clinician may order or pay close attention to RDW if you have symptoms such as:

• Fatigue, weakness, or low energy
• Shortness of breath with activity
• Dizziness, headaches, or lightheadedness
• Pale skin, cold hands/feet, or reduced exercise tolerance
• Heart palpitations (a fast or irregular heartbeat)

RDW is also commonly used when there are risk factors for anemia or nutrient deficiencies, including:

• Heavy menstrual bleeding, gastrointestinal bleeding, or recent blood loss
• Pregnancy, breastfeeding, or rapid growth periods (higher iron and vitamin needs)
• Dietary patterns that increase risk of iron or vitamin deficiency (e.g., limited iron-rich foods, strict vegan diet without B12 supplementation)
• Digestive conditions or surgeries that reduce nutrient absorption (e.g., celiac disease, bariatric surgery)
• Chronic inflammation or long-term illness that can affect red blood cell production
• Monitoring response to treatment for anemia (RDW may rise temporarily as new cells enter circulation)

Clinically, RDW is especially helpful when interpreted alongside MCV. For example, a low MCV (microcytosis) with high RDW can suggest developing iron deficiency anemia, while a high MCV (macrocytosis) with high RDW can point toward vitamin B12 or folate deficiency. Your provider may use these patterns to decide which additional laboratory tests are most appropriate.

• Iron deficiency anemia (often due to low dietary iron, blood loss, or poor absorption)
• Vitamin B12 deficiency anemia
• Folate (vitamin B9) deficiency anemia
• Anemia of chronic disease/inflammation
• Mixed anemia (e.g., iron deficiency plus vitamin B12/folate deficiency)
• Hemolytic anemia (increased breakdown of red blood cells)
• Recent blood loss or recovery after hemorrhage
• Thalassemia (RDW may be normal or mildly elevated depending on type and coexistence of deficiencies)
• Liver disease (can be associated with larger red blood cells and higher RDW)

• Evaluating fatigue and low energy by screening for anemia and nutrient-related causes
• Supporting blood health monitoring as part of routine annual labs or general health screening
• Identifying possible nutritional deficiency (iron, vitamin B12, folate) and guiding diet or supplementation discussions
• Tracking improvement during treatment for iron deficiency or vitamin deficiency anemia
• Assessing overall red blood cell production patterns when other CBC values are abnormal
• Helping clinicians decide when additional testing (iron studies, ferritin, B12/folate levels) may be needed