RDW
Also known as: Red Cell Distribution Width
What Does RDW Measure?
RDW, or Red Cell Distribution Width, is a blood test measurement that reflects the variation in size among your red blood cells (RBCs). Specifically, it quantifies how much your red blood cells differ from one another in volume — a property known as anisocytosis. When red blood cells are relatively uniform in size, the RDW is low; when there is significant size variation, the RDW is elevated. This measurement is typically expressed as a percentage and is routinely included as part of a Complete Blood Count (CBC) panel.
Why Does RDW Matter?
RDW is a valuable diagnostic tool because it helps distinguish between different types of anemia and other blood disorders. For example, iron deficiency anemia typically causes a high RDW, while thalassemia (a genetic condition) often presents with a normal or only mildly elevated RDW despite similarly low hemoglobin — a distinction that can guide treatment decisions. Beyond anemia, emerging research has linked elevated RDW to chronic inflammation, cardiovascular disease, liver disease, kidney disease, and even all-cause mortality. It is increasingly recognized as a marker of systemic stress and nutritional deficiency, making it a surprisingly broad indicator of overall health.
Normal Ranges
Males
11.5% – 14.5%
Females
11.5% – 14.5%
Children
11.5% – 14.5% (may vary slightly by age and laboratory reference)
Causes of High Levels
- Iron deficiency anemia — the most common cause, as iron-deprived red cells are produced in varying smaller sizes
- Vitamin B12 or folate deficiency — leads to production of abnormally large red blood cells alongside normal ones
- Chronic inflammatory conditions (e.g., rheumatoid arthritis, inflammatory bowel disease) — disrupt normal red cell production
- Liver disease or alcoholism — impairs nutrient metabolism and red cell maturation, causing size irregularities
- Hemolytic anemia — accelerated RBC destruction triggers compensatory release of immature, size-variable cells
- Recent blood transfusion — introduces a mixed population of donor and recipient red cells of differing sizes
Causes of Low Levels
- Microcytic anemia of uniform cell size (e.g., some forms of thalassemia trait) — all cells are small but similar
- Well-controlled nutritional status — consistent, adequate nutrient supply supports uniform red cell production
How to Improve Your RDW
Diet
- Increase iron-rich foods such as lean red meat, poultry, lentils, spinach, and fortified cereals to support consistent red cell production
- Consume vitamin C alongside iron-rich plant foods (e.g., bell peppers, citrus fruits) to enhance non-heme iron absorption
- Eat folate-rich foods daily including leafy greens, beans, avocado, and fortified grains to support proper red cell maturation
- Include vitamin B12 sources such as eggs, dairy, fish, meat, or fortified plant-based foods to prevent macrocytic red cell abnormalities
- Limit alcohol consumption, as chronic alcohol use impairs folate metabolism and disrupts normal red cell production
Supplements
- Iron supplementation (e.g., ferrous sulfate 325 mg daily or as directed by physician) if iron deficiency is confirmed by ferritin and serum iron tests
- Vitamin B12 (500–1000 mcg daily, sublingual or oral) for individuals with confirmed deficiency or those on vegan/vegetarian diets
- Folic acid (400–1000 mcg daily) for confirmed folate deficiency or for individuals with poor dietary intake
- A high-quality multivitamin containing iron, B12, folate, and zinc to address potential multi-nutrient deficiencies
Related Biomarkers
Frequently Asked Questions
What does it mean if my RDW is high?
A high RDW means your red blood cells vary significantly in size, which is a condition called anisocytosis. The most common cause is iron deficiency anemia, but it can also indicate vitamin B12 or folate deficiency, chronic inflammation, liver disease, or other conditions affecting red cell production. A high RDW alone doesn't diagnose a specific disease — your doctor will interpret it alongside other blood markers like MCV, hemoglobin, and ferritin to identify the underlying cause.
Can RDW be high without anemia?
Yes, RDW can be elevated even when hemoglobin levels are still in the normal range. This can happen in early-stage nutritional deficiencies (such as early iron, B12, or folate deficiency) before anemia fully develops. Elevated RDW without anemia may also reflect chronic inflammation, early liver disease, or other systemic conditions. It can serve as an early warning signal, prompting further investigation before more significant blood changes occur.
How is RDW used to distinguish different types of anemia?
RDW is often interpreted together with MCV (the average red cell size) to differentiate anemia types. For example, iron deficiency anemia typically shows a HIGH RDW with a LOW MCV, while thalassemia trait often shows a LOW or NORMAL MCV with a NORMAL RDW — this distinction helps avoid unnecessary iron supplementation in thalassemia patients. Vitamin B12 or folate deficiency presents with a HIGH MCV and HIGH RDW. This combination approach makes RDW a critical tool in anemia workups.