What It Measures
RDW measures the degree of variation (heterogeneity) in the size of circulating red blood cells. It is reported as a percentage — a higher percentage means greater variation in red blood cell size, which can indicate problems with red blood cell production, destruction, or nutritional deficiencies affecting cell formation.
Red Cell Distribution Width (RDW) is a measure of the variation in size of your red blood cells (anisocytosis). It is part of a standard complete blood count (CBC) and is an important marker for diagnosing types of anemia, nutritional deficiencies, and chronic disease. Elevated RDW has also emerged as an independent predictor of cardiovascular mortality and all-cause mortality.
Current Value
What High Means
Elevated RDW (anisocytosis) indicates that red blood cells vary significantly in size. This is commonly seen in iron deficiency anemia, vitamin B12 or folate deficiency, mixed anemias, hemolytic anemias, myelodysplastic syndromes, chronic liver disease, and chronic inflammatory conditions. High RDW is also independently associated with increased risk of cardiovascular events, heart failure progression, and all-cause mortality — even in the absence of anemia. Mechanisms include oxidative stress, chronic inflammation, impaired erythropoiesis, and nutritional deficiencies.
Possible Symptoms
Fatigue, weakness, shortness of breath, dizziness, pale skin, cold hands and feet, headaches, chest pain, rapid or irregular heartbeat, brittle nails, pica (unusual cravings), restless legs, poor exercise tolerance. Many of these symptoms overlap with the underlying anemia or deficiency causing elevated RDW.
What Low Means
A low RDW indicates that red blood cells are very uniform in size, which is generally considered normal and not clinically concerning. However, an unusually low RDW in the context of anemia may suggest thalassemia trait or anemia of chronic disease, where red blood cells are uniformly small (microcytic). Low RDW alone is rarely a clinical problem.
Possible Symptoms
Low RDW is generally asymptomatic and considered normal. If RDW is low in the setting of microcytic anemia (e.g., thalassemia trait), symptoms of the underlying condition may be present such as mild fatigue or pallor.
Risk Factors
Iron deficiency anemia, vitamin B12 deficiency, folate deficiency, chronic inflammatory diseases, cardiovascular disease, heart failure, chronic kidney disease, chronic liver disease, celiac disease, inflammatory bowel disease, myelodysplastic syndromes, thalassemia, hemolytic anemias, hypothyroidism, cancer, diabetes, and increased all-cause mortality.
Actionable Advice
Supplements
- •Iron (if deficient)
- •Vitamin B12
- •Folate (methylfolate)
- •Vitamin B6
- •Copper
- •Vitamin A
- •Vitamin C (enhances iron absorption)
- •Omega-3 fatty acids
Diet & Lifestyle
- •Ensure adequate dietary intake of iron-rich foods (red meat, leafy greens, legumes)
- •Consume B12-rich foods (meat, fish, eggs, dairy) or supplement if plant-based
- •Eat folate-rich foods (dark leafy greens, beans, citrus)
- •Pair iron-rich foods with vitamin C sources for better absorption
- •Avoid excessive alcohol which depletes B vitamins and impairs red blood cell production
- •Address chronic inflammation through anti-inflammatory diet (Mediterranean-style)
- •Regular moderate exercise to support healthy erythropoiesis
- •Get tested for celiac disease or malabsorption if RDW is persistently elevated without clear cause
- •Manage chronic conditions (diabetes, kidney disease) that can elevate RDW
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Historical Trend
Related Biomarkers
Resources & Studies
All Readings
| Date | Value | Change |
|---|---|---|
| 2026-01-28 | 15.5 % | +0.3 |
| 2025-09-18 | 15.2 % | +1.7 |
| 2025-08-25 | 13.5 % | -0.9 |
| 2025-07-31 | 14.4 % | +1.1 |
| 2025-06-27 | 13.3 % | — |