RDW (Red Cell Distribution Width)
Variation in red blood cell size
20 of 22 providers
Complete Blood Count (CBC)
11.5-14.5%
What is RDW (Red Cell Distribution Width)?
RDW (Red Cell Distribution Width) measures the variation in red blood cell size. It is expressed as a percentage, with normal range 11.5-14.5%. Higher RDW means greater variation (anisocytosis)—some RBCs are much larger or smaller than others. Low RDW means uniform RBC size (all similar). RDW is calculated from the standard deviation of RBC volume divided by MCV.
RDW helps differentiate causes of anemia and has emerged as an independent predictor of mortality and cardiovascular disease. In iron deficiency anemia, RDW is typically high (>15%) as the bone marrow produces increasingly smaller RBCs as iron becomes scarce. In thalassemia trait, RDW is often normal despite low MCV because all RBCs are uniformly small. In B12/folate deficiency, RDW is high as large macrocytes and smaller cells coexist.
Beyond anemia, elevated RDW (>15%) independently predicts all-cause mortality, cardiovascular events, and poor outcomes in heart failure, sepsis, and many chronic diseases. The mechanism is unclear but may reflect chronic inflammation, oxidative stress, poor nutritional status, or ineffective erythropoiesis. RDW is increasingly recognized as a general health marker, not just an anemia parameter.
Why RDW Differentiates Anemias and Predicts Health Outcomes
- Iron deficiency vs thalassemia:High RDW + low MCV=iron deficiency. Normal RDW + low MCV=thalassemia trait. Distinguishes these common causes without expensive testing
- Anemia classification:Helps categorize anemias beyond MCV alone. Combined with MCV creates diagnostic matrix
- Mortality predictor:RDW >15% independently predicts 1.5-2x higher all-cause mortality in multiple populations, even without anemia
- Heart failure prognosis:Elevated RDW predicts worse outcomes in heart failure, independent of other biomarkers like BNP
- Early nutritional deficiency:RDW rises before anemia develops in iron, B12, or folate deficiency, allowing earlier intervention
- Inflammation marker:Chronic inflammation increases RDW, reflecting oxidative stress and ineffective erythropoiesis
Optimal vs Standard Ranges
Optimal11.5-13.0%▼
- Uniform RBC size indicating healthy, efficient erythropoiesis
- Associated with lowest mortality risk and optimal health outcomes
- Reflects adequate iron, B12, folate stores and absence of chronic inflammation
Borderline High13.0-14.5%▼
- Upper normal range
- May indicate subclinical nutritional deficiency, mild inflammation, or early bone marrow dysfunction
- Check ferritin, B12, folate, CRP
- If anemia present, investigate aggressively
- Monitor every 6-12 months
Elevated14.5-17.0%▼
- High RBC size variation
- Common causes:iron deficiency (high RDW + low MCV), B12/folate deficiency (high RDW + high MCV), mixed deficiencies, hemolytic anemia, recent blood transfusion
- Check complete iron studies, B12, folate, reticulocyte count
- Associated with increased cardiovascular and mortality risk
Severely Elevated>17.0%▼
- Marked anisocytosis indicating significant pathology
- Causes:severe nutritional deficiency, myelodysplastic syndrome, hemolytic anemia, bone marrow disorders
- Requires comprehensive workup including peripheral smear, reticulocyte count, iron studies, B12, folate, possibly bone marrow biopsy
- Very high RDW (>20%) strongly predicts adverse outcomes
Scientific Evidence
RDW in Iron Deficiency vs Thalassemia
RDW distinguishes iron deficiency from thalassemia trait when MCV is low. Iron deficiency has high RDW (>15%) as progressively smaller RBCs are produced. Thalassemia trait has normal RDW (11.5-14.5%) as uniformly small RBCs are produced. This pattern has 85-90% diagnostic accuracy, complementing the Mentzer index.
RDW and All-Cause Mortality
In large population studies, RDW >14.5% predicts 1.5-2x higher all-cause mortality, independent of age, anemia, and other risk factors. Each 1% increase in RDW increases mortality risk by 14%. The relationship is continuous—even high-normal RDW (13.5-14.5%) has higher mortality than optimal (<13%). Mechanisms may include chronic inflammation, oxidative stress, or nutritional deficiency.
RDW in Heart Failure Prognosis
In heart failure patients, elevated RDW is a powerful independent predictor of mortality and hospitalization. RDW >15% is associated with 2-3x higher mortality compared to RDW <13%. RDW adds prognostic value beyond NT-proBNP, ejection fraction, and other established markers. Rising RDW during treatment predicts decompensation.
RDW and Cardiovascular Events
RDW >14% independently predicts cardiovascular events (MI, stroke, cardiovascular death) even in apparently healthy individuals. Each 1% increase in RDW is associated with 14% higher cardiovascular risk. This relationship persists after adjusting for traditional risk factors, suggesting RDW reflects underlying inflammation and oxidative stress contributing to atherosclerosis.
Source:Tonelli M, et al. Relation between red blood cell distribution width and cardiovascular event rate in people with coronary disease. Circulation. 2008;117(2):163-168.
RDW Changes Before Anemia Develops
RDW increases early in nutritional deficiency, often before hemoglobin or MCV become abnormal. In developing iron deficiency, RDW rises first, then MCV falls, finally hemoglobin drops. This allows earlier detection and intervention. RDW >14.5% with normal hemoglobin should prompt ferritin, B12, and folate testing.
Source:Bovy C, et al. Mature erythrocyte parameters as new markers of functional iron deficiency in haemodialysis:sensitivity and specificity. Nephrol Dial Transplant. 2007;22(4):1156-1162.
Which Providers Test RDW (Red Cell Distribution Width)?
Full Provider Comparison
| Provider | Includes | Annual Cost | Biomarkers |
|---|---|---|---|
 Superpower | ✓ | $199 | 100+ (150 with ratios) |
 WHOOP Advanced Labs | ✓ | $349 | 65 |
 Labcorp OnDemand | ✓ | $398 | 30+ |
| ✓ | $486 | 40+ | |
| ✓ | $444 | 288 | |
| ✓ | $349 | 100+ | |
| ✓ | $761 | 54 | |
 Function Health | ✓ | $365 | 160+ |
| ✓ | $250 | 65 | |
| ✓ | $495 | 70+ | |
| ✓ | $895 | 100+ | |
| ✓ | $1950 | 150+ | |
| ✓ | $375 | 80+ | |
| — | $Varies | 75+ | |
| ✓ | $190 | 100+ | |
| ✓ | $99 | 50 | |
| — | $124 | 60 | |
| ✓ | $199 | 50 | |
| ✓ | $499 | 120+ | |
| ✓ | $4188 | 70-80+ | |
| ✓ | $375 | 85 | |
| ✓ | $700 | 128 |
Frequently Asked Questions
What is RDW (Red Cell Distribution Width)?
Variation in red blood cell size
What is the optimal range for RDW (Red Cell Distribution Width)?
The standard reference range for RDW (Red Cell Distribution Width) is 11.5-14.5%. Optimal ranges may differ based on individual health goals and expert recommendations.
Which blood test providers include RDW (Red Cell Distribution Width)?
20 out of 22 blood testing providers include RDW (Red Cell Distribution Width) in their panels. This biomarker is widely available across major providers.
What category does RDW (Red Cell Distribution Width) fall under?
RDW (Red Cell Distribution Width) is categorized under Complete Blood Count (CBC). This category includes biomarkers that help assess related aspects of health and wellness.
Medical Disclaimer
This information is for educational purposes only and is not medical advice. Always consult with a qualified healthcare provider about your specific health needs.
Last reviewed:2026-02-20