Variation in red blood cell size
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Complete Blood Count (CBC)
B12
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.
| Range Type | Level | Significance |
|---|---|---|
| Optimal | 11.5-13.0% | 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 High | 13.0-14.5% | 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. |
| Elevated | 14.5-17.0% | 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. |
Iron deficiency (high RDW + low MCV):Ferrous sulfate 325 mg 2-3x daily. RDW normalizes in 3-4 months as uniformly normal RBCs replace deficient cells
B12 deficiency (high RDW + high MCV):B12 1000 mcg IM weekly, then monthly. RDW decreases as new normal-sized RBCs are produced
Folate deficiency:Folic acid 1-5 mg daily. RDW improves within 8-12 weeks
Mixed deficiencies:Common in elderly or malnourished. Treat all deficiencies simultaneously. RDW may take 3-6 months to normalize
Anti-inflammatory diet:Mediterranean diet, omega-3 fatty acids (2-4g EPA+DHA daily) reduce oxidative stress affecting RBCs
Treat chronic conditions:Rheumatoid arthritis, inflammatory bowel disease, chronic infections increase RDW through inflammation
Antioxidants:Vitamin C (500-1000 mg), vitamin E (400 IU), selenium (200 mcg) may reduce RBC oxidative damage
Exercise:Moderate regular exercise reduces inflammation. Avoid overtraining which increases oxidative stress
Smoking cessation:Smoking dramatically increases RDW through oxidative damage and chronic inflammation
If high RDW with heart failure:Optimize medical therapy (ACE inhibitors, beta-blockers, diuretics). Elevated RDW predicts worse outcomes—may need more aggressive treatment
Correct anemia if present:Even mild anemia worsens heart failure. Treat iron deficiency, consider ESAs if severe
Monitor closely:RDW rising over time in heart failure predicts decompensation. Check every 3-6 months
Address comorbidities:Kidney disease, diabetes, COPD often coexist with heart failure and increase RDW
If RDW >17% with reticulocytosis:Suggests hemolysis or recent bleeding. Check LDH, haptoglobin, indirect bilirubin, Coombs test
Myelodysplastic syndrome:High RDW with unexplained cytopenias and macrocytosis. Requires bone marrow biopsy for diagnosis
Mixed population:Recent blood transfusion creates two RBC populations (donor + native), elevating RDW. Normalizes over 3-4 months
Bone marrow biopsy:If RDW >18% with unclear cause, consider bone marrow evaluation to exclude clonal disorders
Cardiovascular risk:RDW >15% identifies high-risk patients requiring aggressive risk factor management
Sepsis prognosis:RDW >15% on admission predicts higher mortality. May warrant ICU admission and closer monitoring
Pre-operative risk:Elevated RDW predicts post-surgical complications. Optimize nutritional status before elective surgery
Chronic disease monitoring:Rising RDW trend indicates disease progression or emerging complications. Investigate promptly
General health marker:Even without specific disease, RDW >14.5% suggests need for comprehensive health assessment
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.
Source:Bessman JD, et al. Improved classification of anemias by MCV and RDW. Am J Clin Pathol. 1983;80(3):322-326.
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.
Source:Patel KV, et al. Red cell distribution width and mortality in older adults:a meta-analysis. J Gerontol A Biol Sci Med Sci. 2010;65(3):258-265.
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.
Source:Felker GM, et al. Red cell distribution width as a novel prognostic marker in heart failure:data from the CHARM Program and the Duke Databank. J Am Coll Cardiol. 2007;50(1):40-47.
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 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.
| Provider | Includes | Annual Cost | Biomarkers |
|---|---|---|---|
 Superpower | ✓ | $199 | 100+ (150 with ratios) |
 WHOOP Advanced Labs | ✓ | $349 | 65 |
 Labcorp OnDemand | ✓ | $398 | 30+ |
| ✓ | $486 | 40+ | |
| ✓ | $468 | 83 | |
| ✓ | $349 | 100+ | |
 InsideTracker | ✓ | $680 | 54 |
 Function Health | ✓ | $365 | 100+ |
| ✓ | $250 | 65 | |
| ✓ | $495 | 70+ | |
| ✓ | $895 | 100+ | |
| ✓ | $1950 | 150+ | |
| — | $399 | 100+ | |
| — | $Varies | 75+ | |
| ✓ | $190 | 100+ | |
| ✓ | $99 | 50 | |
| — | $125 | 60 | |
| ✓ | $199 | 50 | |
| ✓ | $499 | 120+ | |
| ✓ | $4188 | 80+ | |
| ✓ | $375 | 85 | |
| ✓ | $700 | 129 |
20 providers include this biomarker in their panels
This information is for educational purposes only and is not medical advice. Always consult with a qualified healthcare provider about your specific health needs.
