Test Description

What is RDW?

Red Cell Distribution Width (RDW) is a measurement of the variability in size (volume) of circulating red blood cells. It quantifies the degree of anisocytosis - the presence of RBCs of unequal sizes.

How is RDW Calculated?

RDW is derived from the RBC size histogram generated by automated hematology analyzers. Two forms are commonly reported:

  • RDW-CV (Coefficient of Variation): (Standard deviation of RBC volume ÷ MCV) × 100, expressed as a percentage. This is the most commonly used form.
  • RDW-SD (Standard Deviation): The actual standard deviation of RBC volume in femtoliters. Less affected by MCV changes.

Clinical Significance of RDW

RDW provides additional information beyond MCV alone:

  • Helps differentiate anemia types: When combined with MCV, RDW creates a 6-cell classification of anemias
  • Early deficiency detection: RDW rises before MCV changes in iron or B12 deficiency
  • Distinguishes iron deficiency from thalassemia: Both cause low MCV, but RDW differs
  • Prognostic marker: Elevated RDW associated with worse outcomes in heart failure, sepsis, and other conditions
Peripheral Smear Correlation: RDW corresponds to anisocytosis seen on peripheral blood smear. Normal RDW = uniform cell sizes. High RDW = mixture of small and large cells visible on smear.
Quick Reference
  • Normal Range (RDW-CV): 11.5-14.5% (coefficient of variation)
  • Normal Range (RDW-SD): 39-46 fL (standard deviation)
  • Elevated RDW: >14.5% indicates anisocytosis (variable cell sizes)
  • Primary Use: Distinguish between causes of anemia; prognostic marker in various diseases
  • Sample Type: Whole blood (EDTA tube - purple top)
  • Key Point: Iron deficiency = high RDW; Thalassemia trait = normal RDW
Normal Ranges
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Measurement Normal Range Notes
RDW-CV 11.5-14.5% Most commonly reported; affected by MCV
RDW-SD 39-46 fL Less dependent on MCV; absolute measure

Interpreting RDW Values

  • Normal RDW: Relatively uniform RBC population
  • Elevated RDW: Mixed RBC population with significant size variation
  • RDW >20%: Marked anisocytosis; consider active hemolysis, severe nutritional deficiency, or post-transfusion
Important Note: RDW can only increase, never decrease below normal. A "low" RDW simply means uniform cell size, which is normal. There is no pathologic "low RDW."
Clinical Significance

Elevated RDW - Causes

An elevated RDW indicates a mixed population of RBC sizes. Common causes include:

Nutritional Deficiencies:

  • Iron deficiency anemia: Classic cause of elevated RDW; new microcytic cells mixed with older normal cells
  • Vitamin B12 deficiency: Macrocytic cells mixed with normal cells
  • Folate deficiency: Similar to B12 deficiency
  • Mixed deficiency (iron + B12/folate): Very high RDW with dimorphic population

Hemolytic Anemias:

  • Sickle cell disease: Irreversibly sickled cells mixed with normal cells
  • Hereditary spherocytosis: Spherocytes of varying sizes
  • Autoimmune hemolytic anemia: Reticulocytosis adds larger cells

Other Causes:

  • Recent blood transfusion: Donor and recipient cells of different sizes
  • Myelodysplastic syndrome: Dyserythropoiesis with variable cell sizes
  • Post-treatment response: New cells being produced after treatment of deficiency
  • Liver disease: Variable macrocytosis
  • Chronic kidney disease: Often with reticulocytosis after EPO therapy

Normal RDW - Significance

Normal RDW in the setting of anemia suggests a uniform RBC population:

  • Thalassemia trait: Uniform population of small cells (key differentiator from iron deficiency)
  • Anemia of chronic disease: Often normal RDW
  • Acute blood loss: Before bone marrow response
  • Chronic kidney disease: Before EPO therapy
  • Aplastic anemia: Uniform failure of production

RDW as a Prognostic Marker

Elevated RDW has emerged as an independent prognostic marker in various conditions:

  • Heart failure: Higher RDW associated with increased mortality
  • Acute coronary syndrome: Predictor of adverse events
  • Sepsis: Associated with increased mortality
  • COVID-19: Elevated RDW linked to worse outcomes
  • General population: Higher RDW associated with all-cause mortality
Why RDW Predicts Mortality: The mechanism is not fully understood, but elevated RDW likely reflects underlying inflammation, oxidative stress, poor nutritional status, or impaired erythropoiesis - all of which are associated with adverse outcomes.
Interpretation Guidelines

MCV + RDW Classification of Anemias

Combining MCV and RDW creates a powerful diagnostic framework:

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MCV RDW Normal RDW Elevated
Low (<80 fL) • Thalassemia trait
• Chronic disease (some) 		• Iron deficiency
• Sideroblastic anemia
• Hgb H disease
Normal (80-100 fL) • Chronic disease
• Chronic kidney disease
• Acute blood loss
• Hereditary spherocytosis 		• Early iron/B12/folate deficiency
• Mixed deficiency
• Sickle cell disease
• Myelofibrosis
High (>100 fL) • Aplastic anemia
• Myelodysplasia (some)
• Liver disease (some) 		• B12/folate deficiency
• Autoimmune hemolytic anemia
• Cold agglutinin disease
• Myelodysplasia

Iron Deficiency vs Thalassemia Trait

This is the classic use of RDW - both conditions cause microcytic anemia, but:

Swipe to see more
Feature Iron Deficiency Thalassemia Trait
RDW Elevated (>14.5%) Normal (≤14.5%)
RBC Count Low or normal Normal or elevated
Mentzer Index (MCV/RBC) >13 <13
Ferritin Low Normal
Peripheral Smear Hypochromia, anisocytosis Target cells, basophilic stippling
Interfering Factors

Factors That Increase RDW

  • Recent transfusion: Donor cells of different age/size than recipient cells
  • Cold agglutinins: RBC clumping can affect histogram analysis
  • Marked reticulocytosis: Young RBCs are larger
  • RBC fragments: Schistocytes add to size variability
  • Sample age: Prolonged storage can cause RBC swelling variability

Factors That May Normalize RDW Despite Disease

  • Recent transfusion (paradoxically): Can temporarily "normalize" a mixed population
  • Early/mild deficiency: RDW may not yet be elevated
  • Uniform pathology: Conditions causing uniform cell changes (thalassemia trait)

Technical Considerations

  • Analyzer differences: Different instruments may give slightly different RDW values
  • RDW-CV vs RDW-SD: Know which your lab reports; RDW-SD less affected by MCV
  • Histogram review: When available, the RBC histogram provides visual confirmation of anisocytosis pattern
Clinical Pearls
The "Iron vs Thal" rule: In microcytic anemia, elevated RDW strongly favors iron deficiency; normal RDW favors thalassemia trait. This simple distinction can guide initial workup and reduce unnecessary hemoglobin electrophoresis in iron-deficient patients.
RDW rises early: In nutritional deficiencies, RDW increases before MCV changes and often before anemia develops. An isolated elevated RDW in a non-anemic patient may indicate subclinical iron or B12 deficiency.
Mixed deficiency: If a patient has both iron and B12 deficiency, MCV may be normal (opposing effects), but RDW will be markedly elevated due to dimorphic RBC population. This pattern should prompt checking both iron studies and B12/folate.
Post-transfusion interpretation: RDW is unreliable after blood transfusion because donor cells dilute the patient's native RBC population. Wait at least 2-3 months after transfusion for meaningful RDW interpretation.
Response to therapy: During treatment of iron or B12 deficiency, RDW initially INCREASES further (new normal cells mixed with old abnormal cells), then normalizes over 2-3 months as deficient cells are replaced.
RDW in the ED: Beyond anemia evaluation, elevated RDW is associated with increased mortality in critically ill patients. It's a nonspecific but useful marker of physiologic stress that may help identify high-risk patients.
Look at the histogram: Many analyzers provide an RBC histogram. A bimodal (two-peak) histogram indicates a dimorphic population - either post-transfusion or mixed deficiency. A widened single peak indicates general anisocytosis.
References
  1. Kratz, A., Ferraro, M., Sluss, P. M., & Lewandrowski, K. B. (2004). Laboratory reference values. New England Journal of Medicine, 351, 1548-1564.
  2. Bessman, J. D., Gilmer, P. R., & Gardner, F. H. (1983). Improved classification of anemias by MCV and RDW. American Journal of Clinical Pathology, 80(3), 322-326.
  3. Patel, K. V., et al. (2009). Red cell distribution width and mortality in older adults: a meta-analysis. Journals of Gerontology Series A: Biomedical Sciences and Medical Sciences, 65(3), 258-265.
  4. Salvagno, G. L., et al. (2015). Red blood cell distribution width: A simple parameter with multiple clinical applications. Critical Reviews in Clinical Laboratory Sciences, 52(2), 86-105.
  5. Hoffmann, J. J. M. L., et al. (2015). Effect of age and gender on reference intervals of red blood cell distribution width (RDW) and mean red cell volume (MCV). Clinical Chemistry and Laboratory Medicine, 53(12), 2015-2019.
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  • For Educational Purposes Only: This content is intended for educational reference and should not be used for clinical decision-making.
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The clinical content and references are curated and reviewed by myself; however, AI was used to assist in organizing, paraphrasing, and formatting the information presented.