Red Cell Distribution Width (RDW)
Red Cell Distribution Width (RDW) measures how much variation exists in the size of your red blood cells. In healthy blood, red cells are fairly uniform. Elevated RDW means your cells vary more than normal — some larger, some smaller — indicating something is affecting production. This test is particularly valuable for distinguishing iron deficiency (high RDW) from thalassemia (normal RDW), and for detecting mixed deficiencies that MCV alone would miss.
Red Cell Distribution Width (RDW) measures how much variation exists in the size of your red blood cells. In healthy blood, red cells are fairly uniform in size. When RDW is elevated, it means your cells vary more than normal — some are larger, some smaller — indicating something is affecting red blood cell production.
Why does this matter? RDW is particularly valuable for distinguishing between different types of anemia and detecting mixed nutritional deficiencies. It can reveal iron deficiency earlier than other markers and helps identify when someone has more than one condition affecting their blood. Beyond anemia, elevated RDW has emerged as a marker associated with various health conditions and overall mortality risk.
This measurement is part of every Complete Blood Count (CBC) and adds important information to the picture provided by other red cell indices. When MCV (cell size) appears normal, RDW can reveal underlying problems that would otherwise be missed.
Key Benefits of Testing
RDW helps differentiate between causes of anemia that might otherwise look similar. For example, both iron deficiency and thalassemia cause small red blood cells, but iron deficiency typically elevates RDW while thalassemia usually doesn’t. This distinction guides the right diagnostic path.
This test also detects mixed deficiencies — when someone has both iron deficiency (causing small cells) and B12 deficiency (causing large cells), the MCV might appear normal because the effects cancel out. But RDW will be elevated because there’s a wide range of cell sizes present. This catches problems that MCV alone would miss.
What Does This Test Measure?
RDW quantifies how much red blood cell sizes vary from each other. Think of it as measuring how uniform or diverse your red cell population is. A higher number means more variation; a lower number means cells are more uniform in size.
What Variation Means
Normal RDW: Red blood cells are fairly uniform in size, which is typical when production is steady and stable.
Elevated RDW: Red blood cells vary significantly in size (a condition called anisocytosis). This happens when new cells being produced differ from older cells — often because production conditions are changing (developing deficiency, recovery from deficiency, or multiple conditions present).
Why Cell Sizes Vary
Red blood cells are produced continuously in bone marrow. Under normal conditions, production is consistent and cells are uniform. When something disrupts production — nutritional deficiency, bone marrow problems, or other conditions — the newly produced cells may differ from existing ones, increasing variation.
For example, in early iron deficiency, new cells are smaller than the older, normal-sized cells still circulating. This creates a mixed population with high variation. Once all cells have turned over (about 3-4 months), variation may decrease even though all cells are now uniformly small.
Why This Test Matters
Distinguishes Anemia Types
RDW helps differentiate conditions that cause small cells:
Iron deficiency: Typically causes elevated RDW — cells vary as deficiency develops
Thalassemia trait: Usually has normal RDW — cells are uniformly small due to genetic factors
This distinction matters because treatment differs completely. Iron deficiency needs iron; thalassemia trait typically needs only monitoring.
Detects Mixed Deficiencies
When someone has both iron deficiency and B12/folate deficiency simultaneously, cell size effects may cancel out, making MCV appear normal. But RDW will be high because there’s actually a wide range of sizes — some cells small from iron deficiency, some large from B12/folate deficiency. Elevated RDW with normal MCV is a red flag for mixed deficiency.
Early Detection of Iron Deficiency
RDW often rises before MCV falls in developing iron deficiency. As iron stores deplete, new cells become smaller while older normal cells remain in circulation. This early increase in variation can signal iron deficiency before other markers become clearly abnormal.
Monitors Treatment Response
When treating iron or vitamin deficiency, RDW initially increases as new, healthy cells mix with older abnormal cells. Then it gradually normalizes as the cell population becomes uniform again. This pattern confirms treatment is working.
Broader Health Associations
Research has found that elevated RDW is associated with increased risk of various health outcomes — cardiovascular disease, overall mortality, and other conditions — even in people without anemia. The reasons aren’t fully understood, but RDW may reflect underlying inflammation, oxidative stress, or nutritional status that affects overall health.
What Can Affect Your RDW?
Causes of Elevated RDW
Iron deficiency: The most common cause. Creates size variation as smaller cells are produced while normal cells remain.
B12 or folate deficiency: Creates larger cells, increasing variation when mixed with normal-sized cells.
Mixed deficiencies: Combined iron and B12/folate deficiency creates both small and large cells, significantly elevating RDW.
Recent blood transfusion: Donor cells may differ in size from recipient’s cells.
Recovery from anemia: New healthy cells mixing with remaining abnormal cells increases variation temporarily.
Hemolytic anemia: Increased red cell destruction and compensatory production can create variation.
Myelodysplastic conditions: Bone marrow disorders affecting cell production.
Chronic liver disease: Can affect red cell production and membrane characteristics.
Chronic inflammation: May affect red cell production consistency.
Conditions with Normal RDW
Thalassemia trait: Cells are uniformly small due to genetic factors — low variation.
Chronic disease anemia: Often has normal RDW with normal-sized or slightly small cells.
Acute blood loss: Initially, remaining cells are uniform; RDW rises later as recovery begins.
Testing Considerations
RDW is calculated automatically with every CBC. No fasting or special preparation needed. Recent transfusion affects results. Results should be interpreted alongside MCV and other red cell indices for the complete picture.
When Should You Get Tested?
Anemia Evaluation
When anemia is detected or suspected, RDW helps classify the type and identify underlying causes. It’s particularly useful when MCV is normal or borderline, potentially revealing problems that cell size alone would miss.
Distinguishing Iron Deficiency from Thalassemia
Both conditions cause small cells. If you have low MCV, RDW helps determine whether iron deficiency (high RDW) or thalassemia trait (normal RDW) is more likely — guiding appropriate follow-up testing.
Suspected Mixed Deficiency
If nutritional deficiencies are suspected despite normal MCV, elevated RDW may reveal that multiple deficiencies are present and masking each other.
Monitoring Treatment
When treating iron or vitamin deficiency, RDW changes help confirm response. Initial rise followed by normalization indicates successful treatment.
Comprehensive Health Assessment
Given associations between elevated RDW and various health outcomes, some providers consider it as part of overall health evaluation, though its role in this context is still being researched.
Routine Screening
RDW is included in every standard CBC, providing this information as part of regular health checkups without additional testing.
Understanding Your Results
Your lab provides reference ranges. RDW is most meaningful when interpreted with other CBC values, especially MCV:
Normal RDW: Red blood cells are relatively uniform in size. If anemia is present with normal RDW, consider thalassemia trait, chronic disease, or acute blood loss.
Elevated RDW: Red blood cells vary significantly in size. Suggests iron deficiency, B12/folate deficiency, mixed deficiencies, recovery from anemia, or other conditions affecting production.
Combining RDW with MCV
Low MCV + High RDW: Strongly suggests iron deficiency — small cells with significant variation
Low MCV + Normal RDW: Suggests thalassemia trait or chronic disease — uniformly small cells
High MCV + High RDW: Suggests B12 or folate deficiency — large cells with variation
Normal MCV + High RDW: Red flag for mixed deficiency — opposing effects on size but increased variation
Normal MCV + Normal RDW: If anemia is present, consider blood loss, early chronic disease, or kidney-related causes
Context Is Key
RDW alone doesn’t diagnose specific conditions — it points toward possibilities that other tests confirm. An elevated RDW prompts checking iron studies, B12, folate, and other relevant markers based on the clinical picture.
What to Do About Elevated RDW
Identify the Underlying Cause
Elevated RDW is a clue, not a diagnosis. The next step is determining why variation exists:
Check iron status: Ferritin, serum iron, TIBC — especially if MCV is low or low-normal
Check B12 and folate: Especially if MCV is high or if mixed deficiency is suspected
Review history: Recent transfusion, treatment for anemia, or chronic conditions that might explain findings
Address Nutritional Deficiencies
If iron deficiency is confirmed, improve dietary iron intake and consider supplementation. Address the underlying cause of deficiency.
If B12 or folate deficiency is confirmed, supplement appropriately and identify why deficiency occurred.
Consider Further Evaluation
If nutritional causes are excluded and RDW remains elevated, consider evaluation for chronic disease, bone marrow issues, or other conditions affecting red cell production.
Monitor Over Time
With treatment, RDW may initially rise (as new healthy cells mix with old abnormal ones) then normalize over several months. Persistent elevation despite treatment warrants further investigation.
Related Health Conditions
Iron Deficiency
Most Common Cause of Elevated RDW: As iron depletes, new cells become smaller while older cells remain normal-sized, increasing variation.
Vitamin B12 Deficiency
Causes Large Cells and Elevated RDW: Impaired DNA synthesis creates larger-than-normal cells, increasing size variation.
Thalassemia
Typically Normal RDW: Inherited condition causing uniformly small cells. Normal RDW helps distinguish it from iron deficiency.
Mixed Nutritional Deficiencies
High RDW with Normal MCV: Combined deficiencies create both small and large cells, dramatically increasing variation while size effects cancel out.
Cardiovascular and Overall Health
Emerging Research Area: Elevated RDW is associated with increased cardiovascular risk and mortality even without anemia. Research continues to clarify these associations.
Why Regular Testing RDW Matters
RDW can reveal developing problems early — often before other markers become clearly abnormal. Regular monitoring catches these changes, allowing earlier intervention. For those being treated for nutritional deficiencies, RDW tracks treatment response and confirms when blood cell production has normalized.
As part of routine CBC testing, RDW provides ongoing insight into red cell production without requiring additional testing.
Related Biomarkers Often Tested Together
MCV — Average red cell size. RDW and MCV together classify anemia and identify mixed conditions.
Hemoglobin — Oxygen-carrying protein. Confirms presence of anemia.
Red Blood Cell Count — Number of cells. Part of complete blood picture.
Ferritin — Iron stores. Essential for confirming iron deficiency when RDW is elevated.
Iron and TIBC — Iron status markers supporting iron deficiency diagnosis.
Vitamin B12 and Folate — Check when RDW is elevated with high or normal MCV.
Reticulocyte Count — Young red cells indicating bone marrow response.
Note: Information provided in this article is for educational purposes and doesn’t replace personalized medical advice.
Frequently Asked Questions
RDW measures how much your red blood cells vary in size. A higher number means more variation (some cells larger, some smaller); a normal number means cells are relatively uniform.
The most common cause is iron deficiency. Other causes include B12 or folate deficiency, mixed nutritional deficiencies, recovery from anemia, recent blood transfusion, and various conditions affecting red cell production.
This pattern can indicate mixed deficiency — both iron deficiency (causing small cells) and B12/folate deficiency (causing large cells) present simultaneously. The size effects cancel out, making MCV appear normal, but RDW reveals the underlying problem through increased variation.
Both conditions cause small red cells (low MCV), but iron deficiency typically elevates RDW while thalassemia trait usually has normal RDW. This helps determine which condition is more likely and guides further testing.
Yes — RDW can rise before hemoglobin falls, especially in early iron deficiency. Additionally, elevated RDW has been associated with various health conditions and outcomes even without anemia, though this area is still being researched.
No fasting required. RDW is calculated as part of the routine Complete Blood Count.
When treating nutritional deficiency, RDW often rises initially as new healthy cells mix with remaining abnormal cells. Over several months, as the cell population becomes uniform again, RDW normalizes. This pattern confirms treatment is working.
As part of routine CBC: annually or as recommended. When evaluating anemia: as part of the diagnostic workup. When monitoring treatment: periodically to track response until values normalize.
References
Key Sources:
- Salvagno GL, et al. Red blood cell distribution width: A simple parameter with multiple clinical applications. Crit Rev Clin Lab Sci. 2015;52(2):86-105.
- Buttarello M, Plebani M. Automated blood cell counts: state of the art. Am J Clin Pathol. 2008;130(1):104-116.
- Montagnana M, et al. Red cell distribution width and cardiovascular diseases. Clin Chem Lab Med. 2011;50(4):635-641.
