Prepared by: Omar Adwan, DHA-Licensed Medical Laboratory Technologist | Founder, MedLab Academy
Last Updated: September 17, 2026
RDW, or red cell distribution width, measures how much red blood cell size varies within a blood sample. A high RDW means there is greater variation in red blood cell size, a finding known as anisocytosis. A low RDW generally means the red cells are relatively uniform in size and, by itself, is usually not considered clinically concerning.
The most important point is that an RDW result should never be interpreted in isolation. Its real value comes from combining it with hemoglobin, MCV, RBC count, MCH, reticulocyte count, iron studies and, when indicated, the peripheral blood smear. A high RDW with low MCV suggests a different laboratory pattern from a high RDW with high MCV or a high RDW with normal MCV.
This guide explains what a high RDW means, what a low RDW means, the normal RDW range, RDW-CV vs RDW-SD, RDW and MCV patterns, iron deficiency, thalassemia, vitamin B12 or folate deficiency, recent blood transfusion, reticulocytosis, laboratory limitations and when an abnormal RDW deserves further investigation.
Quick laboratory principle: RDW tells you about variation in red-cell size. MCV tells you the average red-cell size. Looking at both together is much more useful than interpreting either value alone.
1. Quick Answer: What Does RDW Mean?
RDW stands for Red Cell Distribution Width. It is an RBC index included in many complete blood count reports and reflects how different the circulating red blood cells are from one another in size.
| RDW Result | Basic Meaning | Laboratory Interpretation |
|---|---|---|
| Normal RDW | Red cells are relatively similar in size. | Does not exclude anemia. MCV, hemoglobin and other findings still matter. |
| High RDW | Greater variation in RBC size. | May occur with iron deficiency, B12/folate deficiency, mixed anemia, reticulocytosis, transfusion or other disorders. |
| Low RDW | Red cells are very similar in size. | Usually has little diagnostic importance when other CBC results are normal. |
For a broader explanation of all CBC components, see the Complete Blood Count (CBC) laboratory guide.
2. What Is RDW in a Blood Test?
A circulating red blood cell population is never composed of cells that are perfectly identical in size. Some variation is normal. RDW quantifies the degree of this variation.
The laboratory term for unequal red-cell size is anisocytosis. When anisocytosis becomes more pronounced, RDW usually increases.
RDW is particularly useful because MCV is only an average. Two patients may both have an MCV of 88 fL, yet one may have a very uniform red-cell population while the other has a mixture of unusually small and unusually large cells. Their MCV values can look identical even though the underlying red-cell populations are very different.
Laboratory perspective: When an unexpectedly high RDW appears on a CBC, the useful question is not simply “Which disease causes high RDW?” The better questions are: What is the hemoglobin? What is the MCV? Is the RBC count relatively high or low? Are reticulocytes increased? Does the analyzer histogram look abnormal? Is there a recent transfusion? Does the smear show microcytes, macro-ovalocytes, polychromasia or a dimorphic population?
This is why RDW is most useful as part of a pattern rather than as a stand-alone diagnostic test.
3. How Is RDW Measured?
Modern automated hematology analyzers count and characterize large numbers of red blood cells and generate an RBC volume distribution or histogram. RDW summarizes the degree of variation within that distribution.
Depending on the analyzer, RDW may be reported as RDW-CV, RDW-SD, or both.
RDW-CV is related mathematically to the spread of red-cell volumes and the MCV. A commonly described expression is:
RDW-CV (%) = Standard deviation of RBC volume ÷ MCV × 100
RDW-SD, in contrast, represents an absolute measurement of the width of the RBC size-distribution curve and is reported in femtoliters.
Analyzer methodology, software algorithms and validated laboratory reference intervals can differ. For this reason, the reference interval printed on the patient's own laboratory report should always take priority over a generic internet range.
4. RDW-CV vs RDW-SD: What Is the Difference?
| Feature | RDW-CV | RDW-SD |
|---|---|---|
| Unit | % | fL |
| Meaning | Relative variation in RBC size | Absolute width of RBC size distribution |
| Relationship to MCV | Influenced by MCV because MCV is included in the calculation | More directly reflects the width of the RBC distribution curve |
| Common report format | Examples may appear around 12%, 14% or 18% | Examples may appear around 40 fL, 45 fL or 52 fL |
| Clinical use | Commonly used with MCV for anemia pattern recognition | Useful additional measure of red-cell size heterogeneity |
A frequent source of confusion is comparing an RDW-SD result in fL with an RDW-CV reference interval expressed as a percentage. These are not interchangeable values.
Important: If your report shows RDW as a percentage, it is usually RDW-CV. If the result is reported in femtoliters (fL), it is generally RDW-SD. Always use the matching reference interval supplied by the laboratory.
5. What Is the Normal RDW Range?
There is no single universal RDW reference interval for every analyzer, laboratory and patient population.
A commonly encountered adult RDW-CV reference interval is approximately 11.5% to 15%, although narrower or slightly different ranges may be used by individual laboratories.
RDW-SD intervals are analyzer-dependent and commonly fall somewhere around the low-40-fL range, but a numerical RDW-SD value should never be classified as high or low without checking the reference interval printed by the performing laboratory.
Reference ranges may vary because of:
- Analyzer manufacturer and methodology.
- Laboratory validation procedures.
- Reference population.
- Age or clinical population.
- Differences in specimen handling and analytical systems.
Best practice: Interpret the patient's value against the laboratory-specific reference interval, not an isolated range copied from another laboratory.
6. What Does High RDW Mean?
A high RDW means that the circulating red blood cells show greater-than-expected variation in size.
This may develop when the bone marrow is producing a new population of cells that differs in size from older circulating cells, when more than one anemia process is present, or when donor cells and patient cells coexist after transfusion.
Common situations associated with increased RDW include:
- Iron deficiency and iron-deficiency anemia.
- Vitamin B12 deficiency.
- Folate deficiency.
- Mixed iron and vitamin deficiency.
- Recent blood loss with reticulocyte response.
- Hemolytic processes with reticulocytosis.
- Recent blood transfusion.
- Recovery after treatment of nutritional anemia.
- Some chronic inflammatory or systemic illnesses.
- Selected bone marrow disorders.
However, a high RDW is not a diagnosis. It does not tell you by itself whether the patient has iron deficiency, B12 deficiency, thalassemia or another disease.
If the hemoglobin is reduced, see the MedLab Academy guide to low hemoglobin and CBC interpretation.
7. What Does Low RDW Mean?
A low RDW means the red blood cells are relatively similar in size.
Unlike a high RDW, an isolated low RDW is generally not considered an important disease marker. A person can have anemia even when RDW is normal or low because all of the circulating cells may be similarly small or similarly large.
For example, a patient can have microcytosis with relatively uniform red cells. Therefore:
Low or normal RDW does not rule out anemia, thalassemia or another red-cell disorder. Hemoglobin, MCV, RBC count, clinical history and additional testing remain essential.
8. RDW and MCV Interpretation Chart
This is one of the most useful ways to interpret RDW in routine hematology.
| MCV | RDW | Possible Pattern | Useful Next Clues |
|---|---|---|---|
| Low | High | Common in iron deficiency; mixed microcytic populations may also produce this pattern. | Ferritin, iron, TIBC/transferrin, TSAT, RBC count, smear. |
| Low | Normal | May occur in thalassemia trait or other relatively uniform microcytic states. | RBC count, ferritin, iron studies, hemoglobin analysis when appropriate. |
| Normal | High | Early nutrient deficiency, mixed microcytic and macrocytic populations, transfusion, reticulocytosis or evolving anemia. | Smear, ferritin, B12/folate assessment, reticulocyte count, history. |
| Normal | Normal | Does not exclude anemia. May be seen in some normocytic disorders. | Hemoglobin, reticulocytes, renal function, inflammatory context and clinical findings. |
| High | High | May occur with B12 or folate deficiency, mixed anemia, reticulocytosis or some marrow disorders. | B12/folate-related testing, reticulocytes, smear, liver/thyroid context when appropriate. |
| High | Normal | Relatively uniform macrocytosis may occur in selected non-megaloblastic states. | Clinical history, liver testing, medication review, thyroid assessment and smear. |
This table is a pattern-recognition aid, not a diagnostic algorithm. Clinical history and confirmatory testing remain necessary.
For a full framework of anemia classification, see Anemia: Laboratory Diagnosis and CBC Interpretation.
9. Common RDW Laboratory Patterns
High RDW + Low Hemoglobin
This combination means anemia is present together with increased red-cell size variability. Nutritional deficiency is an important consideration, but the pattern is not specific.
The MCV helps refine the next step:
- Low MCV suggests a microcytic pattern.
- High MCV suggests a macrocytic pattern.
- Normal MCV with high RDW may indicate an early or mixed process.
High RDW + Normal Hemoglobin
An elevated RDW can occasionally appear before overt anemia develops. The result should not automatically be labeled as disease, but it may justify review of previous CBC results, MCV, ferritin or nutritional status when clinical circumstances support investigation.
High RDW + Normal MCV
This is one of the most useful and easily overlooked patterns.
MCV is an average. A mixture of small microcytic cells and large macrocytic cells can produce an apparently normal average MCV while RDW becomes markedly elevated.
A peripheral smear can be particularly useful in this situation because the microscope may reveal two red-cell populations that the average MCV hides.
Normal RDW + Low MCV
This pattern may raise consideration of thalassemia trait, particularly when the RBC count is relatively preserved or increased compared with the degree of microcytosis. However, RDW cannot diagnose or exclude thalassemia, and iron deficiency must still be assessed appropriately.
10. RDW in Iron Deficiency and Iron-Deficiency Anemia
Iron deficiency is one of the most important clinical contexts in which RDW is used.
During evolving iron deficiency, newly produced red cells may become progressively smaller while older circulating cells retain their previous size. This creates a wider distribution of red-cell volumes and can increase RDW.
A typical laboratory pattern in established iron-deficiency anemia may include:
- Low hemoglobin.
- Low MCV.
- Low MCH.
- Elevated RDW.
- Low ferritin.
- Reduced serum iron.
- Reduced transferrin saturation.
- Often increased TIBC or transferrin.
- Microcytic, hypochromic cells on peripheral smear.
Iron status should be assessed from the full pattern. See the detailed Iron Studies Blood Test guide.
Practical laboratory point: RDW may become abnormal while the patient is moving from a relatively uniform RBC population toward a more clearly microcytic population. That is why reviewing previous CBCs can sometimes provide more information than looking at one result in isolation.
Ferritin also requires context because it can increase during inflammation. For additional interpretation, see High Ferritin Levels: Causes and Laboratory Interpretation.
11. RDW in Thalassemia: Can RDW Tell the Difference?
One of the most common searches around RDW is whether it can distinguish iron-deficiency anemia from thalassemia trait.
The simplified teaching pattern is:
| Feature | Iron Deficiency | Thalassemia Trait |
|---|---|---|
| MCV | Often low | Often low |
| RDW | Often increased | May remain normal or be less increased |
| RBC count | May be reduced or not disproportionately increased | Often relatively preserved or increased despite microcytosis |
| Ferritin | Often low when uncomplicated iron deficiency is present | Usually not low solely because of thalassemia trait |
| Confirmation | Iron studies and investigation of the cause of iron deficiency | Hemoglobin analysis and/or genetic assessment when clinically indicated |
Important limitation: RDW alone cannot reliably separate iron deficiency from thalassemia. The two conditions can coexist, and RDW values overlap. Never diagnose thalassemia or prescribe iron based solely on RDW.
Thalassemia assessment requires the complete hematological pattern, family and ethnic background, iron status and appropriate confirmatory testing.
12. RDW in Vitamin B12 and Folate Deficiency
Vitamin B12 and folate deficiencies may produce ineffective erythropoiesis and macrocytosis. When macrocytes coexist with older cells of more typical size, RDW may increase.
A megaloblastic pattern may include:
- Low hemoglobin.
- High MCV.
- High RDW.
- Macro-ovalocytes.
- Hypersegmented neutrophils.
- Possible leukopenia or thrombocytopenia in more significant deficiency.
- Low reticulocyte production before effective treatment.
- Possible increases in LDH and indirect bilirubin due to ineffective erythropoiesis.
Not every patient with high MCV has vitamin B12 or folate deficiency. Other causes of macrocytosis include liver disease, selected medications, alcohol exposure, hypothyroidism, reticulocytosis and bone marrow disorders.
Why Can MCV Be Normal in Mixed Deficiency?
Consider a patient who has both iron deficiency and vitamin B12 deficiency. Iron deficiency pushes red-cell size downward while B12 deficiency pushes red-cell size upward.
The average MCV can therefore appear deceptively normal.
RDW may be markedly increased because the circulating blood contains both smaller and larger cells.
This is a classic example of why normal MCV does not always mean normal red-cell morphology.
13. Blood Transfusion, Reticulocytes and RDW
RDW After Blood Transfusion
After red-cell transfusion, donor erythrocytes and the patient's erythrocytes may differ in size. The resulting mixed population can increase RDW and may alter interpretation of MCV and the RBC histogram.
Therefore, recent transfusion history should be reviewed before making strong conclusions from RDW.
RDW and Reticulocytosis
Reticulocytes are generally larger than mature erythrocytes. When the bone marrow produces an increased reticulocyte response after blood loss or hemolysis, the larger young cells can contribute to increased red-cell size variation and may influence MCV and RDW.
In suspected hemolysis, interpretation may include hemoglobin, reticulocyte count, LDH, bilirubin, haptoglobin and peripheral-smear findings.
For pre-analytical and biochemical issues related to hemolyzed specimens, see Hemolysis in Clinical Chemistry.
14. Specimen, Analyzer and Laboratory Considerations
RDW is generated from an automated hematology analysis, which means result interpretation begins with confidence in specimen quality and instrument performance.
Specimen Requirements
CBC testing is generally performed using properly collected EDTA-anticoagulated whole blood according to the laboratory's validated method and analyzer manufacturer instructions.
Pre-Analytical Problems to Review
- Clotted or partially clotted specimen.
- Incorrect patient identification.
- Underfilled or unsuitable collection tube when relevant to laboratory policy.
- Poor sample mixing.
- Prolonged delay before analysis.
- Improper sample storage.
- Extreme specimen conditions that alter cellular morphology.
Analytical Issues
Unexpected results should be evaluated alongside analyzer flags, RBC histogram characteristics, quality-control status, calibration or verification status where applicable and the laboratory's standard operating procedure.
Instrument-specific algorithms can produce differences in RDW-CV and RDW-SD. This is another reason that serial results are best compared when methodology and laboratory conditions are understood.
Peripheral Blood Smear Correlation
The smear is especially valuable when numerical CBC findings do not tell the whole story.
Microscopy can reveal:
- Microcytosis.
- Macrocytosis.
- Anisocytosis.
- Poikilocytosis.
- Dimorphic red-cell populations.
- Macro-ovalocytes.
- Schistocytes.
- Spherocytes.
- Polychromasia.
- Nucleated red blood cells.
For a practical microscopy review, see Peripheral Blood Smear and RBC Morphology.
15. When Should High RDW Be Investigated?
An isolated mildly increased RDW is not automatically an emergency. Clinical significance depends on the rest of the laboratory profile and the patient's symptoms.
Further evaluation becomes more important when increased RDW occurs together with findings such as:
- Low hemoglobin or falling hemoglobin.
- Markedly low or high MCV.
- Abnormal WBC or platelet counts.
- Significant fatigue, weakness, dizziness or shortness of breath.
- Evidence of bleeding.
- Jaundice or suspected hemolysis.
- Neurological symptoms potentially compatible with vitamin B12 deficiency.
- Significant abnormal blood-smear findings.
- Progressive change compared with previous CBC results.
Do not treat the RDW number itself. Treatment should address the underlying cause. Starting iron, vitamin B12 or another therapy solely because RDW is elevated can be inappropriate if deficiency has not been established.
16. Practical Laboratory Cases
Case 1: High RDW + Low MCV
A patient has:
- Hemoglobin: decreased
- MCV: 69 fL
- RDW-CV: 19.0%
- Ferritin: low
- Transferrin saturation: low
Interpretation: The combination of microcytosis, elevated RDW and biochemical evidence of reduced iron stores strongly supports iron-deficiency anemia in the appropriate clinical context.
The laboratory result still does not explain why iron deficiency developed. Menstrual loss, gastrointestinal blood loss, dietary deficiency, malabsorption and other causes may require clinical investigation.
Case 2: Low MCV + Relatively Normal RDW
A patient has:
- Hemoglobin: mildly reduced
- MCV: 64 fL
- RDW: within the laboratory interval
- RBC count: relatively high
- Ferritin: not reduced
Interpretation: The pattern can raise suspicion for a thalassemia trait or another inherited microcytic process, but RDW does not establish the diagnosis. Appropriate hemoglobin testing and clinical assessment are required.
Case 3: Normal MCV + Markedly High RDW
A patient has:
- Hemoglobin: reduced
- MCV: 88 fL
- RDW: markedly increased
- Ferritin: reduced
- Vitamin B12: reduced
- Smear: both microcytes and macro-ovalocytes
Interpretation: The apparently normal MCV is misleading. Small cells and large cells average into the normal range while RDW and microscopy reveal a mixed population.
Human laboratory insight: This is the type of CBC where stopping at the MCV can send interpretation in the wrong direction. When the average looks normal but RDW is high, reviewing the histogram and blood film can reveal a mixed population that the single MCV number hides.
Case 4: High RDW After Treatment
A patient with nutritional anemia begins appropriate treatment. Newer erythrocytes entering circulation may differ in size from the older abnormal population.
Interpretation: RDW can remain increased temporarily during hematological recovery. Serial trends, hemoglobin response, reticulocyte response and the underlying diagnosis are more informative than one isolated RDW measurement.
17. What Tests May Be Checked Next?
Additional investigation depends on the CBC pattern, symptoms and clinical history. Possible tests may include:
| Test | Why It May Help |
|---|---|
| Ferritin | Helps assess iron stores, with interpretation adjusted for inflammatory context. |
| Serum iron, TIBC/transferrin and TSAT | Provide a broader view of iron availability and transport. |
| Reticulocyte count | Evaluates bone-marrow erythropoietic response. |
| Peripheral blood smear | Directly assesses RBC size, shape and mixed populations. |
| Vitamin B12 / folate-related assessment | Useful when macrocytosis or megaloblastic features are suspected. |
| LDH, bilirubin and haptoglobin | May help investigate suspected hemolysis. |
| Hemoglobin analysis | May help investigate thalassemia or other hemoglobin disorders when clinically appropriate. |
| Renal, liver or thyroid-related testing | May be appropriate depending on the anemia pattern and clinical picture. |
When abnormalities involve more than one blood-cell line, evaluation should be broader. See the MedLab Academy guide to pancytopenia and laboratory investigation.
18. Frequently Asked Questions About RDW
What does RDW mean in a blood test?
RDW means red cell distribution width. It measures how much the size of circulating red blood cells varies.
What does high RDW mean?
High RDW means there is increased variation in red blood cell size. It may occur with iron deficiency, vitamin B12 or folate deficiency, mixed anemia, reticulocytosis, recent transfusion and several other conditions. It does not identify the cause by itself.
Is high RDW dangerous?
RDW itself is not a disease and an elevated result is not automatically dangerous. Its significance depends on hemoglobin, MCV, other CBC parameters, symptoms and the underlying cause.
What does high RDW with low MCV mean?
This pattern is commonly seen in iron deficiency, especially when hemoglobin and ferritin are also low. Other causes of microcytosis remain possible and should be assessed from the complete laboratory picture.
What does high RDW with normal MCV mean?
It can occur when differently sized red-cell populations average into a normal MCV. Possible explanations include early deficiency, mixed iron and B12/folate deficiency, recent transfusion or reticulocytosis.
What does high RDW with high MCV mean?
This may occur with vitamin B12 or folate deficiency, reticulocytosis, mixed disorders and selected marrow or systemic conditions. MCV and RDW alone cannot determine the diagnosis.
Does high RDW mean iron deficiency?
No. Iron deficiency commonly increases RDW, but many other conditions can also produce a high RDW. Ferritin and iron studies are more direct tests of iron status.
Can RDW distinguish iron deficiency from thalassemia?
RDW can contribute to pattern recognition but cannot reliably diagnose the difference by itself. RBC count, ferritin, full iron studies and appropriate hemoglobin testing are more important.
What does low RDW mean?
A low RDW means red cells are relatively uniform in size. An isolated low RDW is usually not clinically significant.
Can you have anemia with a normal RDW?
Yes. Some anemias produce a relatively uniform population of abnormal red cells, so RDW may remain within the laboratory reference interval.
Can RDW be high when hemoglobin is normal?
Yes. Increased red-cell size variation may be present before overt anemia or in situations where hemoglobin remains within range. The result should be interpreted with the full CBC and clinical context.
Does RDW change after a blood transfusion?
It can. Donor red cells and the patient's own red cells may differ in size, creating a mixed population and increasing RDW.
Is RDW-CV the same as RDW-SD?
No. Both describe red-cell size variation, but RDW-CV is generally reported as a percentage while RDW-SD is reported in femtoliters. They use different approaches and reference intervals.
Should I take iron if my RDW is high?
Not solely because RDW is high. Iron supplementation should be based on appropriate clinical and laboratory evidence of iron deficiency and professional medical guidance.
19. Related MedLab Academy Guides
Continue learning with these verified MedLab Academy resources:
- Complete Blood Count (CBC): Normal Ranges and Laboratory Interpretation
- Anemia: Classification, Laboratory Diagnosis and CBC Interpretation
- Low Hemoglobin Levels: CBC, MCV, MCH and RDW Interpretation
- Iron Studies: Serum Iron, TIBC, Transferrin Saturation and Ferritin
- High Ferritin Levels: Causes and Laboratory Interpretation
- Peripheral Blood Smear: RBC Morphology and Interpretation
- Thrombocytosis: High Platelet Count and Laboratory Approach
- Thrombocytopenia: Low Platelet Count and Smear Findings
- Pancytopenia: Laboratory Approach to Low RBCs, WBCs and Platelets
- ESR: Erythrocyte Sedimentation Rate Laboratory Guide
- High CRP Levels: C-Reactive Protein Interpretation
- Hemolysis in Clinical Chemistry: Detection, Interference and Prevention
20. Authoritative References
This article was developed using authoritative medical and laboratory resources. Reference intervals and diagnostic decisions should always follow the performing laboratory's validated procedures and the patient's clinical context.
- MedlinePlus — RDW (Red Cell Distribution Width)
- MedlinePlus — Red Blood Cell (RBC) Indices
- Cleveland Clinic — RDW Blood Test
- NCBI Bookshelf — Normal and Abnormal Complete Blood Count With Differential
- PubMed — Red Blood Cell Distribution Width: A Simple Parameter With Multiple Clinical Applications
- NHLBI, NIH — Iron-Deficiency Anemia
- NHLBI, NIH — Thalassemia
Medical and Educational Disclaimer: This article is intended for medical laboratory education and general information. It does not replace patient-specific medical assessment, diagnosis, treatment, institutional laboratory procedures, manufacturer instructions or professional clinical judgment. Reference intervals differ between laboratories and analytical systems.
Take-Home Message: RDW is most valuable when it is treated as part of a pattern rather than as an isolated number. High RDW indicates increased variation in red-cell size, but the combination of RDW + MCV + hemoglobin + RBC count + reticulocytes + iron status + blood-smear morphology is what turns the result into clinically useful laboratory information.