| Marker | Gabriel optimal | Typical lab range | Functional range |
|---|---|---|---|
| RDW | 11 to 12.6 % | 11.6 to 15.4 | 13 or lower |
What RDW measures
Healthy red cells are close to the same size. RDW describes how wide that spread is. A wider spread is called anisocytosis.
RDW is read together with MCV, the average cell size. MCV tells you whether cells are small, normal or large on average; RDW tells you whether they are all alike or mixed. MCH and MCHC add how much hemoglobin the cells carry.
RDW-CV vs RDW-SD
Many reports show two RDW numbers. They measure the same idea in different ways and are not interchangeable:
| RDW-CV | RDW-SD | |
|---|---|---|
| Unit | Percent (%) | Femtoliters (fL) |
| How it is calculated | The spread of cell sizes divided by the average size (MCV) | The width of the cell size curve itself |
| Affected by MCV? | Yes. Small average cells push it up, large ones pull it down | No, it does not depend on MCV |
| Typical lab range | About 12% to 15% | About 38 to 52 fL |
Gabriel's optimal range applies to RDW-CV in percent only. If your report shows only RDW-SD in fL, compare it with your lab's own range.
What a high RDW can mean
A high RDW means a mix of cell sizes. Common causes:
- Iron deficiency. RDW is often high in iron deficiency and usually normal in anemia of chronic disease, which helps tell the two apart. See ferritin levels.
- Low vitamin B12 or folate.
- More than one cause at once. Low iron (small cells) plus low B12 (large cells) can leave MCV looking normal while RDW is high.
- Red cells breaking down early (hemolysis) or recent blood loss, when many young cells enter the blood.
- Long-term conditions, including kidney or liver disease, heart disease, thalassemia, and inflammatory conditions such as Crohn's disease.
RDW can stay high for a while after a deficiency is treated, because new healthy cells mix with older ones. In large studies of older adults, a higher RDW is also linked with higher risk of death from any cause, about 14% higher for each 1% rise. That is a population link, not a diagnosis, but it is one reason Gabriel tracks RDW over time.
What to do with a high result
- Read it with MCV. High RDW with low MCV most often means low iron; high RDW with high MCV points toward B12 or folate; high RDW with normal MCV can mean a mix.
- Ask about ferritin, an iron panel, B12 and folate. A reticulocyte count helps if your clinician suspects blood loss or red cell breakdown.
- Trend it in one lab. RDW varies somewhat between analyzers.
- Recheck after treatment. Give it about three months, since red cells live about 120 days.
See a clinician promptly if a high RDW comes with shortness of breath, chest pain, a racing or irregular heartbeat, fainting, black or bloody stools, yellowing of the skin or eyes, or numbness and tingling in your hands or feet.
Common questions
What is a normal RDW?
Most labs use roughly 12% to 15% for RDW-CV; ranges vary by lab. Gabriel's optimal range is narrower, 11% to 12.6%.
Is a high RDW serious?
It usually points to something common and fixable, like low iron, B12 or folate. Because it is also linked to heart, kidney and liver conditions, an unexplained high RDW is worth following up.
What does a low RDW mean?
A low RDW means your red cells are very uniform in size. On its own it is not a sign of disease or anemia.
Can RDW be high without anemia?
Yes. RDW can be high while hemoglobin is still in range, so ask about ferritin, B12 and folate even if your hemoglobin is normal.
What is the difference between RDW-CV and RDW-SD?
RDW-CV is a percentage that depends on your average cell size (MCV). RDW-SD is in femtoliters and does not. Compare each one only with its own reference range.
Sources
- RDW Blood Test Cleveland Clinic
- Evaluation of Microcytosis (Van Vranken, 2010) American Family Physician
- Normocytic Anemia (Brill and Baumgardner, 2000) American Family Physician
- RDW-SD but not RDW-CV as a potential index for the diagnosis of iron-deficiency anemia in mid-pregnancy women (2021) Open Life Sciences
- Red Cell Distribution Width and Mortality in Older Adults: A Meta-analysis (Patel et al., 2010) The Journals of Gerontology: Series A
- Blood MedlinePlus, U.S. National Library of Medicine
This guide is general information, not medical advice. It does not replace a visit with a licensed clinician. Gabriel membership is not health insurance.