RDW blood test
Red cell distribution width
In short
A typical adult RDW result is 12.2 to 16.1 % for women (RDW-CV) and 11.8 to 14.5 % for men (RDW-CV). A high RDW is common and, on its own, rarely a concern. It is most useful when read with MCV, the average cell size, and hemoglobin. Always read your result against the range printed on your own report.
How much your red blood cells vary in size.
RDW shows how much the red blood cells differ in size from one another. A high RDW means a mix of small and large cells. It is often the first result to change when iron, vitamin B12 or folate run low.
Is my RDW high or low?
Type your result to see where it sits.
Nothing you type leaves this device.
Typical range
| For | Range | Unit |
|---|---|---|
| Female · RDW-CV | 12.2–16.1 | % |
| Male · RDW-CV | 11.8–14.5 | % |
| Adults · RDW-SD | 39–46 | fL |
Typical ranges for adults. Labs differ a little, so the range printed next to your result is the one to go by.
If your RDW is high
A high RDW is common and, on its own, rarely a concern. It is most useful when read with MCV, the average cell size, and hemoglobin.
Common reasons
- Getting older, as RDW tends to creep up with age
- Low iron, often before hemoglobin starts to drop
- Low vitamin B12 or folate
- Recent bleeding or a recent blood transfusion
- Recovery from anemia, as new cells mix with older ones
- A liver condition or long-term kidney disease
- Less often: an inherited red cell condition or a bone marrow problem
If your RDW is low
A low RDW means the red cells are very even in size. It is not known to be a sign of illness, and doctors do not usually act on it.
Common reasons
- Natural variation between people
- Differences between lab machines and methods
- A lab range built from a different group of people
What can throw it off
A recent blood transfusion or recent bleeding can raise RDW for several weeks.
Questions to take to your appointment
- 1What do my MCV and hemoglobin show alongside it?
- 2Should my iron, vitamin B12 or folate be checked?
- 3Is this worth repeating in a few months?
Read together with
Doctors rarely read RDW alone. These usually sit next to it.
- MCVThe average size of your red blood cells.
- HemoglobinThe protein in red blood cells that carries oxygen.
- FerritinThe body's iron-storage protein; its level mirrors your iron reserves.
- Vitamin B12A vitamin needed for healthy nerves and red blood cells.
- FolateA B vitamin the body needs to make red blood cells.
Quick answers
What is the RDW blood test?
RDW (Red cell distribution width): How much your red blood cells vary in size. RDW shows how much the red blood cells differ in size from one another. A high RDW means a mix of small and large cells. It is often the first result to change when iron, vitamin B12 or folate run low.
What is a normal RDW level?
For most adults, RDW is typically 12.2 to 16.1 % for women (RDW-CV) and 11.8 to 14.5 % for men (RDW-CV). Some countries print it in fL, so check the unit first. Labs differ a little, so the range printed next to your result is the one to go by.
What does a high RDW mean?
A high RDW is common and, on its own, rarely a concern. It is most useful when read with MCV, the average cell size, and hemoglobin. Common reasons include getting older, as RDW tends to creep up with age; low iron, often before hemoglobin starts to drop; low vitamin B12 or folate; and recent bleeding or a recent blood transfusion. A doctor reads it together with your other results and how you feel.
What does a low RDW mean?
A low RDW means the red cells are very even in size. It is not known to be a sign of illness, and doctors do not usually act on it. Common reasons include natural variation between people; differences between lab machines and methods; and a lab range built from a different group of people.
What can affect my RDW result?
A recent blood transfusion or recent bleeding can raise RDW for several weeks.
What should I ask my doctor about my RDW?
Questions worth taking to the appointment: What do my MCV and hemoglobin show alongside it? Should my iron, vitamin B12 or folate be checked? Is this worth repeating in a few months?
What this page can’t do
It reads each number on its own.
Doctors read results together — ALT with AST, ferritin with hemoglobin, TSH with free T4. A pattern says more than any single flag.
It doesn't know you.
Your age, your medicines, how you feel and your last result all change what a number means.
It doesn't remember.
Close this page and your results are gone. That's on purpose: nothing was ever sent to us.
We also make Vita: Health Coach, an app for those parts. It reads your whole report with your medicines and history in mind, answers follow-up questions in plain words, and keeps your reports in one place — for you and for the people you look after.
Vita: Health Coach on the App StoreSources
- Mayo Clinic Laboratories — CBC with differential, reference values
- Medscape — Red cell distribution width (RDW) test
Stuck on a word? See the medical words. How we choose our sources. More about Vita Health Coach.