What is RDW?
Your bone marrow makes millions of red blood cells every second, and if it has enough iron, vitamin B12 and folate, they all leave the factory in about the same size. RDW, the red cell distribution width, expresses how much the cells in your blood deviate from each other in size. A low value means they are all about the same size; a high value that small and large cells are mixed together. That makes RDW an early signal. A red blood cell lasts about four months, so if the bone marrow starts making cells of a different size because of a deficiency, the new, deviant cells swim among the old, normal ones for months. The spread then rises while the average, the MCV, hardly moves yet and haemoglobin is still normal. The same happens in reverse during recovery: when normal cells come back among the small ones after an iron course, RDW peaks first before it falls. Your report prints RDW as a percentage, and sometimes as an absolute width in fL. Normal is roughly 12.3 to 14 % for men and 12.4 to 14 % for women, where the labs overlap. A low RDW is naturally low and the healthy outcome; there is no such thing as a deficiency of uniformity.
Why is RDW relevant?
RDW is the number in your blood count that moves first when something changes in the production of red blood cells. With a beginning iron deficiency, through heavy periods, little iron from food or a lot of endurance sport, the spread rises before haemoglobin falls. Anyone who has their blood count done yearly therefore often sees a deficiency in the RDW half a year before the anaemia itself. The second reason is that RDW helps tell causes apart. Small cells with a high RDW is iron deficiency; small cells with a normal RDW fits thalassaemia carriership, where all cells are equally small. Large cells with a high RDW is a deficiency of B12 or folate; large cells with a normal RDW fits alcohol or an underactive thyroid. And a normal MCV with a high RDW is often a mixed deficiency, where small and large cells cancel each other out in the average. The honest limitation is that RDW says something is changing, not what. The number has no cause of its own: it follows the deficiencies and their recovery, and a blood transfusion lifts it because cells from someone else are added. So never read it alone, but as the clue that says which of the other values to look at.
RDW reference ranges
What counts beside the number:
- Your MCV, because that says whether the deviant cells are small or large
- Your haemoglobin, because that says whether there is already anaemia or whether RDW is the early signal
- Your ferritin with small cells, because iron deficiency is the most common cause of a high RDW
- Your vitamin B12 and folate with large cells
- Whether you recently had a bleed, a transfusion or an iron course, because recovery lifts RDW first
- Your previous result, because a rising RDW with an otherwise normal blood count is the earliest signal there is
RDW is usually reported as a percentage, the RDW-CV; some labs also print the absolute width in fL, and those two are not interchangeable. The upper limit differs per lab and per counter, so compare with the band on your own report. There is no lower limit: a low RDW is the healthy outcome, and an RDW that comes out above the band with an otherwise normal blood count is the earliest signal of a deficiency there is.
What Dutch labs actually use
Certe is the only one of the three that splits RDW by sex, and gives women a higher upper limit than men: 15.1 against 14.3. Star-shl keeps it narrowest with 12 to 14, and Clinical Diagnostics publishes no band for RDW at all.
12.4 – 14%
Between 11.8 and 12.4 and between 14 and 15.1 it depends on the lab, because each sets its limits on its own method and population. Above 15.1 every lab calls it abnormal.
every lab calls 12.4 – 14 % normal. Between 11.8 and 12.4 and between 14 and 15.1 it depends on the lab.
Every age, per lab
| Laboratory | Group | Reference range |
|---|---|---|
| Unilabs(Saltro, Medlon, SHO, Atalmedial)2026 | Adults | 11.8–14.3 % |
| Star-shl2026 | Adults | 12–14 % |
| Certe2026 | Men from 18 yrs | 12.3–14.3 % |
| Women from 18 yrs | 12.4–15.1 % |
Each band as the lab publishes it, retrieved in 2026, unless marked otherwise.
RDW high or low: what it means
A low RDW is naturally low and the healthy outcome: your bone marrow makes cells of the same size, and that means it has enough iron, B12 and folate. There is no lower limit and no such thing as a deficiency of uniformity; a low number never needs explaining. A high RDW means cells of different sizes are mixed together. The most common explanation first: iron deficiency, where new small cells swim among the old normal ones, often the first sign before haemoglobin falls; a deficiency of vitamin B12 or folate, where large cells are added instead; recovery after bleeding or a treatment, as normal cells come back among the deviant ones; a blood transfusion, which adds cells from someone else; and a mixed deficiency of iron and B12 or folate at once, where MCV looks normal and RDW gives it away. What you do about it: look at MCV to know which way it goes. If the cells are small, have ferritin measured and replenish an iron deficiency with iron-rich food, such as red meat, legumes and green vegetables with some vitamin C alongside, or with tablets. If the cells are large, have B12 and folate measured, eat more vegetables and legumes and less alcohol, and replenish if needed. Count on three to four months before RDW falls again, because that is how long it takes for the old cells to be replaced; in the first weeks of recovery it even rises first. Read RDW beside MCV, which says whether the deviant cells are small or large, beside haemoglobin, which says whether there is already anaemia, beside ferritin with small cells and beside vitamin B12 and folate with large cells, and beside your previous result, because an RDW that rises while the rest is still normal is the earliest signal there is.
What lowers it
The healthy outcome: all cells the same size.
| Cause | How often |
|---|---|
| Naturally lowYour bone marrow has enough iron, B12 and folate and makes cells of the same size; there is no lower limit and no deficiency of uniformity | Often |
What raises it
Cells of different sizes mixed together, usually through a deficiency that makes the bone marrow produce a different size.
| Cause | How often |
|---|---|
| Iron deficiencyNew small cells swim among the old normal ones; often the first sign, even before haemoglobin falls, and ferritin confirms it | Often |
| Vitamin B12 or folate deficiencyThe bone marrow makes large cells instead, and those mix with the old normal ones; MCV rises with it | Often |
| Recovery after bleeding or treatmentNormal cells come back among the deviant ones, and the spread peaks first before falling over three to four months | Sometimes |
| Blood transfusionCells from someone else mix with yours, and the spread rises temporarily without there being a deficiency | Sometimes |
| Mixed deficiencyIron deficiency and a deficiency of B12 or folate at once: small and large cells cancel each other out in MCV, and only RDW gives it away | Sometimes |
How does a RDW blood test work?
- Referral
- Not needed. RDW is in every blood count, which you can have measured yourself.
- Fasting
- Not needed. RDW does not change with what you eat.
- When
- On an ordinary day, and not in the first weeks after a bleed, a transfusion or the start of an iron course, because then RDW peaks through the recovery.
- The draw
- One tube of blood from a vein in your arm, the same tube as the whole blood count. Request ferritin with it if you suspect it is about iron.
- Repeating
- Once a year at an ordinary check, and three to four months after replenishing a deficiency, because that is how long it takes for the old cells to be replaced. Mind the unit: percentage or fL.
- Which test
- RDW is in the Iron & fatigue test (€99) and in .
- Where to draw
- You can draw at 250+ locations near you, no referral needed. See the locations
Frequently asked questions
What is RDW?
A measure of how much your red blood cells differ from each other in size. Healthy cells all leave the bone marrow in about the same size, and then RDW is low. If the bone marrow starts making smaller or larger cells because of a deficiency, those mix with the old ones and the spread rises. It is in every blood count.
What is a normal RDW value?
Roughly 12.3 to 14 % for men and 12.4 to 14 % for women, where the labs overlap; the exact upper limit differs per lab. A low value is healthy and never needs explaining. Mind the unit, because some labs print an absolute width in fL beside the percentage.
What does a high RDW value mean?
That small and large red blood cells are mixed together, usually because your bone marrow has started making cells of a different size because of a deficiency. Iron deficiency is the most common cause, often before haemoglobin falls; a deficiency of B12 or folate the second. Recovery after bleeding or an iron course, a blood transfusion or a mixed deficiency lift it too. MCV says which way it goes, and ferritin or B12 and folate confirm it.
What does a low RDW value mean?
Nothing: it means your red blood cells are all about the same size, and that is exactly how a healthy bone marrow makes them. There is no lower limit and no such thing as a deficiency of uniformity.
What do I do about a high RDW?
First look at MCV. If the cells are small, have ferritin measured and replenish an iron deficiency with iron-rich food, such as red meat, legumes and green vegetables with some vitamin C alongside, or with tablets. If the cells are large, have B12 and folate measured, eat more vegetables and legumes, drink less alcohol and replenish if needed. Count on three to four months before RDW falls, and do not be alarmed if it rises first in the early weeks: that is the recovery.
Why is my RDW rising when I have just started an iron course?
Because the recovery itself lifts the spread. The bone marrow makes normal cells again, and in the first weeks those swim among the small cells from before the course. RDW therefore peaks first and only falls once the old cells have been replaced after three to four months. A rising RDW just after the start is therefore a good sign.
When is an RDW value worrying?
On its own never; RDW says something is changing, not what. It only becomes a reason to look further when haemoglobin falls too, or when RDW keeps rising without ferritin, B12 and folate showing a deficiency. Then a full blood count with a look at the cells belongs with it.
Do I need to fast?
No. RDW does not change with what you eat. What does count is whether you recently had a bleed, a transfusion or the start of an iron course, because then RDW peaks through the recovery.
Which tests include RDW?
RDW is in the Iron & fatigue test (€99, also as an add-on for €15), in the Biological age test (€69, also as an add-on for €15), in the comprehensive test (€229) and in .
Related biomarkers
Read on
Sources
- 1.NHG guideline on anaemia (M76), Dutch College of General Practitioners. 2024. richtlijnen.nhg.org
- 2.Unilabs, test catalogue (Dutch): RDW. 2026. bepalingenklapper.nl
- 3.Star-shl, test catalogue (Dutch): RDW. 2026. star-shl.nl
- 4.Certe, test catalogue (Dutch): RDW. 2026. bepalingenwijzer.certe.nl
Educational information only, not medical advice. Consult a healthcare professional for clinical decisions.
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