High RDW on a Blood Test: What It Means
Medically Reviewed by Dr. Pedro Yelamo Vera, MD
Licensed Physician No. 484814280 · Official Medical College of Bizkaia (Spain)Last reviewed: 31 October 2026
A high RDW means your red blood cells vary in size. What the normal range is, how RDW and MCV work together, and when a raised result needs a check.
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Dr. Pedro Yelamo Vera is an independent, bilingual GP licensed in Spain. Book a 15-minute video consultation in English or Spanish for €50, with a private electronic prescription issued under Spanish law when clinically appropriate.
A high RDW means your red blood cells vary more in size than usual. On its own it is not a diagnosis. A common reason is iron deficiency, but low B12 or folate, a recent bleed and some long-term illnesses can raise it too. Your doctor reads it with your haemoglobin and MCV.
- RDW stands for red cell distribution width. It measures how much your red cells differ in size.
- Each lab sets its own range. A Spanish paediatric guide gives under 15% as usual.
- High RDW with small cells (low MCV) often points to iron deficiency.
- Small cells with a normal RDW suggest thalassaemia trait instead.
- A low RDW is not a sign of anaemia and is not usually a worry.
- A mildly high RDW with a normal blood count often needs only a repeat test.
Dr. Pedro Yelamo Vera is a GP licensed in Spain (Colegiado No. 484814280).
For a medical emergency, call 112 or go to the nearest local emergency department. Do not wait for a video consultation or an online reply.
What is RDW on a blood test?
RDW is part of a full blood count, which is called a CBC in the US and a hemograma in Spain. MedlinePlus explains that it measures how much the size and volume of your red blood cells vary. When most cells are a similar size, the RDW is low. When sizes are spread out, it is high. Doctors call this mix of sizes anisocytosis.
On a Spanish report, look for ADE (amplitud de distribución eritrocitaria) or simply RDW. Some reports also show RDW-SD, which MedlinePlus lists as another name for the test. The result is usually a percentage. Spanish reports use a comma for decimals, so 14,8 % means 14.8%.
What is a normal RDW range?
There is no single normal range. MedlinePlus says each lab compares your result with its own reference range. Always read the range printed next to your result.
As a guide, Spanish paediatric guidance (AEPap) says RDW is normally under 15%. A star, an "H" or bold type on your report means your value is above that lab's range.
What causes a high RDW?
A high RDW means new red cells are being made at a different size from the older ones. The usual reasons are:
- Iron deficiency. AEPap notes that iron deficiency anaemia comes with a raised RDW.
- Low vitamin B12 or folate. These make large cells, and AEPap notes the RDW is raised here too.
- Mixed causes. Low iron plus low B12 can give a normal MCV with a high RDW.
- Recovery after a bleed or after treatment. Fresh cells join older ones of a different size.
- Long-term illness. MedlinePlus links a high RDW with chronic liver disease, heart disease, diabetes, kidney disease and some cancers, especially colorectal cancer.
The last point sounds worrying. A review in Critical Reviews in Clinical Laboratory Sciences found that a raised RDW is common in many conditions. It is a clue, not a diagnosis. On its own it does not tell anyone which illness, if any, is behind it.
How do RDW and MCV work together?
MCV is the average size of your red cells. Reading RDW and MCV together narrows down the cause. MedlinePlus gives a usual MCV range of about 79 to 95 femtolitres (fL).
- Low MCV, high RDW: small, uneven cells. Iron deficiency is the classic cause.
- Low MCV, normal RDW: small, even cells. AEPap notes the RDW stays normal in thalassaemia, an inherited trait.
- High MCV, high RDW: large cells. Think of low B12 or folate. MedlinePlus also lists liver disease and chemotherapy.
- Normal MCV, high RDW: early iron or B12 deficiency, or two causes at once.
Thalassaemia trait and iron deficiency can look alike. A ferritin test helps tell them apart. It matters, because iron tablets do not help thalassaemia trait.
Got a high RDW and not sure what it means?
You can book a video visit with a GP licensed in Spain, in English or Spanish. He reads your full blood count, not just one line, and explains whether anything needs a further check.
Is a high RDW serious?
Usually not on its own. A slightly high RDW with normal haemoglobin, MCV and ferritin often needs only a repeat test some weeks later. What matters is the rest of your report and how you feel.
The NHS lists tiredness, shortness of breath, a pounding heartbeat, pale skin and headaches as signs of iron deficiency anaemia. It names heavy periods and pregnancy as very common causes. Bleeding from the stomach or bowel is another, for example from ulcers, anti-inflammatory painkillers or, less often, cancer. That is why a doctor may suggest a stool test if your iron is low and there is no clear reason.
What does a low RDW mean?
A low RDW means your red cells are very similar in size. MedlinePlus says this is not a sign of anaemia and is not usually something to worry about.
Is RDW reported the same way in Spain, the UK and the US?
The test is the same, and so is its meaning. The names and ranges differ a little.
- Spain: it appears in the hemograma as ADE or RDW, in percent, with a comma for decimals. Ferritin is ferritina.
- UK: it appears in the full blood count (FBC) as RDW. Haemoglobin is usually given in g/L.
- US: it appears in the CBC as RDW or RDW-CV, and sometimes RDW-SD. Haemoglobin is usually given in g/dL.
Because labs use different machines, compare a result only with the range from the same lab. If you have tests from two countries, bring both reports to your doctor.
What happens next?
- Only the RDW is a little high: often a repeat blood count, sometimes with ferritin.
- High RDW and low MCV or low haemoglobin: iron studies, and a search for the cause of iron loss.
- High RDW and high MCV: B12, folate, liver and thyroid tests.
- Small cells with a normal RDW and normal ferritin: a test for thalassaemia trait.
See a doctor soon if you have blood in your stool, black stools, very heavy periods or weight loss you cannot explain. Call 112 if you vomit blood, have black stools with dizziness or fainting, or have chest pain or severe shortness of breath.
Talking it through with a doctor by video
Dr. Yelamo is a GP licensed in Spain. He sees adults, children and pregnant patients by video, every day from 07:30 to 20:30 (Spain time). In the visit he will:
- Read your full blood count, including RDW, MCV and haemoglobin.
- Ask about periods, pregnancy, diet, medicines and any bleeding.
- Compare older results, even from another country.
- Explain whether you need ferritin, B12, folate or other tests, or a specialist.
He does not promise any test or prescription before the visit. Where clinically appropriate, following a genuine evaluation. Treatment and prescriptions are issued solely at the doctor's discretion based on evidence-based clinical guidelines.
A prescription is never guaranteed: it is issued only where clinically appropriate after a genuine evaluation, as a private electronic prescription under Spanish law. This service is for urgent, non-emergency care. In an emergency, call 112.
See how the Urgent Care video visit works. It lasts 15 minutes and costs €50.
This guide is general information. It does not replace a consultation with your doctor.
Frequently asked questions
What does a high RDW mean?
Your red blood cells vary more in size than usual. Iron deficiency is a common cause, but the doctor reads it with your haemoglobin and MCV.
What is a normal RDW range?
Each lab sets its own. Spanish paediatric guidance gives under 15% as usual. Use the range printed on your report.
Is a high RDW a sign of cancer?
Not on its own. A raised RDW is common in many conditions. It is a clue that the doctor reads with the rest of your results.
Should I worry about a low RDW?
Usually not. A low RDW is not a sign of anaemia and rarely needs any action.
What is ADE on a Spanish blood test?
ADE is the Spanish name for RDW: amplitud de distribución eritrocitaria. It is shown in the hemograma as a percentage.
Can a high RDW go back to normal?
Often, yes. Once the cause, such as low iron, is treated, new red cells are more even and the RDW tends to settle.
Sources
- MedlinePlus: Red cell distribution width (RDW)
- MedlinePlus: RBC indices
- Salvagno et al., Crit Rev Clin Lab Sci: Red blood cell distribution width
- NHS: Iron deficiency anaemia
- AEPap: interpreting the blood count (in Spanish)
Related guides
Medical Citations & Sources
This content is written and reviewed by a licensed physician with reference to the following official health authorities.
