Is my RDW high?
Yes, if it’s above your lab’s top limit, about 15 to 16%. Low is rarely a worry. But RDW can’t name a cause on its own. One other line on your results points the way.
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Is my RDW high?
Yes, if it’s above the range printed beside it. There’s no single UK range. Each lab sets its own.
For adults, the NHS lab in Gateshead uses 12 to 15%. The Liverpool NHS lab uses 11.5 to 15.7%.
- Below 12 %Below this lab’s range. Rarely a worry.
- 12 up to 15 %In this lab’s adult range.
- Above 15 %Above this lab’s range. Read it with your MCV.
Sources: Gateshead NHS pathology; MedlinePlus
So an RDW of 15.5 is above the range in Gateshead but inside it in Liverpool. Always read yours against your own lab’s range.
A high result means your red blood cells vary in size more than usual. It can’t say why on its own.
Ask your GP what it means next to your other results.
What is an RDW blood test?
RDW stands for red cell distribution width. It shows how much your red blood cells vary in size. Normally, they’re about the same size.
It comes as part of a full blood count, so there’s nothing to prepare. Some labs, such as Liverpool, don’t usually print it on your report.
Think of a class of children. Their average shoe size is your MCV, the average size of your red cells. RDW is how much the sizes differ from child to child.
A class can average size 4 with everyone in a 4, or with half in 2s and half in 6s. Only the spread tells them apart. That’s the job RDW does.
So a normal MCV can still hide a mix of small and large cells. Find your MCV on the same report. You’ll need it next.
Why can two high RDW results mean different things?
Because RDW only says your cells are mixed. It doesn’t say whether they’re mostly too small or too large.
That line is your MCV. In Liverpool, the adult range is 80 to 100 fL, a unit for the size of one cell.
| Your MCV | With a high RDW, it can fit | Tests that often follow |
|---|---|---|
| Low (small cells) | Low iron, where RDW is often raised. | Iron tests, such as ferritin (how much iron your body has stored). |
| High (large cells) | Low vitamin B12. A rising RDW is often the first change in the blood count. | Vitamin B12 and folate. |
| In range | A mix of small and large cells that averages out. | Depends on your other results. |
Sources: AAFP; Parakh 2026; Takami 2026; NHS
Small cells with an RDW in range? It can fit an inherited condition called thalassaemia trait. But RDW can’t reliably tell it apart from low iron.
A high RDW can also follow a blood transfusion, or show cells recovering after treatment.
MedlinePlus says it may be a sign of long-term liver disease, heart or kidney disease, diabetes or some cancers, especially bowel cancer. Your doctor will usually read it with your other blood results.
Have your MCV? Read the MCV blood test guide next.
Is a low RDW bad?
Usually not. It means your red cells are much the same size. MedlinePlus says it’s not a sign of anaemia (too few healthy red blood cells) and isn’t usually something to worry about.
A low or normal RDW doesn’t rule anaemia out, though. The Gateshead lab notes that some causes of anaemia come with a normal RDW.
Low haemoglobin on the same report? See the anaemia blood test guide.
What happens after a high RDW?
What often happens after a high RDW
- 1
Read it with MCV
Your doctor looks at RDW next to your other blood count results.
- 2
Check the cause
Iron tests, or vitamin B12 and folate, depending on cell size.
- 3
Find the reason
If iron is low, the GP may look for why, such as heavy periods or bleeding.
- 4
Treat and retest
If iron is low, iron tablets are usually taken for about 6 months. The GP may repeat blood tests.
Sources: MedlinePlus; AAFP; NHS
Prescribed iron or B12? Never stop or change a prescribed medicine without your prescriber.
Read that a high RDW is linked to dying sooner? In a US study of 8,175 adults aged 45 or over, higher RDW was linked to a higher risk of death.
A link isn’t proof. Researchers haven’t shown whether RDW is a cause or just a sign of something else. The tests above look for what’s behind your result.
Before your GP appointment
- Bring your full blood count, including your MCV and haemoglobin, not just your RDW.
- Note any tiredness, shortness of breath, pounding heartbeat, paler skin or headaches.
- Mention heavy periods, pregnancy or any bleeding.
- Say if you’re vegan and don’t take B12 supplements or eat foods with added B12.
- List your medicines, such as stomach acid tablets (PPIs) or medicines for seizures.
- Tell them about any recent blood transfusion.
Sources: NHS: iron deficiency anaemia; NHS: B12 or folate deficiency; Takami 2026
More on the follow-up tests: iron studies, vitamin B12 and folate.
When to speak to a GP
Book an appointment if:
- You have signs of low iron, such as tiredness, shortness of breath, noticeable heartbeats, paler skin or headaches. The NHS says to see a GP.
- You have signs of low B12 or folate, such as extreme tiredness, pins and needles, a sore red tongue, mouth ulcers, muscle weakness, vision problems, or changes in mood or memory.
- You’re vegan, don’t take B12 supplements or eat foods with added B12, and feel very tired.
- Your RDW is flagged and you have questions about it. Your doctor will usually read it with your other results.
Get low B12 checked soon. The NHS says it should be treated as soon as possible, as some problems can be permanent.