Glossary term
RDW (Red Cell Distribution Width)
Also known as: RDW, red cell distribution width, RDW-CV, RDW-SD
Part of: Full Blood Count (FBC)
- Reading time
- 3 min read
- Last reviewed
- Last reviewed January 1970
- Reading level
- Written in plain English
- Reviewed by
- Reviewed by Medipher Editorial Team
A red cell index, reported with a full blood count, describing how much your red blood cells vary in size.
Medipher explains medical wording in plain English — it does not diagnose or replace a clinician. If this is a medical emergency, call your local emergency number immediately.
What is this?
Red blood cells are not all identical in size. RDW — red cell distribution width — is a calculation of how widely their sizes vary in your sample. Lab Tests Online UK groups it with MCV, MCH and MCHC as the red cell indices, which describe the volume, size and haemoglobin content of red cells.
If cells are unusually mixed in size, the RDW rises. If they are fairly uniform, it stays low — whether those cells are small, average or large.
It is calculated automatically by the analyser as part of a full blood count, so it appears on the report whether or not anyone was specifically interested in it.
Two forms exist. RDW-CV is reported as a percentage and is the one most UK reports show; RDW-SD is reported in femtolitres (fL). Check which one your report gives before comparing numbers.
Why is it important?
Together, the red cell indices help in the diagnosis and monitoring of conditions that affect red cell size, shape and haemoglobin content — for example iron or B12 deficiency, anaemia and thalassaemia.
RDW is most useful next to MCV, the average size of your red cells. For example, a low MCV with a raised RDW is a different picture from a low MCV with a normal RDW, and clinicians use that combination as one pointer among several.
Because cell size can change before overall haemoglobin does, a rising RDW is sometimes an early signal that something is affecting red cell production. On its own it does not identify what.
Where you'll see it: Full blood count (FBC) reports, usually next to MCV, MCH and haemoglobin.
How doctors use this result
Clinicians may use this result to:
- Read it alongside haemoglobin, MCV and the rest of the full blood count.
- Compare it with your previous full blood counts to see whether it is changing.
- Use it as one clue among several when deciding whether further tests are worthwhile.
Does an abnormal result always mean something is wrong?
Not necessarily. A number that sits outside the printed range is a prompt for interpretation — not a diagnosis.
- A single result rarely gives the full picture — clinicians interpret it alongside your symptoms.
- Your medical history, medications and other blood tests all shape the meaning of one value.
- Small movements just outside the reference range are common and often not concerning.
- Repeat testing over time is frequently more useful than any single number.
What might happen next?
- Check whether your report gives RDW-CV (%) or RDW-SD (fL) before comparing it with anything.
- Look at RDW next to haemoglobin and MCV on the same report.
- Ask the requesting service whether the pattern across the full blood count needs following up.
- Mention symptoms such as unusual tiredness or breathlessness rather than focusing on one number.
One result rarely tells the whole story. Clinicians look at your symptoms, medical history, medications and other tests together — and a single value that sits slightly outside the reference range is often normal for you. Repeat testing over time is frequently more useful than any one number. Medipher helps you understand what a result means in plain English, but it cannot diagnose medical conditions or replace advice from a doctor, nurse or pharmacist.
When should I speak to a healthcare professional?
Speak with your GP or another healthcare professional if:
- You've been asked to repeat the test.
- You're experiencing new or worsening symptoms.
- Your clinician has flagged the result as needing follow-up.
- You don't understand your results or feel unsure what to do next.
If you have severe symptoms — chest pain, severe breathlessness, fainting, heavy bleeding or feel very unwell — seek urgent help. If this is a medical emergency, call your local emergency number immediately. Medipher is designed to help explain medical information and is not an emergency service.
Frequently asked questions
Related blood tests
Terms clinicians often look at alongside RDW (Red Cell Distribution Width).
- MCVThe average size of your red blood cells.
- HaemoglobinThe protein inside red blood cells that carries oxygen from your lungs to the rest of your body.
- Red Blood CellsThe cells in your blood that carry oxygen. They contain haemoglobin, which does the actual oxygen carrying.
- AnaemiaA condition where you don't have enough healthy red blood cells or haemoglobin to carry oxygen around your body.
Related guides
- Full Blood Count (FBC)What a full blood count measures — haemoglobin, white cells, platelets and MCV — and what unusual results may lead to.
- How to read a reportTest name, result, units, reference range, flags, comments and status — the layout of a blood test report explained with a clearly labelled example.
Still unsure what your result means?
This page explains one medical term. If you have a blood test, GP letter or hospital report, Medipher can explain your entire document in plain English.
- what your results mean
- unfamiliar medical terminology
- what your clinician may be monitoring
- useful questions to ask at your next appointment
Sources
General patient information referenced when writing this page.
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About this page
- Author
- Omer Suliman
- Medical reviewer
- Clinical review pending
- Last reviewed
- January 1970
- Last updated
- January 1970
Evidence. Written from public patient-information sources including the NHS, NICE and the Association for Laboratory Medicine, and reviewed for plain-English accuracy before publication.
Educational content only. Medipher explains medical language in plain English. It does not diagnose, and it is not a substitute for advice from a doctor, nurse or pharmacist. Read the medical disclaimer.
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This glossary entry is general health information, not medical advice. Medipher explains medical wording in plain English and does not diagnose, provide treatment advice, or replace a GP, doctor, nurse or pharmacist. Always discuss your results with a qualified clinician.