What Does RDW Mean in a Blood Test? Causes of High and Low RDW Explained

If you've recently received your blood test results, you may have noticed a measurement called RDW and wondered what it means. Whether your RDW result is marked as high, low or outside the normal range, it's natural to have questions — especially if you've searched online and found conflicting information.

The good news is that RDW is not a diagnosis. Instead, it's a valuable clue that helps doctors understand the health of your red blood cells and investigate conditions such as iron deficiency anaemia, vitamin B12 deficiency, folate deficiency and other disorders affecting blood cell production.

RDW is measured as part of a Full Blood Count (FBC), one of the most commonly requested private blood tests in the UK. While many people focus on their haemoglobin level, RDW provides additional insight into how much your red blood cells vary in size. If you'd like the wider context first, our guide to decoding your blood test results is a good place to start.

What Does RDW Mean in a Blood Test?

RDW stands for Red Cell Distribution Width. It measures how much your red blood cells vary in size.

Healthy red blood cells are usually similar in size, allowing them to carry oxygen efficiently throughout the body. When there's a greater difference between the smallest and largest red blood cells, your RDW increases.

• Normal RDW means most red blood cells are a similar size.
• High RDW indicates greater variation in red blood cell size.
• Low RDW indicates very little variation, which is usually not a cause for concern.

On its own, RDW cannot diagnose a condition. Interpreted alongside other blood test markers — particularly MCV (Mean Corpuscular Volume) — it helps doctors identify the likely cause of abnormal blood results.

Why Is RDW Important?

Doctors use RDW to help diagnose different types of anaemia, distinguish between iron deficiency and inherited blood disorders, detect vitamin B12 or folate deficiency, investigate chronic inflammatory conditions, assess bone marrow function, and monitor recovery after treatment for anaemia. In many cases, changes in RDW appear before other blood test results become abnormal, making it a useful early indicator.

How Is RDW Measured?

RDW is calculated automatically when your blood sample is analysed during an FBC — the lab measures thousands of red blood cells and determines how much they differ in size. No special preparation is needed: you don't need to fast, the draw takes only a few minutes, and results are usually available within a day or two. Your GP may request an FBC if you have symptoms such as fatigue, weakness, dizziness or shortness of breath, or as part of a routine health check.

What Is a Normal RDW Range?

Most UK laboratories consider a normal RDW to be between 11.5% and 14.5%, although reference ranges may vary slightly.

A normal RDW doesn't always mean your blood test is completely normal, and a mildly raised RDW doesn't necessarily indicate a serious condition. Doctors always interpret RDW alongside haemoglobin (Hb), MCV, MCH, MCHC, red blood cell (RBC) count, ferritin, vitamin B12 and folate.

What Does a High RDW Mean?

A high RDW means there is more variation in the size of your red blood cells than expected — some cells much smaller than average, others larger. This mixed population often suggests your body is responding to an underlying condition affecting red blood cell production. Common symptoms associated with the underlying causes include persistent tiredness, fatigue, weakness, breathlessness, dizziness, pale skin, difficulty concentrating and reduced exercise tolerance — usually from the anaemia or deficiency itself, not the raised RDW.

Iron Deficiency Anaemia

Iron deficiency is one of the most common causes of a high RDW. As iron stores deplete, the body produces new red blood cells smaller than the older ones, creating a mixture of cell sizes. Iron deficiency may develop due to heavy menstrual bleeding, pregnancy, poor dietary iron intake, stomach ulcers, coeliac disease, inflammatory bowel disease, or gastrointestinal bleeding. Iron studies and ferritin testing are usually performed to confirm the diagnosis.

Vitamin B12 Deficiency

Vitamin B12 is essential for producing healthy red blood cells. Without enough B12, the bone marrow produces unusually large red blood cells, increasing the variation in cell size and raising the RDW. Alongside fatigue, people may experience tingling or numbness in the hands and feet, poor balance, memory problems, mood changes or a sore tongue. Prompt diagnosis matters, since prolonged deficiency can lead to nerve damage. We offer vitamin B12 injections where clinically appropriate, and our guide on vitamin B12 deficiency: symptoms, food sources and treatment covers this further.

Folate Deficiency

Folate works alongside B12 to support healthy red blood cell production. Low folate may occur due to a poor diet — including a strict vegan diet without adequate supplementation — pregnancy, malabsorption conditions such as coeliac disease, excessive alcohol intake, or certain medications. Like B12 deficiency, it often produces larger red blood cells and a raised RDW.

Mixed Nutritional Deficiencies

Some people have more than one deficiency at once — low iron and B12, for example — creating a mixture of unusually small and unusually large red blood cells that raises RDW significantly even when the average cell size (MCV) looks normal. Recognising this pattern helps doctors look beyond a single deficiency.

What Does a Low RDW Mean?

A low RDW means very little variation in red blood cell size — your cells are all a similar size. Unlike a high RDW, this is rarely clinically significant on its own. If your RDW is low but the rest of your FBC is normal, no further investigation is usually needed, though your GP will still weigh it against your other results and symptoms.

Can You Have a High RDW Without Anaemia?

Yes. A high RDW can appear before anaemia develops, making it a useful early indicator. Your RDW may rise with early iron deficiency, early B12 deficiency, folate deficiency, chronic inflammation, liver disease, or recovery after blood loss or anaemia treatment — which is why your GP may order further tests even if your haemoglobin is still within range.

RDW vs MCV: What's the Difference?

MCV tells you the average size of your red blood cells; RDW tells you how much those cells differ from one another. Together: low MCV + high RDW often suggests iron deficiency anaemia; high MCV + high RDW may indicate B12 or folate deficiency; normal MCV + high RDW can point to an early deficiency or a combination of conditions. This is why doctors rarely interpret one without the other — our companion piece on what MCV means in a blood test covers MCV in full.

What Other Blood Tests Help Interpret RDW?

RDW is most useful alongside haemoglobin (anaemia and severity), MCV (average cell size), ferritin (a high RDW with low ferritin strongly suggests iron deficiency), vitamin B12 and folate, red blood cell count, MCH/MCHC, and sometimes a reticulocyte count to check whether the bone marrow is responding appropriately.

What Can Affect Your RDW Result?

Iron, B12 or folate deficiency, chronic inflammatory disease, liver disease, recent blood transfusions, bone marrow disorders, and recovery after anaemia treatment can all shift RDW. It reflects a pattern, not a diagnosis — identifying the cause often needs additional blood tests and, occasionally, further investigation.

When Should You Speak to Your GP?

Seek advice if you have persistent fatigue or weakness, breathlessness, dizziness, unexplained weight loss, heavy or prolonged menstrual bleeding, blood in your stools (worth reading alongside our piece on bowel cancer risk in younger people if this applies to you), tingling or numbness in your hands or feet, a family history of inherited blood disorders, or repeated abnormal FBC results. You can book a GP appointment or, if it's easier, arrange a home visit.

How Is an Abnormal RDW Treated?

Dr Simon's Advice

"Many patients become understandably anxious when they see an abnormal RDW result highlighted on their blood test report. But RDW is an investigative tool, not a diagnosis. A raised RDW often points to common and treatable conditions — particularly iron, B12, or folate deficiency. Equally, a low RDW is usually of little clinical significance. As a GP, I never assess RDW in isolation — I look at your symptoms, medical history, examination findings and the rest of your FBC before deciding whether further investigation is needed. If your RDW is outside range, don't ignore it, but don't panic either."

Key Takeaways

• RDW (Red Cell Distribution Width) measures how much your red blood cells vary in size.
• It's routinely measured as part of a Full Blood Count (FBC).
• A high RDW often suggests increased cell-size variation and may be linked to iron, B12 or folate deficiency, or chronic illness.
• A low RDW is uncommon and usually not clinically significant.
• RDW is most useful when interpreted alongside MCV, haemoglobin, ferritin, B12 and folate.
• An abnormal RDW is a sign further investigation may be needed, not a diagnosis in itself.
• Many causes of a raised RDW are common, treatable, and improve once the underlying condition is addressed.

Getting Tested

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