Red Cell Distribution Width (RDW)
RDW is a measure of how much your red blood cells vary in size — whether they are all much the same, or come in a wider mix of sizes. A higher result means a wider mix, which can be an early clue to a shortage of iron, vitamin B12 or folate. It is always read alongside your haemoglobin and the rest of your blood count, never on its own.
Standard CareWhat is red cell distribution width (rdw)?
Red Cell Distribution Width (RDW) is a measure of the variation in size among your red blood cells. All red cells are not identical; RDW captures the spread. When the spread is small — cells are similar in size — RDW is lower. When the spread is large — cells range from small to large — RDW is higher. Red blood cells (also called erythrocytes) carry oxygen from your lungs to every tissue in your body, using a protein called haemoglobin. For them to do this job well, they need to be the right size and shape. When the body struggles to produce red cells normally — because of a nutrient shortage, a bone marrow problem, or another condition — cells of uneven sizes appear in the bloodstream, widening the distribution. RDW is expressed in femtolitres (fL) or sometimes as a percentage, and is reported automatically on a full blood count (FBC). It is a supporting index: it adds detail when read alongside haemoglobin, MCV (mean cell volume — the average cell size), and other red cell indices.
Why is it tested?
RDW is part of the full blood count and is used to investigate anaemia and vitamin deficiencies, as well as bone marrow disorders and liver disease. It is most useful as an early or mixed-deficiency clue. For example, in pure iron deficiency anaemia, MCV drops (small cells) and RDW rises (a mix of normal-old and new-small cells). In mixed deficiency — iron and B12 together — MCV may appear deceptively normal because small cells and large cells average out, yet RDW rises because the two populations are very different in size. In that situation, RDW can flag the problem that MCV alone would miss. RDW is not ordered in isolation. It is interpreted alongside haemoglobin, MCV, MCH, ferritin, and vitamin B12 or folate as your GP narrows down the cause of any abnormality.
Reference range (Australia)
Reference intervals for RDW are laboratory- and analyser-specific. Different analysers and reagent sets produce slightly different intervals, so the range printed on your pathology report is the correct one to use for your result. Do not compare your result against a number found online.
Reference intervals vary between laboratories. Always use the range printed on your own report.
What your result means
A low RDW means red cell sizes are very uniform. This is generally a normal finding in the context of an otherwise unremarkable full blood count. It is rarely the focus of clinical concern on its own. Your GP will interpret any low RDW in the context of your other results.
An RDW within the range on your report means red cell sizes are reasonably consistent. This is generally reassuring, though a normal RDW does not by itself rule out anaemia; it is read together with your haemoglobin and the other indices. If haemoglobin and other indices are also normal, no follow-up for RDW alone is typically needed. Your GP will read the full blood count together when advising you.
A raised RDW means there is more variation in red cell size than expected. This is often seen in iron deficiency anaemia, vitamin B12 or folate deficiency, or a mix of both. It can also appear in recovery after treatment for anaemia (as newer, healthier cells enter the bloodstream alongside older ones) and in some chronic conditions. A raised RDW on its own is not a diagnosis — it is a prompt to look further. Your GP will look at haemoglobin, MCV, and nutrient levels alongside your symptoms to work out what is driving the variation.
What can affect your result
Nutritional deficiencies are the most common driver of a raised RDW. To make red blood cells, your body needs nutrients such as vitamin B12, folate, and iron; shortfalls in any of these can disrupt normal, even cell production and widen the size spread. Recent treatment for iron deficiency or B12 deficiency can temporarily raise RDW as new healthy cells enter the circulation alongside older abnormal ones. Chronic disease — including conditions affecting the liver, kidneys, bone marrow, or causing sustained inflammation — can also affect red cell production and size uniformity. Blood transfusion can raise RDW by introducing donor cells of different sizes into the bloodstream. Haemolytic conditions, where red cells are destroyed faster than normal, can also cause a mixed population of cell sizes to appear. Your GP will weigh up medications, recent illness, and your broader medical history when interpreting the result.
When to act
If your RDW is above the range on your report and haemoglobin is also low, speak with your GP. They will typically look at MCV, ferritin, vitamin B12, and folate to find the underlying cause. If RDW is raised but haemoglobin is normal and you have no symptoms, it is still worth mentioning at your next visit — particularly if you have risk factors for nutrient deficiency such as a restricted diet, malabsorption, or heavy menstrual periods. A trend across two or three results is usually more informative than a single reading, and RDW can take weeks to months to normalise even after the underlying cause is addressed. This page is general information and is not a substitute for advice from your GP.
Frequently asked questions
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