Red Cell Distribution Width (RDW-CV): a guide

What it is

RDW (red cell distribution width) is a blood test marker that shows the variation in red blood cell size. More specifically, RDW-CV measures how much the volume of red blood cells differs from one another, and the result is expressed as a percentage. The higher the value, the more varied the red blood cell sizes — this is known as anisocytosis.

This marker is automatically calculated as part of a complete blood count (CBC) and is particularly useful for differentiating types of anaemia. A population of normally sized red blood cells will have a low RDW value, whereas a mixed population — where some red blood cells are too large and others too small — will result in an elevated RDW.

Reference ranges

The reference range for RDW-CV in adults is typically 11.5–14.5 %. This interval may vary slightly depending on the laboratory and the analyser used. A value above 14.5 % indicates increased variation in red blood cell size, while a value below 11.5 % is rare and generally has little clinical significance.

Why it may be elevated

An elevated RDW is a fairly common finding and can have various causes. Iron deficiency anaemia — one of the most common — causes the body to produce smaller red blood cells, which circulate alongside older, normally sized cells, increasing the difference in size.

Vitamin B12 or folate deficiency causes megaloblastic anaemia, in which some red blood cells become unusually large. Mixed anaemias, where both iron and vitamins are deficient simultaneously, raise RDW particularly strongly.

Chronic conditions such as liver disease, chronic kidney disease, or inflammatory processes can also elevate this marker. After a blood transfusion, RDW often rises temporarily because the donor's red blood cells may differ from those of the recipient. Alcohol consumption can directly affect red blood cell maturation and raise RDW.

Why it may be low

A low RDW generally indicates that red blood cells are very uniform in size, and this is usually not clinically significant. In some situations, a very low RDW may occur with certain haemoglobinopathies — for example, in thalassaemia trait, where red blood cells are uniformly small.

It is important to understand that a low RDW is rarely the primary diagnostic criterion — a clinician will more usefully evaluate it alongside other markers, particularly MCV and haemoglobin.

What to do next

If your RDW result falls outside the reference range, this is a signal to consult a doctor, but it is not a diagnosis in itself. The doctor will first evaluate this marker together with other red cell indices — MCV, MCH, MCHC — and haemoglobin and haematocrit values.

Additional tests may include iron metabolism markers, vitamin B12 and folate concentrations in the blood, a reticulocyte count, and a peripheral blood smear. These tests will help determine whether the variation in red blood cell size is caused by a specific nutritional deficiency or another underlying cause.

Review your diet — make sure you are consuming enough foods rich in iron, vitamin B12, and folate. Avoid excessive alcohol consumption, as this can directly affect red blood cell production.

Monitoring over time

RDW is a particularly informative marker when followed over time. For example, if you have started taking iron supplements, RDW may initially even increase (because the body begins producing new, normally sized red blood cells that circulate alongside older, smaller ones), and should then normalise over the following weeks or months.

Regular blood tests allow your doctor to assess the effectiveness of treatment and adjust the plan accordingly. Record the date and result of each test, or use a health tracking app — this will help you see trends and discuss them at your next appointment.

Questions for your doctor

If your RDW-CV value falls outside the normal range, you may wish to discuss the following with your doctor:

• Could my elevated RDW be related to a nutritional deficiency?
• What additional tests would you recommend to determine the cause?
• Should I change my diet or lifestyle?
• How often should I repeat the test to see whether the marker is improving?
• Could any medications I am taking have affected this result?