RDW blood test: what a high RDW means

RDW is one of the red cell indices that are measured by an automated analyser together with a full blood count. It shows how much the size of red blood cells (erythrocytes) differs within one and the same blood sample — the bigger the difference, the higher the RDW. In a healthy person red blood cells are usually of similar volume, so RDW is low. When the bone marrow starts producing red blood cells of unequal size for one reason or another — for example, some of them smaller because of iron deficiency while others are still of normal size — RDW rises.

According to MedlinePlus, an RDW result on its own is not enough to establish a diagnosis: it is always assessed together with MCV (mean corpuscular volume), MCH and MCHC, because different forms of anaemia affect red blood cells differently. For example, red blood cells that are smaller than usual are associated with iron deficiency anaemia or thalassaemia, and larger ones with anaemia from a deficiency of B group vitamins (B12, folic acid) or with liver disease. RDW most often appears in a full blood count report automatically, even if nobody asked for it separately.

• When a full blood count is done as part of a routine health check — RDW is included automatically
• When signs of anaemia are felt: breathlessness, weakness or tiredness, headache, dizziness
• When a heart rhythm disturbance (arrhythmia), pale skin or cold hands and feet is noticed
• When the other red cell indices (MCV, MCH, MCHC) are abnormal and the type of anaemia needs to be clarified
• To follow the course of treatment or of a condition (for example iron or vitamin B12 deficiency) over time

A raised RDW shows that red blood cells differ in size (anisocytosis). According to MedlinePlus, this result on its own is not enough for a diagnosis — it is assessed together with MCV and the other red cell indices, because different forms of anaemia affect red blood cell size differently. MedlinePlus does not separately name specific causes of a raised RDW.

• Iron deficiency anaemia or thalassaemia — MedlinePlus links these to a lower-than-usual MCV (smaller red blood cells), not directly to RDW
• A deficiency of B group vitamins (B12, folic acid) or liver disease — MedlinePlus links these to a higher-than-usual MCV (larger red blood cells), not directly to RDW

Symptoms listed alongside a raised RDW

• Breathlessness
• Weakness or tiredness
• Headache
• Dizziness
• A heart rhythm disturbance (arrhythmia)
• Pale skin
• Cold hands and feet

• MCV, MCH and MCHC — together with RDW they help establish the type of anaemia
• Iron values (ferritin, iron, transferrin saturation), vitamin B12 and folic acid in the blood
• Liver values, if liver disease is suspected
• When to repeat the test, so that the change over time can be seen

The tests used to look into anaemia are described separately.

A low RDW

A low RDW means that the red blood cells are of similar size — this is considered a normal result rather than a disorder in its own right. According to MedlinePlus, RDW itself is interpreted only as normal or abnormal (raised); the source does not single out a separate category of disease for a low RDW.

• A normal, healthy uniformity of red blood cell size
• RDW is not usually assessed as a sign of a disorder when it is low, unlike the other red cell indices

If the other full blood count values — haemoglobin, haematocrit and the red blood cell count — are also normal, further testing because of RDW is usually not needed. If there are symptoms, discuss the other values in the report with your doctor.