RDW Blood Test Results Explained

RDW Blood Test Results Explained


Red cell distribution width is a clinical measure that is used to assess the size of the red blood cells or calculate the volume of the red blood cells. It forms a part of the complete blood count i.e. CBC analysis. In essence, if you see a red-cell distribution width result on the blood panel, it indicates a variation in the size of the red-cell i.e. anisocytosis.

Blood Collection

As is the case with any blood test, a blood sample is obtained from the patient from a large vein such as the cubital vein or any other accessible veins. The blood is collected in a tube that contains an anticoagulant called EDTA potassium. This keeps the blood from clotting allowing a detailed analysis once the sample reaches the laboratory. This sample is then run through a specific machine that measures the red-cell distribution width.

Measurement of RDW

The results are expressed in different statistical manners including coefficient of variation or as a standard deviation.

The standard deviation is expressed in femtolitre (fL). The value is calculated by plotting the various sizes on a statistical histogram and calculating the width at a level that corresponds to 20% of the height of this histogram. Normal values lie between 39 – 46 fL. The red-cell distribution width (standard deviation) is not impacted upon by the size of the red blood cells i.e. the mean corpuscular volume (MCV).

At the other hand, the red-cell distribution width coefficient of variation is expressed as a percentage and is calculated from the standard deviation and the mean corpuscular volume. In other words, the RDW (CV) is dependent upon the size of the red blood cells. Normal values lies between 11.6 – 14.6%.

Clinical Applications

RDW is useful in diagnosing a number of different clinical conditions. These include:

• Nutritional deficiencies such as iron deficiency anemia, folate deficiency and vitamin B12 deficiency. The RDW is elevated in these conditions.

• RDW is useful in differentiating thalassaemia from iron deficiency anaemia. In thalassaemia, the RDW is normal while in iron deficiency anaemia the RDW is elevated. However, additional tests may be required to confirm the diagnosis.

• In conditions such as vitamin B12 deficiency and folate deficiency, the size of the red blood cells can increase. This is called macrocytosis. However, this finding is not exclusive to these conditions and RDW helps differentiated from other causes of macrocytosis.

In combination with MCV, RDW is useful in diagnosing conditions such as anaemia of chronic disease, sickle cell beta thalassaemia, aplastic anemia and chronic liver disease to name but a few. This is discussed in more detail below.

High RDW, Low MCV:
Iron deficiency anemia, sickle cell beta thalassaemia

Normal RDW, Low MCV:
Anemia of chronic disease, haemoglobin E trait

Normal RDW, High MCV:
Aplastic anemia, chemotherapy, chronic liver disease

High RDW, High MCV:
Folate deficiency, B12 deficiency, chronic liver disease, myelodysplastic syndrome.

High RDW, Normal MCV:
Early iron deficiency anemia, chronic liver disease, sickle cell disease


As is seen above, there are a variety of conditions in which the RDW values are valuable. It is therefore important to make sure that the test is conducted in all patients.