RDW – Things to Know

The red cell distribution width (RDW) blood test measures the amount of red blood cell variation in volume and size. Normal red blood cell maintains a standard size of 6 to 8 micrometres in diameter. If cells are larger, your RDW blood test values will have higher numbers. This means you may have a nutrient deficiency, anaemia, or other underlying condition. The RDW test is used to help diagnose types of anaemia and other medical conditions including:

• Thalassemias, which are inherited blood disorders that can cause severe anaemia

• Diabetes mellitus

• Heart disease

• Liver Disease

• Cancer

If anemia is observed, RDW test results are often used together with mean corpuscular volume (MCV) results to determine the possible causes of the anemia.  It is mainly used to differentiate an anemia of mixed causes from an anemia of a single cause. Vitamin B12 deficiency produces a macrocytic anemia (large cell anemia) with a normal RDW.

However, iron deficiency anemia initially presents with a varied size distribution of red blood cells, and as such shows an increased RDW. In the case of a mixed iron and B₁₂ deficiency, there will normally be a mix of both large cells and small cells, causing the RDW to be elevated.

Mathematically the RDW is calculated with the following formula:

       RDW = (Standard deviation of MCV ÷ mean MCV) × 100.  

The determination of the RDW-SD on instruments is an actual measurement of the width of the erythrocyte distribution curve. This measurement is performed at a relative height of 20 % above the baseline.

The wider the curve is spread by erythrocytes of different sizes, the higher the RDW-SD value will be.

Reference values:

               female: 36.4 – 46.3 fL

               male   : 35.1 – 43.9 fL

RDW-CV:

The RDW-CV is calculated from the formula:

                       RDW-CV = 1SD   x 100

                                               MCV

1SD reflects the size variation of the erythrocytes round the mean. As the 1SD is divided by the MCV, the RDW-CV also depends on the mean size (MCV) of the erythrocytes.

Reference values:

      Female: 11.7 – 14.4%

      Male: 11.6 – 14.4 %

The width of the erythrocyte distribution curve RDW reflects the variability in erythrocyte size and is thus a measure of anisocytosis. Erythrocyte indices are important in the classification of anemia or for the early recognition of processes which can cause anemia.

Determination of the MCV is used in the classification into normocytic, microcytic and macrocytic

anemia, which is important in diagnosis. In combination with the RDW, this is the best criterion for the classification of anemias. As the MCV is an arithmetic mean, it does not exclude partial microcytosis – even in the reference range. Only in combination with the RDW, it may indicate dimorphic erythrocytes, for example, in the initial stages of iron deficiency.