red blood cells

red blood cells (Photo credit: Genista)

The red blood cell distribution width (“RDW”) test is normally done when the complete blood count is ordered by the health practitioner.  The RDW test measures the variations in size of red blood cells.  It is used with the MCV (mean corpuscular volume) test results to better determine types of anemia.

Normally red blood cells are between six to eight micrometers in diameter (a micrometer is one millionth of a meter and its symbol is μm).  The RDW is expressed in a percentage using the MCV results.

NORMAL LEVELS

The normal RDW level is 10.2 to 14.5%.

HIGHER THAN NORMAL RESULTS

A higher than normal RDW level means that the red blood cells substantially vary in size.  It is often associated with:

Vitamin B12 deficiency anemia
Iron deficiency anemia
Combination of Vitamin B12 and iron anemia
Liver disease

Hemolytic anemia (where red blood cells are destroyed earlier than normal)
Folic acid deficiency (B complex vitamin)

LOWER THAN NORMAL RESULTS

A lower than normal RDW level means that the red blood cells have little size variation.  It is often associated with:

Macrocytic anemia (not enough red blood cells are produced)