RDW Test

RDW Test (Red Cell Distribution Width)

RDW Test, The red cell distribution width blood test measures the amount of red blood cell variation in volume and size. RDW results are often compared with those of a mean cell volume (MCV) test. RDW may be used to quantitate Anisocytosis in blood film. Even at normal RDW levels, you may still have a medical condition.

Purpose of RDW (Red Cell Distribution Width)

This parameter is used to evaluate the type of anemia and other health conditions like thalassemia, liver disease, cancer, and inherited disease that can cause severe anemia.

Types of RDW (Red Cell Distribution Width)

There are two types of RDW

  • RDW-CV(co-efficient of variance)
  • RDW-SD(Standard Deviation)

Normal Value of RDW-CV and RDW-SD

  1. RDW-SD 40 to 55 Femto liter (fL)
  2. RDW-CV 11.5% to 15%

Causes of High RDW (Red Cell Distribution Width)

  1. Iron, Folate, and Vitamin B12 Deficiency
  2. A high MCV with a high RDW occurs in some macrocytic anemia. This can be due to a deficiency of folate or vitamin B-12.
  3. A low MCV with high RDW occurs in microcytic anemia, Iron deficiency anemia is a common cause of microcytic anemia.

Health Condition with Normal Results of RDW

  • Normal RDW with low MCV may have anemia resulting from thalassemia or a chronic disease, such as that caused by chronic kidney disease.
  • If your RDW result is normal but you have a high MCV, you may have aplastic anemia, Alcoholism, or Liver disease.

Specimen Collection For RDW Test

Collect 3 to 5 ml blood EDTA tube.

The test will be performed by using an automated cell counter.


  • To calculate RDW-SD Automated hematology analyser is required
  • Its calculation is not possible if the CBC test is performed manually

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