This automated method of measurement is helpful in the investigation of some hematologic disorders and in monitoring response to therapy. The RDW is essentially an indication of the degree of anisocytosis (abnormal variation in size of RBCs). Normal RBCs have a slight degree of variation.

11.5%–14.5% coefficient of variation (CV) of red cell size

1. The CV of RDW is determined and calculated by the analyzer.

2. Use the CV of RDW with caution and not as a replacement for other diagnostic tests.

3. Use the following calculation:

1. The RDW can be helpful in distinguishing uncomplicated heterozygous thalassemia (low MCV, normal RDW) from iron-deficiency anemia (low MCV, high RDW).

2. The RDW can be helpful in distinguishing anemia of chronic disease (low-normal MCV, normal RDW) from early iron-deficiency anemia (low-normal MCV, elevated RDW).

3. Increased RDW occurs in:

   1. Iron deficiency

   2. Vitamin B₁₂ (VB₁₂) or folate deficiency (pernicious anemia)

   3. Abnormal Hb: S, SC, or H

   4. S-beta-thalassemia (homogeneous)

   5. Immune hemolytic anemia

   6. Marked reticulocytosis

   7. Fragmentation of RBCs

4. Normal RDW—normal in anemias with homogeneous red cell size

   1. Chronic disease

   2. Acute blood loss

   3. Aplastic anemia

   4. Hereditary spherocytosis

   5. Hb E disease

   6. Sickle cell disease

5. There is no known cause of a decreased RDW.

Pretest Patient Care

1. Explain the purpose and procedure for testing. Assess for possible causes of anemia. No fasting is required.

2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. This test is not helpful for persons who do not have anemia.

2. Alcoholism elevates RDW.

3. Cold agglutinins.