The correlation between RDW and MCV

RDW = Red Cell Distribution Width

The RDW value tells you whether enough of your red blood cells are of normal size and shape.

RDW is useful in the following conditions:

Elevated RDW helps provide a clue for a diagnosis of early nutritional deficiency such as iron, folate, or vitamin B12 deficiency as it becomes elevated earlier than other red blood cell parameters.

• It aids in distinguishing between uncomplicated iron deficiency anemia (elevated RDW, normal to low MCV) and uncomplicated heterozygous thalassemia (normal b, low MCV); however, definitive tests are required.[6, 7]
• It can also help distinguish between megaloblastic anemia such as folate or vitamin B12 deficiency anemia (elevated RDW) and other causes of macrocytosis (often normal RDW).
• RDW can be used as a guidance for flagging samples that may need manual peripheral blood smear examination, since elevated RDW may indicate red cell fragmentation, agglutination, or dimorphic red blood cell populations.

RDW along with mean corpuscular volume (MCV) is helpful in narrowing the cause of anemia: 

Normal RDW and low MCV is associated with the following conditions:

• Anemia of chronic disease
• Heterozygous thalassemia
• Hemoglobin E trait

Elevated RDW and low MCV is associated with the following conditions:

• Iron deficiency
• Sickle cell-β-thalassemia

Normal RDW and high MCV is associated with the following conditions:

• Aplastic anemia
• Chronic liver disease [9]
• Chemotherapy/antivirals/alcohol

Elevated RDW and high MCV is associated with the following conditions:

• Folate or vitamin B12 deficiency
• Immune hemolytic anemia
• Cytoxic chemotherapy
• Chronic liver disease
• Myelodysplastic syndrome

Normal RDW and normal MCV is associated with the following conditions:

• Anemia of chronic disease
• Acute blood loss or hemolysis
• Anemia of renal disease

Elevated RDW and normal MCV is associated with the following conditions:

• Early iron, vitamin B12, or folate deficiency
• Dimorphic anemia (for example, iron and folate deficiency)
• Sickle cell disease
• Chronic liver disease
• Myelodysplastic syndrome

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