Red Cell Distribution Width (RDW)

Reference Range

Red cell distribution width (RDW) is a parameter that measures variation in red blood cell size or red blood cell volume. RDW is elevated in accordance with variation in red cell size (anisocytosis), ie, when elevated RDW is reported on complete blood count, marked anisocytosis (increased variation in red cell size) is expected on peripheral blood smear review.

The reference range for RDW is as follows:

• RDW-SD 39-46 fL^[1]
• RDW-CV 11.6-14.6% in adult^[2]

Reference ranges may vary depending on the individual laboratory and patient's age.

ed cell distribution width (RDW) is a red blood cell parameter that measures variability of red cell volume/size (anisocytosis). Depending on the types of hematology analyzer instruments, RDW can be reported statistically as coefficient of variation (CV) and/or standard deviation (SD), RDW-CV and/or RDW-SD, respectively.^{[1, 3]}

RDW-SD (express in fL) is an actual measurement of the width of the RBC size distribution histogram (see the first image below) and is measured by calculating the width (in fL) at the 20% height level of the RBC size distribution histogram (see the second image below). This parameter is therefore not influenced by the average RBC size (mean corpuscular volume, MCV).

RDW-CV (express in %) is calculated from standard deviation and MCV as follows (see the third image below):

• RDW-CV (%) = 1 standard deviation of RBC volume/MCV x 100%

Of note, since RDW-CV is mathematically derived from MCV, it is therefore affected by the average RBC size (MCV).

RBC size distribution histogram from a Sysmex SE-2100 analyzer from a subject with an MCV of 81.4 fL, RDW-SD of 38.2 fL, and RDW-CV of 12.8%.Determination of RDW-SD measurement. In this example, RDW-SD is 38.2 fL.Calculation of RDW-CV measurement, which is derived from 1SD divided by MCV times 100%. In this example, RDW-CV is 12.8%.

RDW is useful in the following conditions:^{[4, 5, 1]}

• Elevated RDW helps provide a clue for a diagnosis of early nutritional deficiency such as iron, folate, or vitamin B12 deficency 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 RDW, low MCV); however, definitive tests are required.
• 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:^[6]

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
• 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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