Red Cell Distribution Width (RDW) Test

The red cell distribution width (RDW) test measures variation in red blood cell size or red blood cell volume as a part of a complete blood count (CBC). It is used along with other red blood cell (RBC) indices, especially mean corpuscular volume (MCV), to help determine the causes of anemia.

RDW is elevated in accordance with variation in red cell size (anisocytosis); that is, 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.

Indications/Applications

Red cell distribution width (RDW) laboratory test is a part of a standard complete blood count (CBC), and it is used along with other RBC indices, especially mean corpuscular volume (MCV) to help determine the causes of anemia.

Considerations

Elevated RDW provides a clue for heterogenous red cell size (anisocytosis) and/or the presence of 2 red cell populations, since other RBC indices (MCV, MCH and MCHC) reflect average values and may not adequately reflect RBC changes where mixed RBC populations are present, such as dimorphic RBC populations in sideroblastic anemia or combined iron deficiency anemia (decreased MCV and MCH) and megaloblastic anemia (increased MCV). Peripheral blood smear review can help confirm the above findings in these circumstances. ^{[3, 4, 5]}

Red 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, 6]}

RDW-SD (expressed in fL) is an actual measurement of the width of the red blood cell (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 (expressed 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).

RDW blood test is useful in the following conditions: ^{[7, 3, 1]}

• A high 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 RDW, low MCV); however, definitive tests are required. ^{[8, 9]}
• 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: ^[4]

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

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

High RDW and low MCV are associated with the following conditions:

• Iron deficiency
• Sickle cell-β-thalassemia

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

• Aplastic anemia
• Chronic liver disease ^[10]
• Chemotherapy/antivirals/alcohol

High RDW and high MCV are 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

High 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

A study by Moreno-Torres et al indicated that RDW is an independent prognostic factor for mortality in patients with sepsis admitted to the intensive care unit (ICU). The investigators noted that higher RDW values were found during the first week following ICU admission in sepsis patients who died. Moreover, the investigators stated that adding 24-hour and admission RDWs to certain prognostic tools in sepsis—specifically, the Sequential Organ Failure Assessment (SOFA) score, the Logistic Organ Dysfunction System (LODS) score, the Acute Physiology and Chronic Health Evaluation II (APACHE II) score, and the Simplified Acute Physiology Score II (SAPS-II)—bolstered the discriminatory ability of these scores. ^[11]

Collection and panel details for the RDW blood test are as follows:

• Specimen: Whole blood, usually collected by venipuncture

• Collection: EDTA tube (purple/lavender top) containing EDTA potassium salt additive as an anticoagulant (see image below)

  EDTA tubes, purple top.

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• Panels: Complete blood count (CBC)