Red Cell Distribution Width (RDW)

Why test for Red Cell Distribution Width (RDW) at Symbion VIP Diagnostics Ahmedabad ?

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. 

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).

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 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: 

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

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario. For further information about these lab tests contact Symbion VIP Diagnostics pathology lab Ahmedabad at 09429410291