RDW Blood Test

What is RDW blood test?

Red blood cell distribution width (RDW blood test) is a measure or laboratory counting process of the size inconsistency of circulating Red Blood Cells (RBCs) or erythrocytes and is normally describe as a factor of CBC (complete blood count) in the differential diagnosis of anemia. Estimation of hemoglobin can be possible by conducting RDW. (1,2)

RDW blood Test


Normal Range

It is considered that 10.2 % to 14.5% is the normal range of RDW level. But this range is not universally accepted, as this may vary with lab testing machine. Therefore, in the lab report, it is necessary to mention the decided normal range according to the usable machine. (2)

The calculation process of RDW blood test

Electronic medical device is used for calculating the blood cells find in the blood sample. The tool also identifies the pulses, which are prepared by RBCs. In case of increase size of Red blood cells cause production of strong pulses, while for the tiny size of RBCs produce weak pulses. (2)

What does RDW mean in a blood test?

In the blood test, RDW denoted the parameter obtain from numerical counting of red blood cells and indicates the variation of size or volume of erythrocytes present in the blood sample. Various medical devices are available for hematological analysis and able to calculate the RDW . The measure is reported in two ways such as coefficient of variation (CV) or RDW-CV and standard deviation (SD) or RDW-SD.

The measurement of RDW-SD is denoted as “fL” and it is the fundamental measure of the width of red cells size distribution histogram. The calculation basically maintains the width at the 20% elevation plane of the histogram. Therefore the typical size of the RBC does not provide any consequence on this limit and mean corpuscular volume or MCV is calculated.

In addition, RDW-CV is denoted in the percentage value and is measured from standard deviation and MCV as follows:

RDW-CV (in percentage) = 1 SD of RBC volume/MCV x 100% (3,4)


Results – High & Low


High value of RDW

If the value of RDW is higher than the normal range means more than 14.5%, then it indicates the size of the red blood cells are varied too much. This occurs with different medical conditions and need to identify the underlying cause of this variation. It is significant to create an evaluation of mean corpuscular volume or MCV, which indicates the total standard of space occupy by each red cell in the available blood sample.

Both higher value of RDW and MCV

This is usually common with hepatic disorders. The liver is taking part in the excretion of toxic material from blood by filtering them. If the liver is not functioning properly due to hepatic disorder, then toxic elements are remaining in the blood and affects the normal functionality of the blood and possesses the increase size of the RBCs.  This provides higher value of MCV and RDW.

For hemolytic anemia and vitamin B12 deficiency also raises the range of RBC and MCV.

Higher value of RDW with low range of MCV

The lab test report can provide higher value of RDW with a low range of MCV. The different clinical manifestations are responsible for this type of result. Iron deficiency is one of the prime reasons of obtaining this type of result. The iron deficiency leads to iron deficiency anemia, as iron is one of the chief components of hemoglobin. Hemoglobin is responsible for transportation of oxygen in different tissues.

Unavailability of the sufficient iron ion causes insufficient hemoglobin in blood, which resultant is decreasing of MCV volume. But hemoglobin is required for maturation of RBCs, and immature RBCs are larger in size and raises the RDW level.

Thalassemia is another blood disorder, which causes limiting some essential blood elements responsible for hemoglobin production. The blood cells are easily split and decrease the level of MCV, whereas the increased size of immature RBCs causes high RDW.

Higher RDW with normal range of MCV

At the initial stage of iron deficiency anemia, can provide the blood test report with higher RDW with normal range of MCV. This is common when vitamin B12 is deficit in the body, as it plays a great role in hemoglobin production and maturation of the blood.

Low value of RDW

When the RBCs size becomes lesser than the normal, then The RDW level becomes lower than the normal limit. Usually the level is below than the 10.2%. Certain clinical conditions are responsible for providing this type of blood test result.

In macrocytic anemia, the red blood cell count is reduced than the normal and provides low range of the RDW.(2,4)



  1. Yang Wen, (2010), High red blood cell distribution width is closely associated with risk of carotid artery atherosclerosis in patients with hypertension; Retrieve from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2954027/
  2. http://www.medfriendly.com/red-cell-distribution-width.html
  3. Choladda Vejabhuti Curry (2015); Red Cell Distribution Width (RDW); Retrieve from: http://emedicine.medscape.com/article/2098635-overview
  4. RDW High (2014), Retrieve from: http://www.newhealthguide.org/RDW-High.html

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