RDW Blood Test

Red Blood Cell Distribution Width (RDW) Blood Test gives an indication of the variation in Red Blood Cell size. Red Blood Cell Distribution Width is calculated by a machine using the Mean Corpuscular Volume (MCV) and RBC Count. Normally, all the RBCs are about the same size with very little variation. Variations in the width of the RBCs may be helpful when classifying certain types of anemia. The RDW is essentially an indicator of the degree of anisocytosis which is a blood condition characterized by RBCs of variable and abnormal size.


RDW Blood Test is one of the Red Blood Cells Indices. However, RDW can be considered as a test on its own since its method of calculation doesn’t depend on mathematics as it is with the other RBC Indices like Mean Corpuscular Hemoglobin. (MCH) and Mean Corpuscular Volume (MCV). Instead, blood cell counting machines take additional steps in the case of calculating RDW, this includes dividing RBCs into groups that the cells in the same group have the same size then running more complex calculations than those used in calculating the other RBC Indices. RBC Blood Test is ordered routinely as a part of Complete Blood Cell Count.


Advanced and newer models of electronic cell counting machines are able to sort out RBCs according to size and compare those sizes to a histogram. In a healthy person, variation in RBC Distribution Width is small and this creates a histogram with a single narrowed peak.


Certain diseases change the size of some of the RBCs, whereas the less abnormal RBCs are less affected. For example, with Folic Acid Deficiency Anemia or Iron deficiency, the newer RBCs are more significantly affected than the older cells and therefore will be of significantly different size. This creates a histogram with multiple peaks indicating large numbers of cells at variable sizes.



Normal RDW Variations

Human body doesn’t create all Red Blood Cells of the exact same size, variations in RDW are normal but on low levels. The normal RDW variation in adults range between 11% to 14%. It may be possible to see variations less than 11% in very rare cases, but this wouldn’t be an indication of a disease or unhealthy Red Blood Cells.



Causes of High RDW Variations

Increased variation in RDW is caused by a combination of factors in Iron-deficiency Anemia, B12 vitamin Anemia, or Folate-deficiency Anemia. RBC fragmentation alters RBC size and shape. Furthermore, new cells produced when the deficiency was greatest will be markedly different in size and shape than the older RBCs that were produced before the deficiencies were as severe.


Hemoglobinopathies (e.g., sickle cell or C disease): Fragmentation increases RDW variation. Furthermore, different RBCs have different amounts of pathologic hemoglobin and therefore will be affected by fragmentation to varying degrees.


Hemolytic Anemias: Fragmentation increases RDW variation.


Posthemorrhagic Anemias: The marrow’s response to bleeding is to release premature RBCs into the bloodstream. These are larger than mature RBCs and contribute to RDW variation.