Red blood cell distribution width explained

Red blood cell distribution width
Purpose:measure of the range of variation of red blood cell volume that is reported as part of a standard complete blood count.
Synonyms:RCDW

Red blood cell distribution width (RDW), as well as various types thereof (RDW-CV or RCDW and RDW-SD), is a measure of the range of variation of red blood cell (RBC) volume that is reported as part of a standard complete blood count.[1] Red blood cells have an average volume of 80–100 femtoliters, but individual cell volumes vary even in healthy blood. Certain disorders, however, cause a significantly increased variation in cell size. Higher RDW values indicate greater variation in size. Normal reference range of RDW-CV in human red blood cells is 11.5–15.4%.[2] If anemia is observed, RDW test results are often used together with mean corpuscular volume (MCV) results to determine the possible causes of the anemia. It is mainly used to differentiate an anemia of mixed causes from an anemia of a single cause.

Deficiencies of Vitamin B12 or folate produce a macrocytic anemia (large cell anemia) in which the RDW is elevated in roughly two-thirds of all cases. However, a varied size distribution of red blood cells is a hallmark of iron deficiency anemia, and as such shows an increased RDW in virtually all cases. In the case of both iron and B12 deficiencies, there will normally be a mix of both large cells and small cells, causing the RDW to be elevated. An elevated RDW (red blood cells of unequal sizes) is known as anisocytosis.[3]

An elevation in the RDW is not characteristic of all anemias. Anemia of chronic disease, hereditary spherocytosis, acute blood loss, aplastic anemia (anemia resulting from an inability of the bone marrow to produce red blood cells), and certain hereditary hemoglobinopathies (including some cases of thalassemia minor) may all present with a normal RDW.[4]

Calculations

The "width" in RDW is sometimes thought to be "misleading", since it in fact is a measure of deviation of the volume of RBCs, and not directly the diameter. However, "width" refers to the width of the volume curve (distribution width), not the width of the cells. [5] [6]

RDW-SD is calculated as the width (in fL) of the RBC size distribution histogram at the 20% height level. This parameter is, therefore, not influenced by the average RBC size (mean corpuscular volume, MCV).

RDW-CV (expressed in %) is calculated with the following formula:

RDW-CV = (1 standard deviation of RBC volume ÷ MCV) × 100%.[7] Since RDW-CV is mathematically derived from MCV, it is therefore affected by the average RBC size (MCV).

Pathological implications

Normal RDW

Anemia in the presence of a normal RDW may suggest thalassemia. A low Mentzer Index, calculated from CBC data [MCV/RBC < 13], may suggest this disorder but a hemoglobin electrophoresis would be diagnostic. Anemia of chronic diseases show normal RDW.

High RDW

High RDW may be a result of the presence of fragments, groups of agglutination, and/or abnormal shape of red blood cells.[8]

By severity, elevated RDW can be classified as follows:

RDW in adults[9] Severity Most notable causes
14.5% - 18%Mild anisocytosisInfection, vigorous exercise or certain drugs. Iron deficiency anemia in the presence of anemia and microcytosis.
18% - 26%Moderate anisocytosisSevere iron deficiency anemia, megaloblastic anemia.
> 26%Severe anisocytosisSideroblastic anemia

Further reading

Notes and References

  1. Nah EH, Kim S, Cho S, Cho HI . Complete Blood Count Reference Intervals and Patterns of Changes Across Pediatric, Adult, and Geriatric Ages in Korea . Annals of Laboratory Medicine . 38 . 6 . 503–511 . November 2018 . 30027692 . 6056383 . 10.3343/alm.2018.38.6.503 . Seon . and Han-Ik .
  2. Web site: Red Cell Distribution Width . Family Practice Notebook . 18 October 2013.
  3. Evans TC, Jehle D . The red blood cell distribution width . The Journal of Emergency Medicine . 9 . Suppl 1 . 71–4 . 1991 . 1955687 . 10.1016/0736-4679(91)90592-4 .
  4. Book: Kjeldsberg CR, Perkins SL . Table 1.4 . Practical Diagnosis of Hematologic Disorders . 2010 . ASCP Press . Chicago, IL . 978-0-89189-571-8 . 5th . https://web.archive.org/web/20130725024821/http://www.ascp.org/pdf/SneekPeekPracDiagofHemDisorders.aspx . 2013-07-25 .
  5. http://www.ams.edu.pl/files/File/wronog/Metodyki-unit_4c.pdf Pomeranian Medical University > Red blood cell indices (indexes)
  6. http://diaglab.vet.cornell.edu/clinpath/modules/hemogram/rdw.htm Cornell University, Department of Population Medicine & Diagnostic Sciences > Red Cell Distribution Width (RDW)
  7. http://www.surgeryencyclopedia.com/Pa-St/Red-Blood-Cell-Indices.html Encyclopedia of Surgery: A Guide for Patients and Caregivers Red blood cell indices
  8. Web site: Red Cell Distribution Width (RDW): Reference Range, Interpretation, Collection and Panels . Curry . Choladda Vejabhuti . 2017-01-07 . Medscape Reference . 2019-05-18.
  9. Web site: High RDW level in the blood. MrLabTest. Last update: 12/01/2021