Editing Red blood cell distribution width

{{short description|Measure of red blood cell volume variation as part of a standard blood test}}
{{Use dmy dates|date=October 2018}}
{{Infobox diagnostic
| name            = Red blood cell distribution width
| image           = Redbloodcells.jpg
| alt             =
| caption         = Human red blood cells
| pronounce       =
| purpose         = measure of the range of variation of red blood cell volume that is reported as part of a standard complete blood count.
| test of         =
|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]].<ref name=":0">{{cite journal | vauthors = Nah EH, Kim S, Cho S, Cho HI | title = Complete Blood Count Reference Intervals and Patterns of Changes Across Pediatric, Adult, and Geriatric Ages in Korea | journal = Annals of Laboratory Medicine | volume = 38 | issue = 6 | pages = 503–511 | date = November 2018 | pmid = 30027692 | pmc = 6056383 | doi = 10.3343/alm.2018.38.6.503 | first5 = Seon | first7 = and Han-Ik }}</ref> Red blood cells have an [[Mean corpuscular volume|average volume]] of 80–100 femtoliters, but individual cell volumes vary even in healthy blood.<ref name=":0" /> 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%.<ref>{{Cite web |title=Red Cell Distribution Width |url=http://www.fpnotebook.com/hemeonc/lab/RdClDstrbtnWdth.htm |website=Family Practice Notebook |access-date=18 October 2013}}</ref><ref name=":0" /> 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|Vitamin B<sub>12</sub>]] 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 B<sub>12</sub> 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]].<ref>{{cite journal | vauthors = Evans TC, Jehle D | title = The red blood cell distribution width | journal = The Journal of Emergency Medicine | volume = 9 | issue = Suppl 1 | pages = 71–4 | year = 1991 | pmid = 1955687 | doi = 10.1016/0736-4679(91)90592-4 }}</ref>

An elevation in the RDW is not characteristic of all anemias. [[Anemia of chronic disease]], [[hereditary spherocytosis]], [[hemorrhage|acute blood loss]], [[aplastic anemia]] (anemia resulting from an inability of the bone marrow to produce red blood cells), and certain [[hemoglobinopathy|hereditary hemoglobinopathies]] (including some cases of [[thalassemia minor]]) may all present with a normal RDW.<ref>{{cite book | vauthors = Kjeldsberg CR, Perkins SL | chapter = Table 1.4 |title=Practical Diagnosis of Hematologic Disorders |date=2010 |publisher=ASCP Press |location=Chicago, IL |isbn=978-0-89189-571-8 |edition=5th | url= http://www.ascp.org/pdf/SneekPeekPracDiagofHemDisorders.aspx | archive-url = https://web.archive.org/web/20130725024821/http://www.ascp.org/pdf/SneekPeekPracDiagofHemDisorders.aspx | archive-date = 2013-07-25 }}</ref>

==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. RDW-CV "width" refers to the width of the volume curve (distribution width), not the width of the cells. 
<ref>[http://www.ams.edu.pl/files/File/wronog/Metodyki-unit_4c.pdf Pomeranian Medical University > Red blood cell indices (indexes)]{{dead link|date=April 2018 |bot=InternetArchiveBot |fix-attempted=yes }} Retrieved on 5 April 2009</ref><ref>[http://diaglab.vet.cornell.edu/clinpath/modules/hemogram/rdw.htm Cornell University, Department of Population Medicine & Diagnostic Sciences > Red Cell Distribution Width (RDW)] Last Updated: June 2000. Retrieved on 5 April 2009</ref>

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).<ref name="Medscape Reference 2017" />

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

: RDW-CV = (1 [[standard deviation]] of RBC volume ÷ MCV) × 100%.<ref>[http://www.surgeryencyclopedia.com/Pa-St/Red-Blood-Cell-Indices.html Encyclopedia of Surgery: A Guide for Patients and Caregivers Red blood cell indices] Retrieved on 5 April 2009</ref>
Since RDW-CV is mathematically derived from MCV, it is therefore affected by the average RBC size (MCV).<ref name="Medscape Reference 2017" />

==Pathological implications==
===Normal RDW===
Anemia in the presence of a normal RDW may suggest [[thalassemia]]. A low [[Mentzer index|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.{{citation needed|date=July 2020}}

===High RDW===
High RDW may be a result of the presence of fragments, groups of agglutination, and/or abnormal shape of red blood cells.<ref name="Medscape Reference 2017">{{Cite web |title=Red Cell Distribution Width (RDW): Reference Range, Interpretation, Collection and Panels |url=https://emedicine.medscape.com/article/2098635-overview |last1=Curry |first1=Choladda Vejabhuti |date=2017-01-07 |website=Medscape Reference |access-date=2019-05-18}}</ref>
* Iron-deficiency anemia usually presents with high RDW and low MCV.
* Folate and vitamin B<sub>12</sub> deficiency anemia usually presents with high RDW and high MCV.
* Mixed-deficiency (iron + B<sub>12</sub> or folate) anemia usually presents with high RDW and variable MCV.
* Recent hemorrhages typically present with high RDW and normal MCV.
* A false high RDW reading can occur if [[EDTA]] [[anticoagulated blood]] is used instead of [[citrated blood]].  See [[Pseudothrombocytopenia]].

By severity, elevated RDW can be classified as follows:
{|class=wikitable
! RDW in adults<ref name=MrLabTest>{{cite web|url=https://www.mrlabtest.com/high-RDW-blood.htm|website=MrLabTest|title=High RDW level in the blood}} Last update: 12/01/2021</ref> !! Severity<ref name=MrLabTest/> !! Most notable causes<ref name=MrLabTest/>
|-
! 14.5% - 18%
| Mild anisocytosis
| Infection, vigorous exercise or certain drugs. [[Iron deficiency anemia]] in the presence of anemia and microcytosis.
|-
! 18% - 26%
| Moderate anisocytosis
| Severe iron deficiency anemia, [[megaloblastic anemia]].
|-
! > 26%
| Severe anisocytosis
| Sideroblastic anemia
|}

== References ==
{{reflist}}

== Further reading ==
* [https://web.archive.org/web/20180806155333/http://rdwbloodtest.com/ RDW Blood Test - Red Blood Cell Distribution Width]

{{Myeloid blood tests}}
[[Category:Blood tests]]