Empeiria

Erythrocyte distribution width

Also known as RDW, red cell distribution width

derived index · no single molecular formula

LOINC 788-0 code verified whole_blood %

What this substance is

A statistic, not a substance: how much red cells vary in size.
How it is calculated

RDW (%) = standard deviation of red cell volume / MCV x 100

Derived from mcv.

Reference ranges

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This open edition shows a range only where a public-domain or openly licensed source states one, and none does for this marker yet. Compare your result with the range printed on your own lab report.

How it is measured

In practice

RDW comes from the same blood sample as a complete blood count (CBC). A health worker takes blood from a vein in your arm with a small needle. The machine works out the RDW from the sizes of your red blood cells, so it is not a separate test. No special preparation is needed.

RDW is not separately measured: the analyser derives it from the RBC volume histogram of the same EDTA whole-blood CBC sample, as the coefficient of variation (%) of the RBC size distribution. Specimen is venous blood collected by venipuncture into an EDTA tube. No patient preparation is required.

Where: laboratory

Sources: Laboratory Procedure Manual, Complete Blood Count (NHANES 2007-2008), CDC National Center for Health Statistics; RDW (Red Cell Distribution Width), MedlinePlus (U.S. National Library of Medicine) cited · not yet checked by a person

In the lab

calculation

Coefficient of variation of the RBC volume histogram

RDW shows how much your red blood cells vary in size. A higher number means the cells are more mixed in size.

RDW is derived from the impedance RBC volume histogram as the coefficient of variation (standard deviation divided by the mean, as a percentage) of the red cell size distribution.

  • Read by: None directly; derived from the RBC volume histogram

What can throw it off

  • Very high white cell count
  • Very large or clumped platelets
  • Red cell agglutinates
Why labs disagree: RDW is calculated from the RBC size histogram, so it inherits how each analyser builds and analyses that curve; there is no standardisation of RBC distribution curve analysis, of anticoagulant choice, or of whether RDW is calculated from the standard deviation or the coefficient of variation. RDW also varies between populations and age groups.

Sources: Aslam H et al. The Role of Red Cell Distribution Width as a Prognostic Marker in Chronic Liver Disease: A Literature Review. Int J Mol Sci 2023 (PMC9967940, open access); CDC NHANES 2011-2012 Laboratory Procedure Manual: Complete Blood Count using Coulter HMX cited · not yet checked by a person

What it reports on

Associated symptoms

Conditions this marker features in

When high

Ordered as part of

cbc