Wednesday, August 31, 2016

Study group discussion: Red cell Distribution Width (RDW)

Let's discuss Red cell Distribution Width (RDW) today!

RDW represents variation in RBC size.
If the RDW is large, it says that the cells are different in sizes.
Analogy: If RDW is large, we know the RBCs are all unity in diversity. Like India :P
What's the clinical significance?

To distinguish causes of microcytic hypochromic anemia, for example, iron deficiency anemia (IDA) or thalessemia.
- If RDW is high, likely to be IDA.
- If it is normal, likely to be thalassemia.

Yup, that is correct, but why?

- In thalassemia, all cells are uniformly decreased in size, hence there's uniformity.
Analogy: They're all small. (Like a country of little people.)
- But in IDA, different cells are at different stages of development and are not equal in size.
Analogy: Like a country with some little people and some taller ones.

- Low MCV, high RDW is IDA
- Low MCV, normal RDW is thalassemia.

If the patient has IDA and also B9 / B12 deficiency, that is, dimorphic anemia, then what will the RDW be?

*We had different opinions, some said high, some said normal, then a brilliant mind resolved our confusion*

*le representative of team high*: 
Since RDW is the distribution range, the 'width' should be highest in dimorphic. Because there's too many RBCs on both ends of the size spectrum.

*le representative of team normal*: 
Like in MCV, microcytic and macrocytic is normocytic. The same should apply for RDW!
*le brilliant mind*: 
In dimorphic anemia, it is important to note what are the units you are expressing RDW in. It can be expressed as:

- Femtolitre: Calculated from standard deviation (SD) of size of RBCs.
- Coefficient of variation:
CV is calculated from MCV.
- Since RDW in CV is derived from MCV, it is not high in dimorphic. It is normal.
- But RDW in SD is not derived from MCV, therefore, it is high in dimorphic.

Most places, you will find it written that RDW is normal in dimorphic. This is the reason. Because mostly, we express it in CV. So varied opinion on RDW in dimorphic can be explained :)

Thank you, for this study group discussion: Swadeep Raj, Divya and A. P. Burkholderia!


  1. Another good way to differentiate Thalassemia from IDA is by Reticulocyte Count.
    Higher Retic Count would highly support the Diagnosis of Thalassemia because your spleen is sequestering all those abnormally formed RBCs. WHile in IDA, there is normal to low value Retic Count as there is not much abnormal RBCs to be sequestered in the first place.

    :) THanks for this discussion!

    1. Really? Because I thought reticulocyte count increases in IDA as well


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