Q. About the red cell distribution width (RDW), I don’t understand it! The formula is: RDW= (MCV standard deviation/ MCV) x 100. So now if the standard deviation is a fixed number, why does the RDW increase whether MCV is increased or decreased? I understand that in both iron deficiency and megaloblastic anemia it should be increased cause it shows the volume differentiation but it is mathematically obscure to me.

A. Good question! The standard deviation of the mean actually does change depending on what type of anemia the patient has.

Normally, the cells in our blood are all about the same size. So the standard deviation of the mean is fairly low. Meaning that if our MCV is 90 fL, there might be a few red cells that are 88 or 89, and a few that are 91 or 92, but basically, there’s little deviation from the mean – almost every cell is very close to 90 fL in size.

In some types of anemia, there is a huge variation in the size of the red cells. In iron-deficiency anemia, for example, each new wave of iron-depleted cells is smaller than the last (because there is less and less iron around). So the older red cells are bigger than the newer red cells. If the overall MCV in a particular case is 70 fL, there are going to be some cells (the older ones) that might be close to 80 fL, and other cells (the newest ones) that might be around 60 fL. So the deviation from the mean is large, and the RDW is high.

If you think of it in terms of test scores (a topic we all know well!), it might help. The mean score for the class might be, say 80. But it’s also useful to know if everyone scored right around 80 (meaning that the standard deviation was low), or if there were a wide range of scores from 60 to 100 (meaning that the standard deviation was high). Same thing with a blood smear: if all the red cells are roughly the same size, the standard deviation (and RDW) is low. If there is a wide range of sizes, the standard deviation (and RDW) is high.

By the way, the place that the RDW is most useful (in my humble opinion) is in differentiating between iron-deficiency anemia (IDA) and mild to moderate thalassemia. In IDA, as we just talked about, the RDW is high. In mild-moderate thalassemia, the RDW is not elevated. The cells in mild-moderate thalassemia are all basically the same size – probably because the defect in thalassemia is static (unlike the situation in IDA, where the defect worsens over time, so the cells keep getting smaller and smaller).

*More pathology!*

This makes a lot of sense. Thanks for this explanation Dr Krafts.

hi mam I want to ask you one question I am very keen to know or more experience to read blood morphology is it possible you to give me address somewhere near wher I can get training by y own expence

I am 60 year old and more then 20 year experience in this field

Hello Dr Kristine Krafts,

I just joined your website. I enjoyed your discussion of the RDW.

Are you familiar with the book “Cells, Tissues, and Disease” by Guido Manjo and Isabelle Joris? I just recently bought the first edition (the new second edition was out of my price range). I have always used Robbin’s Pathologic Basis of Disease, but I do like the book by Manjo.

Love your website.

How are things in Minnesota. I did my infectious diseases fellowship in Iowa City, and took the board exam at the Mayo Clinic.

Best wishes,

Michael Martin, MD, FACP.

Thanks, Michael, and welcome! So glad you liked the discussion of the RDW.

No – I haven’t heard of that book – but will check it out. Robbins has had the market on pathology for so long – it’s good to look at other path books too 🙂

Things are hot right now in MN, but that’s better than winter, as you may recall!

Best to you too.

very informative.explained in the simplest way.RDW is an important automated hematological parameter.

THanks,,so EXPLICIT

First of all I am big fan of your posts. Wow. Superb. Some teacher explained it very simply, RDW signifies variation in size of RBC so if there is a gene defect then all the RBC which will be formed will be of similar size so RDW will be normal. But in nutritional deficiencies, which varies day to day, like a wave of iron depleted so the cells progressively becomes smaller or larger as in case of megaloblastic anaemia. Nutritional deficiencies vary so does size of RBC and so in all nutritional deficiency we see increased RDW. It also explains increased RDW in megaloblastic anaemia, so just thought to share it here

very simple explanation thank u very much..

I have done my post grad in path after Mbbs from India, I have read the standard books in Hematology during my post grad Nd I have read few charts regarding RDW Nd MCV based classification of Anaemia but ur explanation Dr Krafts is just amazing . So simple nd logical .

Thank you very mach for very simple explanation

nice n simple ! keep it up… but what happens in severe Fe def or severe B12 DEF… where we report a microcytosis 4+ or macrocytosis 4+ …the anisocytosis is not great .. so RDW won’t be much affected ??