Q. I don’t understand the red cell distribution width (RDW)! The formula is: RDW= (MCV standard deviation/ MCV) x 100. 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).