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Understanding RBC Indices

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Published on April 12, 2015

Dr. Susan Leclair continues her living well series on understanding your lab reports, discussing red cell indices, more commonly referred to as CBC.  The CBC is an efficient and focused analysis of patients who have trouble with their red blood cells.  One of the measurements that the red cell index quantifies is the mean cell volume (MCV) and Dr. Leclair explains why, what, and how the test helps your physician develop your specific plan of care.

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Transcript | Understanding RBC Indices

Please remember the opinions expressed on Patient Power are not necessarily the views of our sponsors, contributors, partners or Patient Power. Our discussions are not a substitute for seeking medical advice or care from your own doctor. That’s how you’ll get care that’s most appropriate for you.

Dr. Leclair:

Okay.  Once upon a time, physicians knew that people had anemias.  They had no idea how to go about classifying them or trying to figure out what the cause was.  So if you ever saw a physician, they would have to order 20, 30, 40 tests just to find out what was going on.  So something was needed.

And in the 1930s—this is going to sound familiar—a physician by the name of Max Wintrobe invented something that he called the red cell indices that on your CBC form you see as MCB, MCH, MCHC, and a late add-on, RDW.  So I'm going to talk about those indices and what he did to allow for efficiency and focus in analysis of people who have trouble with red blood cells.

So we're going to start at the beginning.  What is an MCV?  Mean  cell volume.  And sadly we're going to go back to 5th grade math.  So once you get over the shudder of remembering who your math teacher was and seeing her stand there in front of you, the question I need answered is what is the size of the average red cell?  Okay.

If I have 12—16, my math isn't that good.  If I have 16 apples and they fill a one-pound bag—pretty small apples—how much volume of that one-pound bag does one apple take?  So that's all I'm doing is I'm taking the number of cells, dividing it into the volume that they take up, the space that they take up, and I come up with an average size.  That's the MCV.  It's pretty simple actually.

Well, why is that important?  Well, it turns out that cells that have trouble making hemoglobin, oh, like iron deficiency cells, are smaller than they should be.  So if I have an MCV that's on the small side, I'm going to think of the possibility of iron deficiency, of damage to the hemoglobin-making process in the bone marrow.  That can happen from exposure from heavy metals like lead or cadmium or something like that.  It can be due to a change of using iron in hemoglobin mostly to using iron being used in the inflammation process, so I have something called an anemia of chronic inflammatory disease.  All of those come about because of damage to the hemoglobin production and hemo—damage to the hemoglobin production makes for small cells.

So now all of a sudden if I have someone who’s got a 75 MCV, I don't have to worry about whether or not they have an abnormal hemoglobin or that they have problems with albumin or they have problems with their liver.  I know they have problems with iron or exposure to heavy metals or chronic inflammatory disease or, rarely, an inherited disease, which means I'm not going to pick it up in somebody who is under the age of 10.  I'm mostly going to pick it up at birth or maybe less than five years old, called thalassemia.  All right.          

So what happens if I have bigger cells, if my MCV is greater than a hundred?  That's typically where the reference ranges will fall.  Well, these are big cells.  How do I get to have a big cell?  I get to have a big cell because I miss mitotic divisions.  I skip one or two in the bone marrow.

What are things that cause mitotic divisions to fail?  Well, the  most important thing about mitotic divisions is the amount of DNA you have.  You have to have enough DNA in one cell to make two.  So I'm constantly from a developing cell making DNA.  What do I need to make DNA?  Well, I need phosphates.  I need the bases.  I need deoxyribose as—structures.  I need two vitamins, B12 and folic acid.  And if I don't have enough B12 and folic acid, I don't make DNA.

Now, the chances are you're probably going to have enough phosphates, and you're probably going to have enough deoxyribose, and you're probably going to have enough of the bases, but folate and B12, those come from food.  Those come from your environment, and sometimes you can become deficient in one or both of them.  It's highly unusual to be deficient in both.  It's usually one or the other.

So now all of a sudden if you look at your MCV and it's greater than it should be, two possibilities are do you have a B12 deficiency, or do you have a folate acid deficiency?

There's a third iffy kind of situation here, and that basically involves reticulocytes.  Now, we spoke about reticulocytes two times ago, I think it was.  Retics come out of the bone marrow. They're very young cells.  They're slightly larger than a normal red cell because, remember, they have to go through the spleen and get stuff taken out of them so that they lose membrane and a little volume.

So if you have a lot of retics, um, you were in a car accident a week-and-a-half ago, and you lost a unit of blood. And everybody said, good, you'll make more.  Well, you will make more, and what will happen is all of those retics will come out faster than they should, actually, in order to replenish that blood in the bloodstream. And so now all of a sudden your MCV, which should be in one place, now got moved a little bit higher, because you have a mixture of your normal cells and these younger, slightly larger cells.

I'm not really too sure if you want to call that a condition. Everybody pretty much knows that. And, of course, it goes away because as those cells go through the spleen and they get older, they get back down to their normal cell.  So that's kind of a temporary phenomenon.

Another potential condition might be liver disease.  It turns out that the membrane of the cell is controlled by your liver, and the thinner the membrane, that is, the stretchier, the thinner it is, the more likely it is that cell is going to look larger than it is.

For those of you who are cooks, you got pancakes.  You know exactly what that looks like when you put it on the griddle. That mix is thick.  It's kind of lumpy.  It doesn't really pour easily.  You have to kind the place it down there. That's—that's--call that a typical red cell membrane.

Now what happens when you make crepes?  It pours.  It's thin. It's less viscous, so it's going to spread more easily when you put it on the pan.  So sometimes if the liver is in trouble and it's not making good enough membranes, those cells can be slightly larger than they should.  But now all of a sudden with this one number you're going to look at this and say, oh, I have   a hemoglobin problem if my cells are small.  I have a B12 or folic acid problem, maybe liver, if my cells are large, and those are the only tests that I have to focus on to determine what the cause is.

I don't have to test for B12 in small cells.  I don't have to test for iron in large cells.  It's a focused, controlled way of looking at cells.

So if you have any questions please, don't hesitate to write to questions@patientpower.info.  And remember, knowledge is a wonderful weapon.  See you next time.

Please remember the opinions expressed on Patient Power are not necessarily the views of our sponsors, contributors, partners or Patient Power. Our discussions are not a substitute for seeking medical advice or care from your own doctor. That’s how you’ll get care that’s most appropriate for you.

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