Friday, January 25, 2008


The mean corpuscular volume is the most important of the red cell indices. It measures the average size of the red cells.

A red cell is mainly made of hemoglobin with a membrane to go round it. Small round cells are caused by a lack of hemoglobin. There are only three causes of small red cells - there are two causes of too little hem and one cause of too little globin. The amount of hem is controlled by iron and the amount of globin is controlled by the genes you inherit from your parents. If you inherit dodgy globin genes you have thalassemia. There are several sorts, but they all result in small red cells.

If you have too little iron you can't make hem and you end up with small red cells. Too little iron may be the result of too little iron in the body - either because of not enough iron in the diet, which is very rare in most Western countries, or because of excessive iron loss - mainly due to bleeding. But too little iron for the red cells can be caused by the macrophages greedily hanging on to it and not releasing it to the forming red cells in the bone marrow. This happens with a wide variety of chronic disorders such as Crohn' disease, ulcerative colitis, rheumatoid arthritis, chronic infection and some cancers.

Raised MCVs have more causes. Everybody knows about megaloblastic anemia due to a deficiency of either folic acid or vitamin B12. These are relatively easy to diagnosed and it is important to rule them out by measuring B12 and folate levels, but in actual fact, these are relatively rare cause of macrocytosis, as too are the floppy membrane diseases - conditions where the size of the red cells is increased because there is too much membrane. Examples would be liver disease and thyroid deficiency. Alcoholics have large red cells, probably for a variety of reasons, and probably the commonest cause in older people is MDS.

However, we sometimes see raised MCVs in CLL and none of these reasons apply. I think that whatever fills the bone marrow spaces that shouldn't be there causes an increase in the MCV. You certainly see it in myeloma, but why this should be is unknown.


Anonymous said...

And I would have guess the RBC would have been the most important number.

Terry Hamblin said...

I seldom look at the red count. It is one of the primary measurements, although cell counters have to make an adjustment for the possibility of two cells going through an orifice together and being counted as one. However, for full information of interpretation of a CBC we meed to know not only how many red cells there are, but how much hemoglobin each one holds, and this is largely determined by how big it is. an index like RDW tells us how much variation there is around the mean. In olden days the means of measurment brought in artefacts that distorted teh meaning of indeces like MCHC and MCH, but MCV has retained its value, and since about 1968 has been measured directly by the automated counters. My practice has aleays been to look at Hb, MCV, WBC, platelets and differential white count.

David Arenson said...

How does AIHA affect MCV? When I was hemolyzing the MCV increased; now, following treatment and the end of hemolysis, it is gradually reducing and is getting close to nornal again.

Terry Hamblin said...

AIHA causes an outpouring of young red cells into the blood. These are larger than older cells.