Mean Corpuscular Volume

Mean corpuscular volume (MCV) is a laboratory measure of the average volume of circulating red blood cells. It is one of the red cell indices routinely reported in a complete blood count (CBC). MCV helps classify anemia according to cell size, but does not independently establish its cause or determine whether anemia is present. (medlineplus.gov)

Measurement and units

MCV is expressed in femtoliters (fL), with one femtoliter equal to $10^{-15}$ liters. Automated hematology analyzers assess red cell volume using electrical impedance or optical methods. MCV can also be calculated from the hematocrit—the proportion of blood volume occupied by red cells—and the red blood cell count: (ncbi.nlm.nih.gov)

For example, a hematocrit of $45\%$ and a red cell count of $5.0\times10^{6}/\mu L$ correspond to an MCV of $90\;\mathrm{fL}$. In many automated systems, MCV is measured and hematocrit is calculated from MCV and the cell count, rather than the reverse. (ncbi.nlm.nih.gov)

MCV differs from mean corpuscular hemoglobin (MCH), which describes the average amount of hemoglobin per red cell, and mean corpuscular hemoglobin concentration (MCHC), which describes its concentration within the cells. These measurements characterize related but distinct properties. (medlineplus.gov)

Reference intervals and classification

A widely used adult classification defines red cell populations as follows:

These are conventional classification boundaries, not universal laboratory reference limits. Laboratories may report narrower or otherwise different intervals, and age affects interpretation. “Microcytic anemia,” for example, describes anemia accompanied by a low MCV; a low MCV alone is not equivalent to anemia. (merckmanuals.com)

Biological basis and clinical significance

Low MCV

Microcytosis commonly accompanies impaired hemoglobin synthesis. Important causes include iron-deficiency anemia and thalassemia. Some cases of anemia associated with chronic disease also become microcytic. These conditions overlap in their MCV values, so cell size alone cannot distinguish them. (merckmanuals.com)

Normal MCV

Anemia may occur despite a normal average red cell volume. Examples include acute blood loss and anemia associated with kidney failure. Consequently, a normal MCV does not exclude a clinically important reduction in hemoglobin. (medlineplus.gov)

High MCV

Macrocytosis may result from impaired DNA synthesis, including that caused by vitamin B12 or folate deficiency and certain medicines. It also occurs with alcohol-related effects and liver disease. Defective nuclear maturation can produce large oval red cells; other mechanisms produce enlarged cells without this maturation defect. (merckmanuals.com)

Reticulocytes, newly released red blood cells, are larger than mature erythrocytes. A substantial increase in their number can therefore raise MCV, including during the response to blood loss or increased red cell destruction. (merckmanuals.com)

Interpretation and limitations

MCV is an average: it does not describe the full distribution of cell sizes. A mixture of unusually small and unusually large cells may yield a normal result. Red cell distribution width (RDW) measures variation in cell volume and can reveal this heterogeneity. A peripheral blood smear supplies additional information about cell shape and morphology that MCV cannot provide. (merckmanuals.com)

Interpretation also incorporates hemoglobin concentration, red cell count, reticulocyte count, clinical history, and other laboratory findings. Tests such as ferritin, vitamin B12, and folate measurements address possible causes rather than cell size itself. (merckmanuals.com)

Measurement artifacts can produce misleading results. Red cell agglutination may cause an analyzer to count cell clusters as larger particles, falsely increasing MCV while reducing the reported cell count. Marked hyperglycemia can also increase measured MCV through cell swelling during sample dilution. These analytical effects are distinct from true enlargement of circulating red cells. (ncbi.nlm.nih.gov)

References

1. MCV (Mean Corpuscular Volume): MedlinePlus Medical Testmedlineplus.gov
2. RBC indices: MedlinePlus Medical Encyclopediamedlineplus.gov
3. Evaluation of Anemia — Merck Manual Professional Editionmerckmanuals.com
4. Red Cell Indices — Clinical Methods — NCBI Bookshelfncbi.nlm.nih.gov
5. Red Cell Indices — Clinical Methods — NCBI Bookshelfncbi.nlm.nih.gov