Mean corpuscular volume

The mean corpuscular volume, or mean cell volume (MCV), is a measure of the average volume of a red blood corpuscle (or red blood cell). The measure is attained by multiplying a volume of blood by the proportion of blood that is cellular (the hematocrit), and dividing that product by the number of erythrocytes (red blood cells) in that volume. The mean corpuscular volume is a part of a standard complete blood count. In a laboratory test that computes MCV, erythrocytes are compacted during centrifugation.

In patients with anemia, it is the MCV measurement that allows classification as either a microcytic anemia (MCV below normal range), normocytic anemia (MCV within normal range) or macrocytic anemia (MCV above normal range). Normocytic anemia is usually deemed so because the bone marrow has not yet responded with a change in cell volume. It occurs occasionally in acute conditions, namely blood loss and hemolysis.

If the MCV was determined by automated equipment, the result can be compared to RBC morphology on a peripheral blood smear. Any deviation would usually be indicative of either faulty equipment or technician error, although there are some conditions that present with high MCV without megaloblastic cells.

For further specification, it can be used to calculate red blood cell distribution width.

Calculation

To calculate MCV, the hematocrit (Hct) is divided by the concentration of RBCs ([RBC])

Normally, MCV is expressed in femtoliters (fL, or 10⁻¹⁵L), and [RBC] is the quantity expressed in millions per microliter (10⁶ / μL). The normal range for MCV is 80–100 fL.

For example, if the Hct = 42.5% and [RBC] = 4.58 million per microliter (4,580,000/μL), then

Use of volume-sensitive automated blood cell counters, such as the Coulter counter. In this type of apparatus, the red cells pass one-by-one through a small aperture and generate a signal directly proportional to their volume. Other automated counters measure red blood cell volume by means of techniques that measure refracted, diffracted, or scattered light.^[1]

Interpretation

The normal reference range is typically 80-100 fL.^[2]

High

In pernicious anemia (macrocytic), MCV can range up to 150 femtolitres. An elevated MCV is also associated with alcoholism^[3] (as are an elevated GGT and a ratio of AST:ALT of 2:1). Vitamin B12 and/or folic acid deficiency has also been associated with macrocytic anemia (high MCV numbers).

Low

The most common causes of microcytic anemia are iron deficiency (due to inadequate dietary intake, gastrointestinal blood loss, or menstrual blood loss), thalassemia, sideroblastic anemia or chronic disease. In iron deficiency anemia (microcytic anemia), it can be as low as 60 to 70 femtolitres. In some cases of thalassemia, the MCV may be low even though the patient is not iron deficient.

Worked example

Derivation

The MCV can be conceptualized as the total volume of a group of cells divided by their number. For a real world sized example, imagine you had 10 small jellybeans with a combined volume of 10 µL. The mean volume of a jellybean in this group would be 10 µL / 10 jellybeans = 1 µL / jellybean. A similar calculation works for the MCV.

1. You measure the RBC index in cells/µL. Take the reciprocal (1/RBC index) to convert it to µL/cell.

1/5E6 (cells/µL) = 2E-7 (µL/cell).

2. The 1 µL is only made of a proportion of red cells (e.g. 45%) with the rest of the volume composed of plasma. Multiply by the hematocrit to take this into account.

2E-7 (µL/cell) * 0.4 (hematocrit has no units) = 8E-8 µL/cell.

3. Finally convert the units of uL to fL by multiplying by 10^9. The result would look like this.

8E-8 µL/cell* (1E9 fL / 1 µL) = 80 fL/cell

Note: The shortcut proposed above just makes the units work out: 10*40/5=80

References

See also

• Anemia: Differential Diagnosis
• Full blood count
• Mean corpuscular hemoglobin