The mean corpuscular volume, or "mean cell volume"(MCV), is a measure of the average
volume of a red blood corpuscle.
The measure is attained by multiplying a volume of blood by the proportion of blood that is
cellular), and dividing that product by the number of erythrocytes 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, normocytic anemia or macrocytic anemia.
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.
To calculate MCV, which is expressed in femtoliters, hematocrit as a decimal is divided by RBC.
RBC is the quantity of erythrocytes expressed in millions per microliter.
The normal range for MCV is 80–100 fL.
For example, if the hematocrit equals 42.5% and RBC = 4.58 million per microliter, then
the following equation would obtain.
MCV = Hrt/RBC.
MCV = 0.425/([4.58 * 106]/10–6 L).
Then, multiplying both the numerator and the denominator of the divisor by 10–6, the
divisor becomes/(10–6*10–6 L).
MCV = 0.42510–12 L).
Then, after inverting the divisor and multiplying it by the dividend, the result is/4.58, which
= 0.0928*10–12 L, which = 92.8*10–15 L, which = 92.8 fL.
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.
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.
Interpretation The normal reference range is typically 80-100
fL.
High In pernicious anemia, MCV can range up to
150 femtolitres.
An elevated MCV is also associated with alcoholism and myelofibrosis.
Vitamin B12 and/or folic acid deficiency has also been associated with macrocytic anemia.
Low The most common causes of microcytic anemia
are iron deficiency, thalassemia, sideroblastic anemia or chronic disease.
In iron deficiency 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 uL.
The mean volume of a jellybean in this group would be 10 uL / 10 jellybeans = 1 uL / jellybean.
A similar calculation works for the MCV. 1.
You measure the RBC index in cells/uL.
Take the reciprocal to convert it to uL/cell.
1/5E6 = 2E-7.
2.
The 1 uL is only made of a proportion of red cells with the rest of the volume composed
of plasma.
Multiply by the hematocrit to take this into account.
2E-7 * 0.4 = 8E-8 uL/cell.
3.
Finally convert the units of uL to fL by multiplying by 10^9.
The result would look like this.
8E-8 uL/cell* = 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