What does MCV mean in a blood test?
MCV is the abbreviated form of Mean Corpuscular Volume (or Mean Cell Volume), which is an important component of the common blood test called Full Blood Count (FBC or CBC). Mean corpuscular volume (MCV) is a measure of the average volume of red blood corpuscles (or red blood cells – RBC) in a person’s body. Thus, it gives an idea about the average size of each red blood cell inside someone’s body.
How is MCV Calculated?
There are several different methods to calculate the MCV. Most of the modern blood analyzer equipment calculate the MCV automatically, when a sample of blood is introduced to it.
Other manual methods of calculating the MCV using the values of haematocrit (or packed cell volume -PCV) and red blood cell count (RBC), are as follows:
- MCV = (PCV [in L/L] / RBC count [in ×1012/L]) × 1000
- MCV = PCV [in L/L]/ RBC count [in millions/ml]
- MCV = PCV (%) × 10 / RBC count [in 1012/L]
The value of MCV is usually expressed in femtoliters (fL) which is equivalent to 10-15 litres. It should be noted that the above equations are to express the value of MCV in femtoliters.
Normal Range of MCV
The normal range for MCV is usually considered as 80-95 fL (per red blood cell) in adults. If the MCV is within this normal range, the red blood cells are said to be normocytic(normal in size); if the MCV is lesser than 80 fL, the red blood cells are considered to be microcytic (smaller than usual); and if it is greater than 95fL, the red blood cells are considered to be macrocytic (larger than usual).
Possibilities for Abnormal levels of MCV
The size of red blood cells can fall outside the normal range – hence resulting in abnormal levels of MCV – due to many reasons. Some of these may be due to normal variations in the physiological condition of a person, or due to variations in age. Other reasons include a variety of pathological conditions (i.e. disease conditions) ranging from anaemias to cancers.
There are considerable variations of MCV in newborn babies (neonates) and infants, when compared to the normal range in adults. Pregnancy is also another situation where the MCV may fall outside the normal range.
The most common clinical entity related to abnormalities in MCV is anaemia (abnormally low haemoglobin concentration in blood). There are 3 categories of anaemia in relation to red blood cell size (MCV) and red blood cell haemoglobin concentration (mean corpuscular haemoglobin – MCH). These types of anaemia include:
- Normochromic, normocytic anaemia – with normal cell size and mean corpuscular haemoglobin (MCH)
- Hypochromic, microcytic anaemia – with smaller cell size and low MCH
- Macrocytic anaemia – with larger cell size
Each of these types of anaemia could be caused by a variety of disease conditions which will be dealt with in detail in the following sections.
Causes for low MCV
Iron deficiency anaemia
This is the most common cause of anaemia in the world, and is an important cause for hypochromic microcytic anemia with low MCV and MCH. Red blood cells contain haemoglobin (Hb) molecules which are responsible for transporting oxygen. Each of these haemoglobin molecules consists of two parts; a protein part made of α and β chains and an iron containing porphyrin part known as ‘haem’ which contains iron as Fe 2+. Thus, reduction in body iron levels causes reduced haemoglobin formation and results in hypochromic microcytic red blood cells, and a reduced MCV.
Iron deficiency can be due to several reasons such as:
- Insufficient intake (vegan diet,limited diet, malnutrition)
- Decreased absorption (high gastric acidity, gastric surgeries, vitamin C deficiency)
- Increased body needs (adolescence, menstruation, pregnancy, lactation, cancer)
Thalassemia is a hereditary disease which is caused by a defect in the synthesis of α or β globin chains of haemoglobin molecules. In thalassemia major there are either fewer or no β chains available and in α thalassemia there are either fewer or no α chains available. Both types of thalassemia causes defective haemoglobin synthesis and smaller red blood cell size, which results in low MCV.
Any long standing inflammatory conditions (arthritis, systemic lupus erythematosus etc.) or infectious conditions (pulmonary abscesses, tuberculosis,osteomyelitis, pneumonia, bacterial endocarditis etc.) or non-infectious conditions (sarcoidosis, Crohn’s disease, Gaucher’s disease) or malignant conditions (carcinoma, lymphoma, sarcoma) or chronic kidney diseases can cause disturbed red blood cell production due to many reasons such as:
- Decreased availability of iron
- Decreased level of erythropoietin, a hormone which increases the red cell production
- Mild decrease in the life span of red blood cells to 70-80 days (normally 120 days)
- Cytokines ( interleukin 1 and 6) , tumor necrosis factor (TNF – α ) produced by the abnormal cells causes destruction of red blood cell precursors and decrease the number of eryhropoitein receptors in the progenitor cells.
This condition arises due to a defect in the incorporation of iron(Fe2+) into the haem part of the haemoglobin molecule which leads to defective formation of haemoglobin and thus results in low MCV. This condition can arise due to congenital reasons (mitochondrial diseases) or acquired reasons (myelodysplastic syndrome, deficiency of copper and vitamin B6, lead poisoning, alcohol, drugs such as anti-tuberculosis, hypothermia etc.)
Infants and young children also have a low level of MCV, which rises gradually with age.
Causes for high MCV
High MCV is usually caused by conditions that lead to macrocytic anaemia. Macrocytic anaemia is commonly caused by deficiency of vitamin B12(cobalamin) or folate.Some abnormalities in the metabolism of vitamin B12 or folate(e.g. transcobalamin deficiency, use of antifolate drugs) also cause macrocytic anaemia. Genetic or acquired enzyme deficiencies that lead to defective DNA synthesis (e.g. orotic aciduria, heavy alcohol use) may also cause macrocytic anaemia.
Deficiency of vitamin B12 or folate, and associated abnormalities in their metabolism, lead to defective DNA synthesis within red blood cell precursors (erythroblasts) in the bone marrow. This leads to abnormally large volume of cytoplasm remaining in such cells (within bone marrow) which are called as megaloblasts.Therefore, the macrocytic anaemia due to aforementioned causes is also known as megaloblastic anaemia.The peripheral blood film (or blood picture) in this condition shows oval macrocytes, and the MCV is usually higher than 98 fL, or may even reach up to about 120-140 fL in severe cases.
Other causes of macrocytic (non-megaloblastic) anaemia leading to high MCV include:
- Heavy use of alcohol(may cause high MCV even in the absence of anaemia)
- Liver disease
- Myelodysplastic syndrome
- Cytotoxic drugs (e.g. methotrexate, 6-mercaptopurine, cyclophosphamide, trimethoprim etc.)
- Aplastic anaemia
Normal variations in the physiological condition of a person may also sometimes give rise to a high MCV. Pregnancy is one such important condition where the MCV is raised beyond the usual normal range. In an uncomplicated pregnancy, the MCV may rise by up to 4-5 fL than the normal level.
Newborn babies also have a high MCV during their initial few weeks, starting at an average level of about 120 fL. This drops progressively until about one year. Infants and small children usually have a low level of MCV (around 70-80 fL), which rises gradually with age and reaches normal adult levels at puberty