The Mean Corpuscular Volume (MCV) test is a standard component of the Complete Blood Count (CBC), one of the most frequently ordered laboratory tests. The MCV specifically measures the average size of the red blood cells, which transport oxygen throughout the body. Abnormalities in the size of these cells often signal an underlying medical condition, prompting further investigation.
What the MCV Test Measures
The MCV is a calculated value derived from other CBC measurements, primarily the hematocrit and the total red blood cell count. This value represents the average physical size of circulating red blood cells and is expressed in femtoliters (fL).
The primary purpose of the MCV test is to classify the type of anemia a patient may have. Anemia is defined by low levels of hemoglobin or red blood cells, and the size of the cells helps pinpoint the underlying cause. Normal red blood cells, termed normocytic, typically fall within a reference range of 80 to 100 fL for adults.
If the MCV is within this range, the anemia is classified as normocytic, which can be seen in cases of acute blood loss or the early stages of a chronic condition. A low or high MCV suggests the body is producing red blood cells that are either too small (microcytic) or too large (macrocytic).
Interpreting Low MCV Results (Microcytosis)
A mean corpuscular volume below the reference range (typically less than 80 fL) indicates microcytosis, meaning the red blood cells are smaller than average. This reduction in size is usually due to a defect in hemoglobin production, resulting in smaller cells. Microcytosis is frequently associated with microcytic anemia.
Iron Deficiency Anemia (IDA)
The most common cause of microcytosis globally is IDA, where a lack of iron prevents the proper synthesis of hemoglobin. To differentiate IDA, further iron studies are performed. These studies characteristically reveal low serum ferritin (the body’s main iron storage protein), low serum iron, and an elevated total iron-binding capacity (TIBC).
Thalassemia
Thalassemia is a group of inherited disorders affecting the production of globin chains necessary for hemoglobin synthesis. In these conditions, the MCV can be extremely low, sometimes falling below 70 fL. Unlike iron deficiency, the iron studies in thalassemia are typically normal, and diagnosis often requires specialized testing like hemoglobin electrophoresis.
Anemia of Chronic Disease (ACD)
ACD is a third possibility, though it often presents as normocytic anemia. In some cases, the red blood cells may become microcytic, though the MCV rarely drops below 70 fL. In ACD, inflammation causes iron to be sequestered, resulting in low serum iron and transferrin but a normal or elevated ferritin level.
Interpreting High MCV Results (Macrocytosis)
When the MCV result exceeds the normal range (typically 100 fL), it is called macrocytosis, meaning the red blood cells are larger than average. This suggests the red blood cells are not maturing correctly within the bone marrow, leading to the release of oversized cells. Macrocytosis is primarily subcategorized into megaloblastic and non-megaloblastic causes.
Megaloblastic Macrocytosis
Megaloblastic macrocytosis occurs due to impaired DNA synthesis during cell division, resulting in the production of large, immature red blood cell precursors called megaloblasts. The most common drivers are deficiencies in Vitamin B12 (cobalamin) or folate (Vitamin B9), both necessary for genetic material creation. Diagnostic follow-up involves direct measurement of serum B12 and folate levels to confirm the nutritional deficiency.
Non-Megaloblastic Macrocytosis
Non-megaloblastic macrocytosis involves different mechanisms that cause cell enlargement without affecting DNA synthesis. This category includes several distinct causes:
- Chronic alcohol use is a frequent cause, as alcohol can directly affect the red blood cell membrane or interfere with nutrient absorption.
- Liver disease can also lead to macrocytosis by altering the lipid composition of the red blood cell membrane, sometimes resulting in cells that resemble target shapes.
- An increase in reticulocytes, which are young, slightly larger red blood cells, can temporarily elevate the MCV, often seen after significant blood loss.
- Certain medications, such as those used in chemotherapy, can also cause cells to enlarge.
- Hypothyroidism is recognized as a condition that can result in non-megaloblastic macrocytosis, requiring a thyroid panel for further investigation.