Mean corpuscular volume, or MCV, is a measurement of the average size of your red blood cells. It’s reported in femtoliters (fL), and a normal value falls between 80 and 100 fL. MCV is part of a standard complete blood count (CBC), one of the most commonly ordered blood tests, and it helps doctors figure out why you might be anemic or flag conditions you didn’t know you had.
How MCV Is Measured
Modern blood analyzers measure MCV directly by passing individual red blood cells through a tiny opening with an electrical current running across it. As each cell passes through, it briefly disrupts the current. The size of that disruption is proportional to the cell’s volume, so the machine can measure each cell individually. Some analyzers use light scatter instead of electrical impedance, or a combination of both. The machine counts thousands of cells, measures each one, and produces a histogram showing the distribution of sizes. The MCV on your lab report is the average of all those measurements.
Your doctor can also calculate MCV from two other values on your CBC: hematocrit (the percentage of your blood occupied by red cells) divided by the red blood cell count in millions. But in practice, the automated measurement is what appears on your results.
What a Normal MCV Looks Like
For adults, the normal range is 80 to 100 fL. Red blood cells below 80 fL are considered small (microcytic), and those above 100 fL are considered large (macrocytic). During pregnancy, MCV rises by an average of 4 fL in healthy women with adequate iron intake, so a value slightly above your usual baseline isn’t automatically concerning if you’re pregnant.
It’s worth knowing that certain conditions can produce falsely elevated readings. Cold agglutinins, which are antibodies that cause red blood cells to clump together at low temperatures, are one well-known culprit. When cells clump, the analyzer reads the clump as a single large cell, artificially inflating the MCV.
Low MCV: Small Red Blood Cells
The two most common causes of microcytosis are iron deficiency anemia and thalassemia trait. Less common causes include lead toxicity and chronic disease.
Iron deficiency anemia develops when your body’s iron supply can’t keep up with demand. That mismatch can come from inadequate dietary intake, increased needs (like during pregnancy), blood loss, or poor absorption in the gut. Blood loss and malabsorption tend to produce more significant anemia than diet alone. On lab work, a ferritin level below 15 ng/mL in an otherwise healthy person strongly suggests iron deficiency, while a level above 100 ng/mL generally rules it out. In people with chronic inflammation, the cutoff shifts upward to around 50 ng/mL because inflammation artificially raises ferritin.
Thalassemia trait is a genetic condition in which the body underproduces one of the protein chains that make up hemoglobin. In beta-thalassemia trait (the most common form), the anemia is usually mild, with hemoglobin rarely dropping below 9.3 g/dL. But the MCV can dip much lower than it does with iron deficiency alone. Alpha-thalassemia trait, caused by the deletion of two genes, often produces small red blood cells without any anemia at all.
Telling Iron Deficiency From Thalassemia
Both conditions shrink red blood cells, so the MCV alone can’t distinguish between them. One simple tool doctors use is the Mentzer index: MCV divided by the red blood cell count. In iron deficiency, the body produces fewer red blood cells and they’re small, so the index tends to be above 13. In thalassemia, the body produces a normal number of cells but they’re smaller and more fragile, pushing the index below 13. This is a screening shortcut, not a definitive test. A firm thalassemia diagnosis requires hemoglobin electrophoresis, a test that separates out the different types of hemoglobin in your blood.
High MCV: Large Red Blood Cells
Macrocytosis has two broad categories, and the distinction matters because the causes and treatments are very different.
Megaloblastic Causes
The most common reason for large red blood cells is megaloblastic anemia, which happens when cells can’t properly copy their DNA. The cell keeps growing (because other cellular machinery works fine) but divides slowly, producing fewer, oversized red blood cells. Vitamin B12 deficiency and folate deficiency are the classic triggers.
Dietary B12 deficiency is rare in most populations and typically shows up in elderly people eating very limited diets or in strict vegans. More often, deficiency results from absorption problems: pernicious anemia (an autoimmune condition that destroys the stomach cells needed to absorb B12), prior stomach surgery, or diseases affecting the small intestine. Folate deficiency, meanwhile, can stem from poor diet, pregnancy (which increases folate requirements), alcohol use disorder, or conditions that cause rapid red blood cell turnover like sickle cell disease.
Certain medications also impair DNA synthesis and drive MCV up. This includes some chemotherapy drugs and the HIV medication zidovudine. In fact, doctors sometimes monitor zidovudine compliance by checking whether a patient’s MCV has risen, since an increasing value suggests the person is actually taking the drug.
Non-Megaloblastic Causes
Not all macrocytosis involves faulty DNA synthesis. Liver disease and obstructive jaundice cause large red blood cells through a completely different mechanism: excess cholesterol and fat molecules deposit onto the cell membrane, expanding its surface area. Alcohol use disorder can cause macrocytosis through this pathway even when folate levels are normal. Chronic obstructive pulmonary disease (COPD) is another recognized cause.
Drug-induced macrocytosis without anemia is actually the most common cause of elevated MCV in people who don’t drink heavily. Metformin (widely prescribed for diabetes), certain seizure medications, and even oral contraceptives can raise MCV. In most of these cases, the large cells don’t cause symptoms or require treatment on their own.
How Doctors Use MCV Alongside Other Tests
MCV doesn’t work in isolation. One of the most useful companion values is the red cell distribution width, or RDW, which measures how much variation there is in the size of your red blood cells. A normal RDW means your cells are fairly uniform in size. A high RDW means there’s a wide spread, with some cells much bigger or smaller than others.
Combining MCV and RDW helps narrow down the cause of anemia. For example, a low MCV with a high RDW points toward iron deficiency, where the body is producing progressively smaller cells as iron stores deplete. A low MCV with a normal RDW is more consistent with thalassemia trait, where cells are uniformly small. This pairing is a starting framework, not a final answer. Doctors typically follow up with additional blood work, including iron studies, B12 and folate levels, or a peripheral blood smear (where a technician examines your blood cells under a microscope to look at their shape and appearance) depending on which direction the initial results point.
What Your MCV Result Means for You
A single MCV value outside the normal range isn’t a diagnosis. It’s a signal that something is worth investigating. A mildly low MCV in a menstruating woman is a very different clinical picture than a very low MCV in a child from a region where thalassemia is common. Similarly, a mildly elevated MCV in someone taking metformin is expected and usually harmless, while a significantly elevated MCV with symptoms like fatigue, numbness, or difficulty concentrating could point to a B12 deficiency that needs treatment.
If your MCV comes back abnormal, the next step is usually a targeted set of follow-up tests based on whether the value is high or low, how far outside the range it falls, and what other CBC values look like. The combination of these results, not the MCV alone, is what tells the story.