MCV blood test: what high or low MCV means

What MCV measures

Your red blood cells carry oxygen. A CBC counts them and measures them, and MCV is the average volume of one cell, reported in femtoliters (fL).

MCV is one of the red cell indices, read together with MCH (how much hemoglobin each cell carries), MCHC (how concentrated that hemoglobin is) and RDW (how much cell sizes vary). Together they help a clinician tell one kind of anemia from another.

What a low MCV can mean

An MCV below 80 fL is called microcytosis: smaller than usual red cells. Common causes:

• Iron deficiency. The most common cause in the United States. Check it with ferritin and an iron panel; see ferritin levels.
• Thalassemia trait. An inherited difference in hemoglobin. MCV is often low while the red cell count is normal or high.
• Anemia of chronic disease. Long-term inflammation or illness can limit how the body uses iron.
• Less common: lead exposure and sideroblastic anemia.

MCV is a late sign of low iron. Ferritin falls first, and MCV usually drops only once hemoglobin is falling too, so a normal MCV does not rule out low iron. A low-normal MCV is still worth a ferritin check, which is one reason Gabriel tracks the optimal range as well as the lab range.

What a high MCV can mean

An MCV above 100 fL is called macrocytosis: larger than usual red cells. Common causes:

• Low vitamin B12 or folate. Both are needed to make red cells. Long-term metformin use and acid-reducing medications can lower B12.
• Alcohol. Long-term heavy drinking is one of the most common causes.
• Medications. Some chemotherapy, anti-seizure and HIV medications raise MCV.
• Liver disease and an underactive thyroid.
• A recent rise in young red cells (reticulocytes), for example after blood loss.
• Less common: bone marrow conditions such as myelodysplasia, which a clinician rules out when other causes don't fit.

What to do with an abnormal result

1. Read it with the rest of the CBC. Hemoglobin and hematocrit show whether you are anemic; RDW, MCH and MCHC narrow down the cause.
2. Ask about the follow-up tests that fit. For a low MCV that usually means ferritin and an iron panel. For a high MCV, vitamin B12, folate, thyroid (TSH) and liver tests.
3. Bring your medication list and drinking habits to the conversation. They explain many high results.
4. Retest after a change. Red cells live about 120 days, so MCV moves slowly. A recheck after about three months shows whether a fix is working.

See a clinician promptly if an abnormal MCV comes with shortness of breath, chest pain, fainting, black or bloody stools, or numbness and tingling in your hands or feet.