MCV Blood Test High: 8 Serious Causes You Shouldn’t Ignore
Many patients are surprised when their blood test results show MCV blood test high. A high MCV does not directly indicate any diagnosis — it is just more of a diagnostic clue.
In this guide I will explain what MCV blood test high means and what its clinical significance is, based on my experience and peer-reviewed resources.
What is MCV?
MCV (Mean Corpuscular Volume) measures the average size of your red blood cells. It is one part of the complete blood count (CBC), also sometimes referred to as a full blood count (FBC), which is used to assess blood health.
A normal MCV is 80–100 femtolitres (fL) [1]. When your results show MCV blood test high, it means your red blood cells are larger than normal — a condition called macrocytosis [2].
Macrocytosis is not always pathological, but some vitamin deficiencies, heavy alcohol consumption, and certain bone marrow diseases may cause it.
Why Does MCV Blood Test High Matter?
Red blood cells carry oxygen to the tissues. When macrocytosis reflects underlying disrupted erythropoiesis or is accompanied by anemia, it can impair adequate oxygen delivery to tissues [3].
We doctors treat MCV blood test high as an early warning sign. It’s not specific to any disease and it doesn’t directly diagnose anything, but it prompts further testing [3].
Typically, a high MCV is associated with a deficiency in vitamin B12 or folate. Other causes of high MCV include alcohol intake, liver or thyroid disorders, certain medications, or bone marrow conditions [3][4].
What Counts as “High MCV”?
Although laboratory reference ranges vary slightly, typical cutoffs are:
- 80–100 fL: considered “normal.” These reference value limits are defined so that the large majority of the healthy population falls within this range.
- >100 fL: considered macrocytosis or “high MCV”
- >115 fL: particularly suggestive of megaloblastic anemia, often associated with vitamin B12 or folate deficiency or significant bone marrow changes [1][3]
Mild macrocytosis often does not require special measures, but monitoring and repeat testing are recommended.
It is important to be aware that a mixed situation — for example, simultaneous deficiency of iron and vitamin B12 and/or folate — can have opposite effects and mask the MCV value, making it appear normal.
Common Causes of MCV Blood Test High
Macrocytosis can be divided into megaloblastic and non-megaloblastic:
- Megaloblastic macrocytosis is caused by disruption of DNA synthesis. Common causes include vitamin B12 and folate deficiency.
- Non-megaloblastic macrocytosis is due to causes other than disruption of DNA synthesis. Common causes include alcohol use, liver disease, hypothyroidism, certain medications, bone marrow disorders, and reticulocytosis.
1. Vitamin B12 Deficiency
In clinical situations, I often encounter megaloblastic anemia caused by vitamin B12 deficiency. This is one of the most common causes of MCV blood test high [5].
Vitamin B12 is required for DNA synthesis and red blood cell formation. When DNA synthesis is disrupted, cell division slows down, although hemoglobin production continues. In this case, the dividing cells become larger and contain more hemoglobin.
Common causes:
- Vegan-focused diet without supplements
- Pernicious anemia — a condition in which vitamin B12 is not absorbed due to a deficiency of intrinsic factor
- Long-term use of metformin or acid-reducing medications [6]
I occasionally see patients who follow a completely vegan diet. In my experience, they are often aware of the lack of vitamin B12 in a vegan diet and know how to compensate. On the other hand, pernicious anemia has often come as a surprise to my patients, and megaloblastic anemia may reveal a vitamin B12 deficiency.
2. Folate (Vitamin B₉) Deficiency
Folate deficiency also interferes with DNA synthesis, which can cause macrocytosis showing up as a high MCV blood test.
Causes:
- Lack of vegetables in the diet
- Medications (e.g. methotrexate, anticonvulsants)
- Excessive alcohol consumption
- Rapid cell turnover (such as pregnancy, hemolysis, chronic bleeding, lactation, inflammation, malignancies)
- Malabsorption (e.g. celiac disease, Crohn’s disease) [7]
It is important to remember that B12 and folate deficiency may occur simultaneously. Therefore, an accurate differential diagnosis before replacement therapy is essential.
In my work, I encounter patients whose risk of folate deficiency is increased by methotrexate and other cytostatic drugs used to treat severe autoimmune diseases such as rheumatoid arthritis. Rheumatologists in Finland most often routinely start patients on folate supplements, which reduces the risk.
3. Alcohol Use
Alcohol causes macrocytosis through several mechanisms. First, alcohol is a bone marrow toxin, and second, it disrupts cell membranes [8][9].
MCV elevation has been used clinically as an indirect crude measure of alcohol consumption, although CDT (Carbohydrate-Deficient Transferrin), GT (Gamma-glutamyltransferase) and PEth (Phosphatidylethanol) are now more commonly used due to their sensitivity.
After abstinence, the high MCV blood test typically normalises within a few months, though the exact timeline varies between individuals. Liver values tend to return to normal more quickly.
From time to time, a patient’s alcohol use comes up during routine examinations. Understandably, patients who consume alcohol do not always want alcohol-specific tests such as CDT and PEth, so a high MCV is often precisely what raises suspicion first.
4. Liver Disease
Fatty liver, hepatitis, or cirrhosis can alter the red cell membrane, producing MCV blood test high values, as excess lipids accumulate on the surface of red blood cells, increasing the cell surface area without increasing the amount of hemoglobin [3].
Fatty liver is a fairly common finding in my practice, and during regular check-ups the condition is usually revealed by slightly elevated liver values. In my patients, these are often dietary in origin.
5. Hypothyroidism
Hypothyroidism suppresses bone marrow function, slowing erythropoiesis, which may lead to MCV blood test high [11]. The exact mechanism is multifactorial and not fully established.
Hypothyroidism is a common underlying condition in my patients, often identified by thyroxine medication on the medication list. Often, a high MCV also corrects after starting thyroxine if the macrocytosis is due to hypothyroidism alone.
6. Medications
Some drugs directly slow down red blood cell DNA synthesis or affect the bone marrow directly, such as chemotherapy agents, some anticonvulsants, and some antiretrovirals [12][3]. You should consult your doctor about your medications if your MCV blood test high persists.
Some of my patients who receive cytostatic drugs also receive folate supplements, as some of these drugs interfere with DNA synthesis, which can also manifest as macrocytosis and an increase in MCV.
7. Bone Marrow Disorders
Especially in elderly patients, macrocytosis can signal myelodysplastic syndromes (MDS), which affect blood cell production [13]. Early recognition allows for appropriate risk stratification and treatment planning. These typically require comprehensive care from a hematologist.
In my practice, I have mainly encountered myelodysplastic syndromes in elderly patients, as they are rare in young patients.
8. Reticulocytosis
In anemia and hemolysis, the bone marrow produces more new red blood cells, seen as reticulocytosis. Reticulocytes are larger than mature red blood cells and can temporarily make MCV blood test high [3].
I have occasionally encountered reticulocytosis in bleeding patients. Most often the chronic bleeding is due to gastrointestinal bleeding, though heavy menstruation can also be a cause.
How Doctors Evaluate MCV Blood Test High
Evaluation usually includes:
- Full blood count (for anemia pattern and RDW)
- Vitamin B₁₂ and folate levels
- Liver enzymes and thyroid tests
- Alcohol and medication review
- Peripheral smear examination (oval macrocytes vs. round macrocytes)
- Bone marrow studies if malignancy or MDS is suspected [3][13]
It is worth noting that many patients who consume large amounts of alcohol tend to downplay their intake, so careful history-taking is important.
Symptoms Associated with MCV Blood Test High
You may have no symptoms at all, or you might notice signs from the underlying cause:
- Fatigue or weakness
- Tingling or numbness (B12)
- Jaundice/Icterus or abdominal fullness (liver)
- Weight gain or sluggishness (thyroid)
- Cognitive or mood changes
Every now and then I encounter patients with jaundice. I usually refer them to the hospital for acute evaluation, as the cause has often been malignancy, cholestasis or acute hepatitis.
What To Do If You Have MCV Blood Test High
- Usually there’s no need to panic: mild elevations are common.
- Improve diet: increase foods rich in B₁₂ (meat, eggs, fish) and folate (leafy greens, legumes).
- Reduce alcohol intake.
- Review medications with your doctor.
- Re-test after 6–12 weeks or sooner if symptoms persist.
In practice, I usually arrange repeat laboratory testing after 1–2 months if there is no clear explanatory factor. Fortunately, it is often just a harmless incidental finding.
Is It Dangerous?
MCV blood test high itself isn’t harmful — the cause might be. Fortunately, most of the time a single high MCV value does not lead to any serious diagnoses.
Nutritional deficiencies and thyroid or liver problems are treatable. Unaddressed B12 deficiency or marrow disorders require more urgent attention.
How to Lower MCV Blood Test High
| Underlying Cause | Treatment |
|---|---|
| Vitamin B₁₂ deficiency | Oral or injectable B₁₂ therapy |
| Folate deficiency | Dietary folate or supplements |
| Alcohol-related | Abstinence or reduction |
| Hypothyroidism | Thyroxine replacement |
| Drug-induced | Dose review or substitution |
| Liver disease | Treat underlying cause |
MCV usually returns to normal within 1–4 months once the cause is corrected.
Prognosis
Most cases of MCV blood test high are reversible. Nutritional and alcohol-related causes typically resolve relatively quickly, while chronic diseases may take longer. Early recognition of MDS allows for appropriate management [13].
Unfortunately, many patients downplay their alcohol consumption and are often not ready to give up alcohol, while dietary changes are often easier to implement.
How MCV Relates to Other Blood Tests
A high MCV is often an incidental finding. In practice, MCV is almost always part of a complete blood count (CBC), in which other blood cell-related parameters are routinely measured.
When your MCV blood test high result appears, it often helps to look at related values:
- MCH (Mean Corpuscular Hemoglobin): High MCH often accompanies high MCV because larger cells hold more hemoglobin.
- MCHC (Mean Corpuscular Hemoglobin Concentration): May stay normal even when MCV is elevated.
- RDW (Red Cell Distribution Width): A high RDW with a high MCV can suggest mixed deficiencies or recovery from anemia.
- MPV (Mean Platelet Volume): Measures average platelet size — a separate parameter from MCV. People curious about both can read my closely related guide on MPV blood test.
Together, these parameters help clinicians determine whether a high MCV reflects nutritional deficiency, marrow stress, or medication effects.
Frequently Asked Questions
1. What does MCV blood test high mean? Your red blood cells are larger than normal. Common causes include vitamin B12/folate deficiency, alcohol use, liver disease, or thyroid disorders.
2. Is it serious? Usually not by itself, but persistent elevation needs evaluation to rule out treatable causes.
3. Can dehydration cause MCV blood test high? Dehydration affects plasma volume rather than red cell size, so it is not a recognised cause of true macrocytosis.
4. Can medications or alcohol raise MCV? Yes. Anticonvulsants, chemotherapy drugs, and alcohol can enlarge red cells.
5. Can iron deficiency hide MCV blood test high? Yes — mixed iron and B₁₂/folate deficiency may normalise MCV, so both iron and vitamin levels should be tested.
6. How long does it take to normalize? Usually 1–4 months after treatment.
7. When should I repeat my test? In 6–12 weeks, or earlier if symptoms worsen.
References
[1] https://www.ncbi.nlm.nih.gov/books/NBK545275/
[2] https://www.ncbi.nlm.nih.gov/books/NBK560908/
[3] https://pmc.ncbi.nlm.nih.gov/articles/PMC1570488/
[4] https://pubmed.ncbi.nlm.nih.gov/7234509/
[5] https://pubmed.ncbi.nlm.nih.gov/22254022/
[6] https://pubmed.ncbi.nlm.nih.gov/23301732/
[7] https://www.ncbi.nlm.nih.gov/books/NBK560908/
[8] https://pubmed.ncbi.nlm.nih.gov/4132788/
[9] https://pubmed.ncbi.nlm.nih.gov/8747507/
[10] https://pmc.ncbi.nlm.nih.gov/articles/PMC1570488/
[11] https://pubmed.ncbi.nlm.nih.gov/1257398/
[12] https://academic.oup.com/cid/article-abstract/29/2/459/274474
