High MCV On Your Blood Test? What It Means (And What Nobody Told You)

That little "H" next to MCV means your red blood cells are bigger than they should be. On its own it isn't a diagnosis, and it usually isn't an emergency. But it is a clue, and it's one that often gets skipped.

"You're not anemic. It's nothing to worry about." We've read that line in thousands of stories from people trying to make sense of their blood work, and most of them were still tired, still pale, still stopping halfway up the stairs. If that's you, here's what your MCV is telling you, what usually causes it, and the three questions worth taking to your next appointment.

What does high MCV mean?

MCV (mean corpuscular volume) measures the average size of your red blood cells. In adults, an MCV above 100 femtoliters (fL), a unit made for something as small as a cell, is called macrocytosis: red cells larger than normal. Cleveland Clinic gives 80 to 100 fL as the normal range, but your lab prints its own beside your result, so check yours.

Macrocytosis isn't a condition in itself. It's a description, like a flag on a map. The useful question is what put it there.

Where do I find MCV on my blood test?

Look in the red cell section of your complete blood count (CBC). A high result is usually marked "H", in bold, or in red. Three numbers sit close together:

• MCV: the average size of your red cells.
• MCH: the average amount of hemoglobin, the part that carries oxygen, in each red cell.
• RDW: how much your red cells differ in size. Healthy red cells are usually about the same size.

My MCV and MCH are both high. Should I worry?

It usually points to one thing, not two. A bigger red cell tends to carry more hemoglobin, and MedlinePlus, the US National Library of Medicine's health site, lists anemia caused by a vitamin B deficiency as a reason for both large red cells and a higher MCH. Treat it as one question to follow up: why are my cells big?

What causes a high MCV?

The number tells you the cells are big. It doesn't say why. Cleveland Clinic and MedlinePlus list these causes:

• Low vitamin B12 or folate. One of the most common reasons, and one of the first to check.
• Drinking too much alcohol, and liver disease.
• An underactive thyroid (hypothyroidism).
• Some medicines, including certain cancer drugs, HIV medicines and hydroxyurea.
• A rush of young red cells, which are larger than mature ones, after heavy blood loss or when red cells break down too early.
• Less often, a bone marrow condition such as myelodysplastic syndrome.

Small red cells usually point the other way, toward low iron. And if iron and B12 or folate run low at the same time, the small and the big can average out, so your MCV can look normal.

Does high MCV mean cancer?

Usually not. Cleveland Clinic puts it plainly: "Often, macrocytosis isn't a sign of something serious." Most high MCV results come from common, checkable causes like the ones above.

A bone marrow condition is one of the less common causes, and that's a good reason to follow a high MCV up instead of shrugging it off. A few simple blood tests usually rule the common causes in or out first.

Is a high MCV serious? What MCV level is too high?

On its own, a high MCV is a clue, not an emergency. Cleveland Clinic notes that a very high MCV, from about 110 to 115 fL, often signals a more severe form of vitamin-related anemia, called megaloblastic anemia, so the further above the range you are, the sooner it's worth looking into.

Go back to your doctor sooner if your white cells or platelets are also off, or if you notice tingling, numbness, balance problems or a sore, smooth tongue.

Can you have a high MCV without anemia?

Yes. Cleveland Clinic reports that about 60% of people with macrocytosis also have anemia, which means many don't. It's more common without anemia in older adults.

That's why "you're not anemic" doesn't close the question. Big cells can show up before hemoglobin drops. The reverse matters too: a normal MCV doesn't rule out low B12, and in studies of people with B12-related nerve problems, many had a normal blood count.

Does high MCV make you tired?

The size of your cells doesn't make you tired. The cause behind it can. When low B12 or folate leads to anemia, MedlinePlus lists shortness of breath, weakness or fatigue, headache, dizziness and a heartbeat that feels off as common signs.

Why does low B12 or folate make red cells big?

Your bone marrow makes new red cells all day, every day. Before each new cell splits in two, it copies its DNA, and folate supplies the building blocks for that copy.

Your blood carries folate in a locked form. B12 is the key that opens it so the cell can use it. If folate runs short, or B12 isn't there to open it, the copying stalls while the cell keeps growing. It splits late and comes out too big. Many of these oversized cells never make it out of the marrow, and that's how the shortfall can end in fewer working red cells and that heavy, tired feeling on the stairs.

It isn't your age, and it isn't that you're not trying hard enough. Your cells may be missing a piece. Read more about the lock and the key.

What should I ask my doctor about a high MCV?

Take these three with you:

1. "Can we check my B12 and folate?" These are the first two to rule in or out.
2. "If those look normal but I still have tingling, numbness or tiredness, can we check MMA or active B12?" MMA (methylmalonic acid) builds up when your cells are short on B12. Active B12 is the part your cells can take in. Kidney problems can raise MMA too, so your doctor reads it alongside your other results.
3. "Could my thyroid, my liver, alcohol or any of my medicines play a part?"

If you already take B12 or folate, mention it before your blood is drawn. Supplements can change the result.

Can a high MCV be reversed? How long does it take?

Often, yes. Cleveland Clinic notes that addressing the cause often reverses macrocytosis. If a shortfall of folate or B12 was the cause and it's put right, the marrow starts making cells at the right size again.

It takes time, though. Red cells live about 120 days, so the old big ones are replaced slowly. Hemoglobin often recovers over weeks, while cell size takes longer to settle. Judge it by your next blood test, not by how you feel on day three.

Questions people ask

Is an MCV of 100 high?
It's the line Cleveland Clinic uses: above 100 fL is macrocytosis. Your lab's own range is printed beside your result.

Why would MCV be only slightly high?
Mild rises have the same list of causes, and sometimes no clear cause is found, especially without anemia. It's still worth a B12 and folate check.

Should I start B12 before my blood test?
Ask your doctor first. Testing before you start gives the clearest answer, because supplements can change the numbers.

Can food bring a high MCV down?
Food folate and B12 matter. But if the problem is getting B12 in, which gets harder as we age or on long-term acid reflux pills or metformin, food alone may not fix it. See 7 daily habits for healthy red cells.