What MCV measures
MCV is the average size of your red blood cells. It's the most useful single number for working out WHY you're anemic — because different causes make cells that are too small, normal, or too large.
Why the test is done
As part of a CBC. When hemoglobin is low, MCV is what points a clinician toward the cause.
Typical reference ranges
| Band | What it may mean |
|---|---|
| Below typical range (microcytic) | Red cells that are smaller than average. In women this most often means iron deficiency — the body cannot make full-sized, hemoglobin-packed cells without enough iron, and the usual driver is blood loss from heavy periods. Thalassaemia trait, an inherited condition, is the other common cause and is frequently mistaken for iron deficiency for years. Ferritin is the test that tells them apart. |
| Within typical range | Roughly 80–100 fL is the common adult reference range. A normal MCV does not rule out iron deficiency: early on, before stores run down enough to shrink the cells, MCV can still read normal while ferritin is already low. It can also sit in range when a low-MCV cause and a high-MCV cause are present at the same time. |
| Above typical range (macrocytic) | Red cells that are larger than average. Think B12 or folate deficiency first — both are more common with age, with a plant-based diet, after weight-loss surgery, and on long-term metformin or acid-reducing medicines (PPIs), all of which reduce B12 absorption. Alcohol and an underactive thyroid also raise MCV. A high MCV is worth pursuing: B12 deficiency can cause nerve damage that becomes permanent if it's left too long. |
Ranges shown are typical adult values from MedlinePlus, National Library of Medicine (NIH); your own lab's printed range applies to you. View source.
Frequently asked questions
What does a low MCV mean?
A low MCV (below about 80 fL) means your red blood cells are smaller than average — 'microcytic'. In women the most common cause by far is iron deficiency, usually from heavy periods. Thalassaemia trait is the other common cause; a ferritin test is what distinguishes them, so ask for one rather than assuming.
What does a high MCV mean?
A high MCV (above about 100 fL) means larger-than-average red cells — 'macrocytic'. The usual suspects are vitamin B12 or folate deficiency, alcohol, and an underactive thyroid. B12 deficiency is worth chasing promptly because prolonged deficiency can cause nerve damage that doesn't fully reverse.
Can MCV be normal if I'm iron deficient?
Yes. MCV falls late in iron deficiency — ferritin drops first, then hemoglobin, and cell size shrinks as stores stay empty. A normal MCV with a normal hemoglobin still doesn't rule out iron deficiency, which is why ferritin is the test to ask for when you're fatigued.