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Yuria Ito: Does an MCV Below 100 Mean There Is No Vitamin B12 Deficiency?

Yuria Ito/LinkedIn

Oct 14, 2026, 18:20

Yuria Ito: Does an MCV Below 100 Mean There Is No Vitamin B12 Deficiency?

Yuria Ito, Nutrition Therapist, shared a post on LinkedIn:

”Does an MCV below 100 mean there is no vitamin B12 deficiency?

MCV (mean corpuscular volume) is a blood test parameter indicating the average volume of a red blood cell. It is used to classify anemia.

Textbook classifications are:

Microcytic: MCV less than 80 fL
Normocytic: MCV 80 to less than 100 fL
Macrocytic: MCV greater than or equal to 100 fL

(Some sources define macrocytosis as MCV greater than 100 fL.)

Vitamin B12 and folate deficiencies are well-known causes of macrocytic anemia. Both nutrients are involved in DNA synthesis, and deficiency can impair red blood cell maturation, causing the cells to enlarge.

But can we rule out these deficiencies when MCV is below 100 fL?

The answer is Nօ.

MCV can remain normal even with vitamin B12 deficiency, especially when iron deficiency coexists.

For example:

Ferritin: 8 ng/mL
MCV: 98 fL

Ferritin below 10 ng/mL suggests severely depleted iron stores.

Iron deficiency generally tends to make red blood cells smaller.

Yet this person’s MCV is 98 fL—within the normocytic range, but near its upper limit.

Rather than simply saying ‘MCV is normal, so there is no problem,’ I hope we ask:

‘Why is MCV as high as 98 when ferritin is this low?’

‘If iron deficiency tends to lower MCV, might it otherwise be even higher?’

In other words, iron deficiency may mask the tendency toward larger red blood cells caused by vitamin B12 or folate deficiency.

Of course, these values alone cannot establish either deficiency, and iron deficiency does not always cause an MCV below 80 fL.

But they can prompt us to consider multiple nutrient deficiencies.

MCV is only an average.

When small and large red blood cells coexist, the average can fall within the normal range.

In a 2015 study of patients with oral mucosal diseases, 108 of 149 patients (72.5 percent) with both iron and vitamin B12 deficiencies had a normal MCV.

So even with both deficiencies, MCV does not necessarily reach 100 fL.

RDW (red cell distribution width) and peripheral blood smears can also help with assessment.

The UK’s 2024 NICE guideline on vitamin B12 deficiency explicitly states that deficiency should not be ruled out solely because anemia or macrocytosis is absent.

Neurological symptoms, such as numbness and gait disturbances, may appear before anemia becomes evident. In dentistry, glossitis can also be a clue.

Macrocytosis can also result from alcohol use, liver disease, hypothyroidism, medications, or bone marrow disorders.

Assessment should therefore consider hemoglobin, red blood cell count, RDW, ferritin, vitamin B12, folate, symptoms, and medical history together.

An MCV of 100 fL or more suggests macrocytosis.

But an MCV below 100 fL does not rule out vitamin B12 or folate deficiency.

I hope this becomes more widely understood.

Some may think, ‘Isn’t that obvious?’

But not everyone knows it, so I wanted to share.

This is deeply relevant to dentistry, too, isn’t it?”

Yuria Ito: Does an MCV Below 100 Mean There Is No Vitamin B12 Deficiency?