What Does a Low MCV Mean?

Is a low MCV always iron deficiency?

No, but iron deficiency is the most common cause by a distance. Red cells are essentially bags of haemoglobin, and haemoglobin needs iron — short of iron, the body builds smaller cells. The giveaway pattern is a low MCV with a low ferritin, and often a high RDW because the cell sizes become more varied as the deficiency develops.

Could it be thalassaemia trait?

Thalassaemia trait — carrying one altered haemoglobin gene — is common in people with Mediterranean, Middle Eastern, South Asian and South-East Asian ancestry. It typically produces a strikingly low MCV with a normal ferritin and a haemoglobin that is normal or only slightly reduced, and it stays remarkably stable year after year. It is a trait, not a disease: it needs no treatment, but knowing about it prevents years of unnecessary iron tablets and matters for family planning if your partner is also a carrier. A haemoglobinopathy screen confirms it.

Low MCV but normal haemoglobin — what does it mean?

Two main possibilities. Early iron deficiency can shrink cells before the haemoglobin falls — worth catching at this stage, before anaemia develops. Alternatively, a lifelong stable low MCV with normal iron points towards thalassaemia trait. Old results are gold here: a low MCV that has been identical for a decade tells a very different story from one that is new.

Which tests come next?

When to see a doctor

Iron deficiency in a man, or in a woman without heavy periods, always deserves a conversation about where iron is being lost — occasionally the answer is slow bleeding from the gut, which needs investigating rather than just topping up iron.

Written by Dr Ibrar Ahmed — MBChB MRCP MRCGP PhD PG Dip (Cardiology)
Specialist interest: Cardiovascular Medicine · OneMedicine, CQC-registered clinic