Mean corpuscular volume (MCV)
Blood count (CBC) ·Also called: MCV, mean cell volume, red cell size
What it is
MCV is the average volume of a single red blood cell, calculated by the analyser from the cell count and the haematocrit. It is one of the red cell indices, alongside MCH and MCHC, and it is only interpreted properly in the context of whether an anaemia is present.
Why your vet measures it
When a pet is anaemic, MCV helps sort the cause into categories before any expensive testing. Large cells suggest a marrow actively replacing losses. Small cells suggest iron is limiting, which redirects the search towards chronic bleeding.
What a high result can mean
- A regenerative anaemia — young red cells are bigger than mature ones, so active replacement raises the average
- Miniature and toy Poodles, some of which carry a harmless inherited macrocytosis
- FeLV infection in cats, which can enlarge cells without any regeneration
- Storage artefact in a sample that sat too long before running
What a low result can mean
- Iron deficiency from slow chronic blood loss — fleas, gut bleeding, or a bleeding tumour
- A portosystemic shunt, a liver blood-vessel abnormality seen in young dogs
- Akitas, Shiba Inus and other Japanese breeds, which naturally have smaller red cells
Dogs and cats differ
Cats have smaller red cells than dogs across the board, so the two species have entirely separate reference intervals and the numbers are not comparable. Breed matters more than usual here: a low MCV in a Shiba Inu and a low MCV in a Beagle mean quite different things.
Questions worth asking your vet
- Given my pet's breed, is this size difference expected?
- If the cells are small, should we look for a source of slow blood loss?
- Does the reticulocyte count agree with what the MCV is suggesting?
Related terms
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