What is MCV?

Size is settled as the bone marrow builds each cell and never changes afterwards. A red cell lives about 120 days, so the average describes what the marrow had to build with up to four months earlier, not what it has to build with now.

MCH and MCHC both describe the haemoglobin inside a red cell, one as an amount per cell, the other as how densely that amount is packed. MCV supplies the size those two are measured against. The same amount of haemoglobin sits more densely in a smaller cell, so it takes all three to tell a cell that is simply small from one that was left short of haemoglobin at normal size.

Without enough iron the marrow builds red cells smaller than usual. Vitamin B12 and folate matter for a different reason: cells need them to divide, and when either runs short the precursors divide fewer times and the cells produced are larger than usual.

The average changes only as one generation of red cells is replaced by the next. A measurement a year on therefore comes well after a change in the marrow's sizing would have worked through the whole population, so what it reports is a settled average rather than a half-replaced one. This is Do Health's monitoring schedule, not a general recommendation for everyone.