In response to a DEN that came up with a patient’s results, I’ve been refreshing my memory on the causes of macrocytosis. Briefly, these are:
- Physiological – macrocytosis is normal in pregnancy (usually mild but not impossible for it to be marked), in situations where the body is replacing blood loss (because of the high reticulocyte count – reticulocytes are larger than mature RBCs and push up the average MCV), and in Down’s syndrome. It’s also normal in the newborn, though that’s far less likely to come up in my line of work.
- Megaloblastic – these are the B12/folate deficiencies
- Alcohol consumption
- Liver disease
- Thyroid disease (normally only accounts for mild macrocytosis – if your patient has an MCV > around 104, look elsewhere for the cause)
- Myelodysplasia – look for cytopaenias and consider a blood film. Haematology referral may be necessary. Myelodysplasia is rare in younger patients but usually rapid and devastating when it happens, so don’t miss it.
- Drug-induced, especially the cytotoxics.
There are also a bunch of rare causes, including some causes of apparent macrocytosis (such as cold agglutinins, which cause red cell clumping and make the cells look bigger than they are; other causes are severe leucocytosis or hyperglycaemia, or storage of blood for several hours at room temperature before testing). The general view seems to be that if no signs of any of the above are showing up, it’s reasonable to leave it at that and not go searching for an obscure cause. In some people, it simply seems to be normal.