Hi,
azathioprine often causes a macrocytosis (this won't cause harm) - which means large blood cells - other causes of a macrocytosis are B12 and folate deficiency - so it may be that your doc has found your blood cells to be increasing size due to the aza and to rule out all problems wanted to check the B12 and folate - I wouldn't worry too much - if there was a big problem with your blood count he wouldn't wait a month.