An increase in the mean corpuscular volume (MCV), while not diagnostic of vitamin B12 or folate deficiency, is a valuable, readily available screening test. I discuss six instances where an increased MCV was recorded months prior to diagnosis and could, if noted, have led to an earlier diagnosis.
Hall CA. Vitamin B12 Deficiency and Early Rise in Mean Corpuscular Volume. JAMA. 1981;245(11):1144-1146. doi:10.1001/jama.1981.03310360036019