Unexplained anaemia (low haemoglobin) and/or macrocytosis (enlarged red blood cells) have traditionally been the indications used for suspicion of vitamin B12 deficiency.
However, the article by Lindenbaum et al ( N Engl J Med 1988;318:1720-1728) demonstrated that more than 25% of patients with neurological manifestations of vitamin B12 deficiency have either a normal haematocrit or mean cell volume (MCV), or both.
In fact elevated MCV are in most cases related to other conditions (alcoholism, liver disease, drug use). Also, even when there is metabolic evidence of B12 deficiency, MCV often is within the normal reference range.
Absence of macrocytosis in B12deficiency may also be encountered in patients with concurrent iron deficiency.
The message is that we should not rely solely upon anaemia and/or macrocytosis as our entry criterion for further investigation of vitamin B12 deficiency.
Normal red blood cells
Oversized red blood cells