To the Editor. —I read with interest the recent report by Carmel1 and the accompanying editorial by Herbert2 addressing the likely underestimated occurrence of clinically significant cobalamin deficiency. This occurs even in the presence of only mildly depressed serum cobalamin levels, normal or low mean corpuscular volumes, and normal hemoglobin levels.
Another piece of information that would have been interesting to know in this series is the red blood cell volume distribution width. The red blood cell volume distribution width is the SD of the mean corpuscular volume of the red blood cell population in question divided by the mean corpuscular volume and expressed as a percentage.3 Higher values reflect more heterogeneous (anisocytotic) populations of cells. Patients with anemia due to cobalamin deficiencies alone can be expected to have high mean corpuscular volume levels with high red blood cell volume distribution widths, and patients with mixed iron