Mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC) are measures of the amount and volume of hemoglobin in the average cell. The MCH average range is 27-35 picograms. The MCH results from dividing total hemoglobin by total RBCs. The average range from MCHC is 31-37%.
http://www.aegis.com/pubs/bala/1994/ba940309.html
Looks like a way for the doc to determine wether or not you are anemic. It doesn't seem to measure just B12 deficiency, but also iron deficiency etc. They will use a couple other blood test results to determine wether or not you might be anemic.