Elevation of methylmalonate levels is seen in vitamin B12 deficiency but not in folate deficiency. In the absence of vitamin B12, methylmalonyl-CoA accumulates and then decomposes forming methylmalonate. The other laboratory values make it unlikely that the anemia is one caused by iron deficiency, chronic disease or hemolysis. In iron deficiency anemia, both serum ferritin levels and transferrin saturation would be low. In the anemia of chronic disease, we would expect serum ferritin levels to be high and transferrin saturation to be somewhat low.
Hemolysis is likely to result in elevated serum bilirubin levels.
Many people over the age of 60 are at risk for vitamin B12 deficiency because their absorption of the vitamin is impaired either by reduced stomach acidity or by reduced production of intrinsic factor. The patient s use of a drug that reduced his stomach acidity may also have contributed to the deficiency. Negative nitrogen balance indicates a net loss of tissue protein. In general, negative nitrogen balance can be caused by a variety of things including inadequate dietary protein, lack of one or more essential amino acids, and stress or trauma. The cause in this case is the surgery.