Hi Rich, It's a lot more complicated than that. The very definition of b12 deficiency is severely faulty. Most of the definition of b12 deficiency in the USA and UK revolve around Pernicious Anemia. It has absolutely nothing to do with a person having symptoms corectable by b12 of one form or another. In the USA a person can literally be dying or severely disabled by b12 deficiencies and yet not test as b12 deficient. So that diagnosis of exclusion of b12 deficiency has to do with test results indicating deficiency, not whether the person has symtoms that correct with b12. For instance in a study of methylb12 as treatment for peripheral neuropathy, people were accepted into the study based on symptoms. Of those who responded to methylb12, 63% were not deficient according to any test results; serum b12, uMMA or Hcy and would have been excluded from a study using test results as entry criteria. The mean serum b12 level in responsive people prior to treatment was over 700pg/ml with some exceeding 1500pg/ml. Further, other research has indicated that those with CFS and FMS have a cerebral spinal fluid cobalamin level compared to blood serum level of 25% of the healthy controls. Except in studies, nobody has cerebral spinal fluid drawn to test for cobalamins. Further there is no definition for anybody of what constitutes "low" CSF cobalamin levels. The tests as currently applied only cause a few percent of those who have symptoms that are responsive to b12s to be diagnosed as b12 deficient. In Japan for instance anything below 550pg/ml is "low". Their "low" flag is designed to catch more of the neurological symptoms than low being set at 170pg/ml. Even 550pg/ml isn't high enough to prevent neurological symptoms in most people. With testing standards like these no wonder CFS/FMS is mysterious.