Each Medicare Intermediary has "interpretations" for MC policies.
This is a link to CAHABA's policy re HHA + RN administering B12:
Some causes of vitamin B12 deficiency include pernicious anemia, megaloblastic anemias,macrocytic anemias, fish tapeworm anemia, gastrectomy, some malabsorption syndromes such
as sprue and idiopathic steatorrhea, surgical and mechanical disorders, and certain neuropathies.
Skilled nurse visits to administer Vitamin B12 injections are payable for specified anemias,specified gastrointestinal disorders, and certain neuropathies when there is documentation of
an abnormal laboratory test. These include one of the following lab tests:
* an abnormally low serum B12 (generally below 200 pg/ml)
* an elevated serum methylmalonic acid (MMA) early in the course of vitamin B12 deficiency
* an elevated homocysteine (HCYS) early in the course of vitamin B12 deficiency
* an abnormal Schilling test
When the initial lab values cannot be obtained, a physician's statement is acceptable to support coverage for a non-reversible cause of vitamin B12 deficiency such as pernicious anemia. This
statement needs to be in the medical record and must be available to Medicare on request.
For a patient with pernicious anemia caused by a B12 deficiency, intramuscular or subcutaneous injection of vitamin B12 at a dose of from 100 to 1000 micrograms no more frequently than once monthly is the accepted reasonable and necessary dosage schedule for maintenance treatment. More frequent injections would be appropriate in the initial or acute phase of the disease until it has been determined through laboratory tests that the patient can be sustained on a maintenance dose.