What laboratory tests to order to fully evaluate for possible vitamin B12 deficiency

One of my outpatients, an older adult with a depressive disorder, hypertension, dyslipidemia, and mild, unexplained, cognitive impairment, among other problems. She had been previously identified as having vitamin B12 deficiency and, for a long time, has been taking a vitamin B12 supplement. More recently—definitely after the vitamin B12 deficiency was identified—the patient has adopted…