used to treat morbid obesity. Roux-en-Y gastric bypass surgery (RYGB) but not sleeve gastric bypass surgery or gastric banding, increases the risk of vitamin B12 deficiency and requires preventive post-operative treatment with either injected or high-dose oral supplementation. For post-operative oral supplementation, 1000 μg/day may be needed to prevent vitamin deficiency. Diagnosis According to one review: "At present, no 'gold standard' test exists for the diagnosis of vitamin B12 deficiency and as a consequence the diagnosis requires consideration of both the clinical state of the patient and the results of investigations." The vitamin deficiency is typically suspected when a routine complete blood count shows anemia with an elevated mean corpuscular volume (MCV). In addition, on the peripheral blood smear, macrocytes and hypersegmented polymorphonuclear leukocytes may be seen. Diagnosis is supported based on vitamin B12 blood levels below 150–180 pmol/L (200–250 pg/mL) in adults. However, serum values can be maintained while tissue B12 stores are becoming depleted. Therefore, serum B12 values above the cut-off point of deficiency do not necessarily confirm adequate B12 status. For this reason, elevated serum homocysteine over 15 micromol/L and methylmalonic acid (MMA) over 0.271 micromol/L are considered better indicators of B12 deficiency, rather than relying only on the concentration of B12 in blood. However, elevated MMA is not conclusive, as it is seen in people with B12 deficiency, but also in elderly people who have renal insufficiency, and elevated homocysteine is not conclusive, as it is also seen in people with folate deficiency. In addition, elevated methylmalonic acid levels may also be related to metabolic disorders such as methylmalonic acidemia. If nervous system damage is present and blood testing is inconclusive, a lumbar puncture may be carried out to measure cerebrospinal fluid B12 levels. Medical uses Repletion of deficiency Severe vitamin B12 deficiency is corrected with frequent intramuscular injections of large doses of the vitamin, followed by maintenance doses of injections or oral dosing at longer intervals. In the UK, standard initial therapy consists of intramuscular injections of 1000 μg of hydroxocobalamin three times a week for two weeks or until neurological symptoms improve, followed by 1000 μg every two or three months. Injection side effects include skin rash, itching, chills, fever, hot flushes, nausea and dizziness. Cyanide poisoning For cyanide poisoning, a large amount of hydroxocobalamin may be given intravenously and sometimes in combination with sodium thiosulfate. The mechanism of action is straightforward: the hydroxycobalamin