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Interpretation of Vitamin B12 Status After a Roux-en-Y Gastric Bypass

Educational Objective
Based on this clinical scenario and the accompanying image, understand how to arrive at a correct diagnosis.
1 Credit CME

A 37-year-old woman with a history of diabetes, hypertension, obstructive sleep apnea, depression, and Roux-en-Y gastric bypass (RYGB) 16 years prior presented to an ambulatory care clinic with fatigue, loss of appetite, and hair loss. She lost 50.8 kg from the time of the procedure to presentation, her body mass index at presentation was 23, and her preoperative comorbidities were in remission. She did not have any nausea, abdominal pain, or change in bowel habits, but her periods had been more irregular over the past 2 to 3 months and, when present, lasted for 7 to 10 days. She also noted a tingling sensation in her hands and feet. She was under some financial stress after recently losing her job, and reported drinking 2 to 6 beers within a 2-hour period on the weekends. She took a gummy multivitamin sporadically and self-reported a daily total calorie intake of less than 500 kcal and less than 20 g of protein. On physical examination, although she reported having paresthesias in her fingertips and toes, her neurological examination findings were nonfocal. She exhibited bilateral flattening of the temporalis muscle and thenar eminence. Her vital signs were normal, and serum tests were ordered (Table).

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C. The patient has both iron and vitamin B12 deficiencies

The patient has B12 deficiency and concomitant iron deficiency anemia. From her history, this is likely secondary to nonadherence with postoperative vitamin supplementation recommendations.

Vitamin B12 (cobalamin) is a water-soluble vitamin, necessary for neurologic functioning, hematopoiesis, and DNA synthesis. It is obtained exclusively from animal proteins and absorbed in the ileum, facilitated by intrinsic factor.

Serum B12 is measured with a competitive-binding immunoenzymatic assay. Intrinsic factor conjugate and paramagnetic particles coated with monoclonal immunoglobulin G anti–intrinsic factor are added to the sample. B12 binds to the intrinsic factor conjugate, blocking it from binding with the paramagnetic particles. A magnetic field holds the particle-bound intrinsic factor in place while unbound materials are washed away. Finally, a chemiluminescent substrate is added and the light is measured with a luminometer. The light production is inversely proportional to the concentration of vitamin B12 in the sample. Sensitivity for deficiency detection is 95% at levels less than 200 pg/mL (<150 pmol/L), but specificity is limited to 50%. Serum B12 less than 400 pg/mL is considered suboptimal because 5% to 10% of patients with neurologic symptoms related to B12 insufficiency present with levels from 200 to 400 pg/mL.1 At levels greater than 200 pg/mL, sensitivity is reduced and can be bolstered by serum methylmalonic acid (MMA). During the Krebs cycle, B12 mediates the enzymatic conversion of methylmalonyl–coenzyme A to succinyl–coenzyme A; therefore, MMA levels greater than 0.75 μmol/L are suggestive of B12 deficiency.2

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Article Information

Corresponding Author: Sameer B. Murali, MD, MSHS, Southern California Permanente Medical Group, 17296 Slover Ave, Fontana, CA 92337 (sameer.b.murali@kp.org).

Published Online: July 26, 2019. doi:10.1001/jama.2017.18945

Conflict of Interest Disclosures: None reported.

References
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