Holo-Transcobalamin Concentrations and Transcobalamin Saturation Reflect Recent Vitamin B12 Absorption Better than Does Serum Vitamin B12

Background: We evaluated whether measurement of vitamin B12-saturated transcobalamin (holo-TC) concentrations or TC saturation (holo-TC:total TC) reflects active vitamin B12 absorption in healthy individuals and patients after vitamin B12 intake. Methods: We obtained blood samples from 31 healthy individuals (age range, 25–57 years) before (days 1 and 0) and after (days 1, 2, and 6) oral administration of three 9g doses of vitamin B12. The blood samples from seven patients (age range, 22–39 years) suspected to have decreased vitamin B12 absorption were obtained before and 1 day after the vitamin B12 intake. The blood samples were analyzed for vitamin B12, total TC, and holo-TC. The TC saturation was calculated. Results: Intraindividual variation was <13% for all measured values, as calculated from samples removed on day 1 and 0. In healthy individuals (n 31) after intake of vitamin B12, the maximum median (range) increase (as percentages and absolute values) was in TC saturation [52 ( 2% to 128)% and 0.04 (0–0.23) as a fraction], closely followed by holo-TC concentrations [39 (0–108)% and 34 (0–149) pmol/L]. All but one healthy individual had an increase of >15% in these markers. Serum vitamin B12 showed a smaller increase [14 ( 8 to 51)% and 36 ( 27 to 290) pmol/L]. After vitamin B12 intake, three patients with Crohn disease had the lowest increases in holo-TC concentration (3, 7, and 14 pmol/L) and in TC saturation (0.004, 0.01, and 0.01) among patients and 30 healthy individuals. Conclusion: Holo-TC concentrations and TC saturation reflect normal vitamin B12 absorption better than does serum vitamin B12. © 2004 American Association for Clinical Chemistry