CobaSorb, a New Non-Radioactive Vitamin B₁₂ Absorption Test.

After diagnosing vitamin B₁₂ deficiency, the cause of the deficiency has to be clarified. For many years Schillings test was used for this purpose. However, Schillings test has almost disappeared from the marked. Recently, a new vitamin B₁₂ absorption test, CobaSorb, was introduced. CobaSorb is based on oral intake of non-radioactive vitamin B₁₂ followed by measurement of an increase in holotranscobalamin (holoTC) 24 hours later. In the present study, we further explored the design of CobaSorb to identify the best marker for reflection of vitamin B₁₂ absorption and to determine the duration of the test. Seventy-eight healthy individuals (age 21–81 years, 40 males) were included and three oral doses of 9 microgram vitamin B₁₂ per day were administered for 5 successive days. Non-fasting blood samples were collected on days 1 to 5 before administration of vitamin B₁₂ and in the morning day 8. Plasma cobalamins were assayed on the Advia Centaur (Bayer A/S, Germany), and holoTC was measured by an in-house sandwish ELISA after removal of the apoTC with vitamin B₁₂ coated beads. Baseline levels were 151–528 pmol/L for plasma cobalamins and 19–33 pmol/L for holoTC. We found a significant positive association between age and baseline holoTC (p=0.03), whereas no association was found between baseline plasma cobalamins and age (p=0.55). Baseline levels were not associated with sex. Because a vitamin B₁₂ absorption test is relevant only in individuals with borderline or low levels of holoTC or plasma cobalamins, we analysed in detail the pattern of absorption in those having baseline levels below the 75% percentiles. We used a change greater than 2×CV_day-to-day in holoTC (22%) and plasma cobalamins (12%) to indicate a change caused by absorption of vitamin B₁₂. Among individuals with a baseline holoTC below the 75% percentile (<75 pmol/L, n=57), the increase in holoTC from baseline (day 1) to day 3 was > 22% for 56 (98%). Only 41 (72%) of the individuals with baseline plasma cobalamins below the 75% percentile (<335 pmol/L, n=57) had an increase in plasma cobalamins >12% from day 1 to day 3. We concluded that in healthy individuals with baseline holoTC <75 pmol/L, vitamin B₁₂ absorption was well reflected by an increase in holoTC-but not plasma cobalamins-after 2 days administration of oral vitamin B₁₂. We suggest that future use of CobaSorb is done based on this design.