Fewer US adults had low or transitional vitamin B12 status based on the novel combined indicator of vitamin B12 status compared with individual, conventional markers, NHANES 1999-2004.

BACKGROUND Elevated plasma methylmalonic acid (MMA) and/or total homocysteine (tHcy), as well as low serum vitamin B12 and/or holotranscobalamin (holoTC) are indicative of vitamin B12 deficiency. Combined indicators (cB12), which pool some or all 4 markers into an index, may be a more reliable diagnostic tool to overcome inconclusive diagnoses with individual markers. OBJECTIVES We aimed to describe different cB12 score combinations and estimate the prevalence of low or transitional vitamin B12 status compared with individual markers. DESIGN Using cross-sectional data for B12, MMA, and tHcy in persons ≥20 y participating in NHANES 1999-2004 (n = 12,335), we examined raw and covariate-adjusted regression models to assess determinants of 3cB12 (all 3 markers) and combinations of 2cB12 (2 markers). RESULTS 3cB12 was significantly associated with B12 (Spearman r = 0.75), MMA (r = -0.70), and tHcy (r = -0.59). The 3cB12 reference interval (2.5th to 97.5th percentile) was -0.538 to 1.60. In covariate-adjusted models, we found no association of 3cB12 with age; adult females and users of B12 supplements had higher, while adults with advanced chronic kidney disease had lower 3cB12 levels regardless of race-Hispanic origin group (self-reported). Only 2.7% of adults had low or transitional vitamin B12 status using the proposed cB12 cutoff of ≤-0.5, while the prevalence of low (or low-normal) status depended on the selected individual marker and its cutoff: 2.2% and 13% for B12 13 µmol/L. The reference intervals for B12, MMA, and tHcy overlapped from the low ( -0.5) cB12 categories. CONCLUSIONS Vitamin B12 deficiency may be overestimated among US adults when individual, conventional markers are used. When only 2 markers are available, the combination of B12 and MMA provides results comparable to 3cB12.