Holotranscobalamin (active B12)
Holotranscobalamin is the part of blood vitamin B12 that cells can take up. Published adult reference intervals differ by assay, starting anywhere from 19 to 42 pmol/L. Studies place likely deficiency below 20 to 26 and a grey zone above that. It separated deficiency better than total B12 in older adults. Two cohorts linked lower levels to cognitive decline.
- Status
- A blood test ordered by a clinician. FDA regulates it under its classification for vitamin B12 test systems, which are class II devices [1]. FDA cleared a first holotranscobalamin assay in 2004 and automated versions in 2006 and 2011 [2].
- Measures
- The concentration in serum of vitamin B12 bound to the carrier protein transcobalamin, about one quarter of circulating B12 and the part available to the body's cells [3].
- Also called
- holoTC, HoloTC, holo-TC, active B12, Active-B12, active vitamin B12, holotranscobalamin II, transcobalamin-bound B12
- Tested for
- vitamin B12 deficiency, borderline B12 result, vegan or vegetarian diet, B12 status in pregnancy, fatigue, numbness and tingling, checking a B12 supplement
Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on
Linked in research1
Reported risks0
No risks sourced yet.
Reference ranges
Each range is the one its source published, in pmol/L. Where sources differ, each is shown.
Reference values in blood donors, Refsum and colleagues, 2006
Adult blood donors aged 18 to 69 · [4]
| Range | What the source calls it |
|---|---|
| below 42 pmol/L | No name given |
| 42 to 157 pmol/L | "reference values" |
| above 157 pmol/L | No name given |
From 500 blood donors, measured by a microbiological assay the authors developed. They report that values differed by age and sex, with women having lower holotranscobalamin than men only at age 45 or younger, and propose considering separate reference intervals for younger women. The paper gives no meaning for a value above the range [4].
Reference interval for an automated immunoassay, Brady and colleagues, 2008
Adults, as the 95% reference interval of the Abbott AxSYM Active B12 assay · [5]
| Range | What the source calls it |
|---|---|
| below 19 pmol/L | No name given |
| 19 to 134 pmol/L | "reference interval" |
| above 134 pmol/L | No name given |
Established with samples from 292 people the authors describe as healthy, in the paper that introduced the assay, written by its developers. Its lower limit is less than half that of the microbiological assay interval in blood donors [4][5].
Swiss SENIORLAB study reference interval, Risch and colleagues, 2015
Adults aged 60 and older, both sexes combined · [6]
| Range | What the source calls it |
|---|---|
| below 23 pmol/L | No name given |
| 23 to 109.3 pmol/L | "reference interval" |
| above 109.3 pmol/L | No name given |
The 2.5th to 97.5th percentiles in 1,143 Swiss adults aged 60 and older whom the authors describe as "subjectively healthy", measured on the AxSYM assay. Levels did not change with age across this group. Separate intervals were 24.0 to 115 in women and 22.3 to 94.4 in men aged 60 to 79, and 21.3 to 118.2 at 80 and older, and the authors combined them because the limits overlapped [6].
Swiss SENIORLAB study decision limits, Risch and colleagues, 2015
Adults aged 60 and older · [6]
| Range | What the source calls it |
|---|---|
| below 25.8 pmol/L | "probable vitamin B12 deficiency" |
| 25.8 to 56.9 pmol/L | "grey zone" |
| 57 or above pmol/L | "probable vitamin B12 sufficiency" |
Cut-offs chosen by the authors from the same adults, set against a combined index of four B12 markers that itself includes holotranscobalamin. The authors call the middle band a grey zone of diagnostic uncertainty. It is shown here on the low side, where it lies, and not as in range: the source does not clear a result in it [6].
Three-zone partition in older adults, Valente and colleagues, 2011
Adults aged 63 to 97 · [7]
| Range | What the source calls it |
|---|---|
| below 20 pmol/L | "positive zone" |
| 20 to 30 pmol/L | "indeterminate zone" |
| above 30 pmol/L | "negative zone" |
The authors defined the indeterminate zone as 20 to 30 pmol/L. Its boundaries were the values that gave a 60% probability of deficiency after the test and a 98% probability of no deficiency. Deficiency was defined as red blood cell cobalamin below 33 pmol/L, which is not an accepted reference method. Of the results, 14% fell in the indeterminate zone. It is shown here on the low side, where it lies, and not as in range: the source does not clear a result in it [7].
Reference limits in pregnancy, British Columbia, Schroder and colleagues, 2019, first trimester
Pregnant women sampled between 8.3 and 13.9 weeks of pregnancy · [8]
| Range | What the source calls it |
|---|---|
| below 29.5 pmol/L | "may indicate B-12 deficiency" |
| 29.5 or above pmol/L | No name given |
The lower limit of the central 95% interval in 723 pregnant women of European and South Asian ethnicity in Canada, sampled at a median 11.4 weeks. The lower limits differed significantly by ethnicity. In a separate analysis against methylmalonic acid, the authors found a higher probability of impaired B12 status below 62.2 pmol/L, with no difference between ethnicities. They call for the cut-offs to be validated. The upper limit is not given in the abstract read here [8]. A Danish study that followed women through pregnancy found holotranscobalamin unchanged while total B12 fell [9].
Reference limits in pregnancy, British Columbia, Schroder and colleagues, 2019, second trimester
Pregnant women sampled between 14.9 and 20.9 weeks of pregnancy · [8]
| Range | What the source calls it |
|---|---|
| below 26 pmol/L | "may indicate B-12 deficiency" |
| 26 or above pmol/L | No name given |
From the same 723 women, sampled again at a median 16.1 weeks. Against methylmalonic acid, the authors found a higher probability of impaired B12 status below 67.5 pmol/L [8].
Reference intervals in Danish children, Knudsen and colleagues, 2023, ages 2 to 10
Children aged 2 to 10 · [10]
| Range | What the source calls it |
|---|---|
| below 46 pmol/L | No name given |
| 46 to 206 pmol/L | "reference interval" |
| above 206 pmol/L | No name given |
The 95% limits from serum samples of 400 children aged 18 or younger across both age groups. The authors found that holotranscobalamin changes with age and report analytical variation of 7.9% to 15.7% for the assay [10].
Reference intervals in Danish children, Knudsen and colleagues, 2023, ages 11 to 18
Children and adolescents aged 11 to 18 · [10]
| Range | What the source calls it |
|---|---|
| below 30 pmol/L | No name given |
| 30 to 178 pmol/L | "reference interval" |
| above 178 pmol/L | No name given |
Derived the same way from the same 400 samples [10].
Reference intervals in Norwegian schoolchildren, Strand and colleagues, 2025, ages 6 to 8
Children aged 6 to 8 · [11]
| Range | What the source calls it |
|---|---|
| below 56 pmol/L | No name given |
| 56 or above pmol/L | "reference interval" |
From 1,320 schoolchildren aged 6 to 12 in three Norwegian municipalities, mainly Caucasian, measured on the Abbott Architect assay. The authors give a lower limit only. It is 10 pmol/L above the Danish lower limit for ages 2 to 10 [10][11].
How it compares with the reference method
Mixed results4
Detecting vitamin B12 deficiency, against total B12
Mixed results: holotranscobalamin separated deficiency better than total B12 in older adults and in a Dutch hospital study, and no better in men and younger women.
The numbers and sources
- Compared with
- Red cell cobalamin, methylmalonic acid, or a combined index of four B12 markers, depending on the study
- Effect
- In 700 older adults, the area under the curve for detecting deficiency was 0.90 (95% CI 0.86 to 0.93) for holotranscobalamin, 0.80 (0.75 to 0.85) for serum B12 and 0.78 (0.72 to 0.83) for methylmalonic acid, and 14%, 45% and 50% of results fell in an indeterminate zone [7]. In 360 samples from five Dutch hospitals, holotranscobalamin had a larger area under the curve than B12 against methylmalonic acid, and a cut-off of 32 pmol/L detected methylmalonic acid above 0.45 µmol/L with 83% sensitivity and 60% specificity [15]. In Swiss adults aged 60 and older, the areas were 0.923 for holotranscobalamin and 0.884 for B12 against a combined index [6]. In 11,833 patient samples the areas were 0.92 and 0.90 overall, with no significant difference in men or in women under 50 [16]. An expert panel reports that cut-offs around 35 to 45 pmol/L are often used in practice and research, and cites a study in which deficiency was likely below 25 and unlikely above 50 [17].
- Population
- 700 adults aged 63 to 97 [7]. 360 patients suspected of B12 deficiency [15]. 1,143 Swiss adults aged 60 and older [6]. 11,833 samples from a mixed patient population, median age 56 [16].
- Sources
- [7], [15], [6], [16], [17]
- Caveats
- No accepted reference test for B12 deficiency exists, so each study chose its own definition [7]. The combined index used in two of the studies includes holotranscobalamin itself, which favors it in the comparison [6][16]. A total B12 result is on a different scale and cannot be placed against the ranges on this page.
Reduced kidney function
Mixed results: an expert panel reports holotranscobalamin is raised in impaired kidney function, while two studies of older adults found little effect.
The numbers and sources
- Compared with
- People with normal kidney function
- Effect
- An expert panel states that holotranscobalamin is elevated in people with impaired renal function [17]. In Swiss adults aged 60 and older, kidney function affected methylmalonic acid and homocysteine and not holotranscobalamin (p = 0.12) [6]. In 700 older adults, renal dysfunction compromised the diagnostic accuracy of methylmalonic acid and not that of holotranscobalamin [7].
- Population
- 1,143 Swiss adults aged 60 and older [6]. 700 adults aged 63 to 97 [7].
- Sources
- [17], [6], [7]
- Caveats
- The studies state no threshold for agreement. The panel gives no figure for the size of the rise [17].
Agreement between assays
Limited evidence: two assay studies found close correlation with an earlier method, while their published reference intervals start at 19 and 42 pmol/L.
The numbers and sources
- Compared with
- A commercial radioimmunoassay for holotranscobalamin
- Effect
- A microbiological assay correlated with the radioimmunoassay at r = 0.95, with between-day variation of 4% to 9% [4]. The AxSYM immunoassay gave results equal to 0.98 times the radioimmunoassay result plus 4.7 pmol/L in 204 samples, with total imprecision of 6.3% to 8.5% [5]. The reference intervals published with the two assays are 42 to 157 and 19 to 134 pmol/L, drawn from different groups of people [4][5]. A review of the three ways of measuring holotranscobalamin describes them as giving similar results [3].
- Population
- 500 blood donors [4]. 204 serum samples and 292 reference individuals [5].
- Sources
- [4], [5], [3]
- Caveats
- The studies were run by the developers of each assay. A result near a cut point may be classed differently by another method or against another laboratory's interval [4][5].
Week to week variation in one person
One small study found holotranscobalamin varied about 13% from week to week within the same person over ten weeks.
The numbers and sources
- Compared with
- The same person's results on 10 weekly draws
- Effect
- Within-person variation was 13.0% (95% CI 11.5 to 15.0), variation between people was 24.6% (16.3 to 37.7) and analytical variation was 2.4%. The authors calculated that two results in one person need to differ by 36.6% to exceed expected variation. For total B12 in the same samples, within-person variation was 6.7% [18].
- Population
- 15 volunteers aged 22 to 66, 12 women and 3 men, sampled on the same weekday for 10 weeks [18].
- Sources
- [18]
- Caveats
- Fifteen people and one laboratory. The study states no threshold for agreement, so the result is given as numbers only [18].
What studies tie it to
Linked in research1
Cognitive decline and Alzheimer disease in older adults
Limited evidence: two cohorts linked lower holotranscobalamin to faster cognitive decline and to more Alzheimer disease over 7 to 10 years.
The numbers and sources
- Effect
- In an Oxford cohort, a doubling of holotranscobalamin from 50 to 100 pmol/L went with a 30% slower rate of cognitive decline over 10 years, and the association remained significant after adjustment for the other vitamin markers [12]. In a Finnish cohort, each 1 pmol/L higher holotranscobalamin at baseline had an odds ratio for Alzheimer disease of 0.980 (95% CI 0.965 to 0.995), and 0.977 (0.958 to 0.997) after adjustment for confounders that included age, sex, education, the APOE ε4 allele, smoking, stroke and blood pressure [13].
- Population
- 1,648 older adults in Oxford, United Kingdom, tested on 3 or more occasions over 10 years [12]. 271 Finnish adults aged 65 to 79 without dementia, followed for 7 years [13].
- Sources
- [12], [13], [14]
- Caveats
- Observational. In a meta-analysis of 11 trials with 22,000 participants, B vitamins given to lower homocysteine had no significant effect on global cognitive function (z score difference -0.01, 95% CI -0.03 to 0.02). Those trials did not select people by holotranscobalamin and were not trials of treating B12 deficiency [14].
Related
Sources
- US Food and Drug Administration. 21 CFR 862.1810, Vitamin B12 test system. Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-21/section-862.1810
- US Food and Drug Administration. 510(k) premarket notification records K030655 (HoloTC RIA, decided January 29, 2004), K062467 (AxSYM HoloTC, decided November 22, 2006) and K112443 (ARCHITECT Active B-12, decided December 19, 2011), read from the openFDA device 510(k) database. https://api.fda.gov/device/510k.json?search=k_number:K112443
- Nexo E, Hoffmann-Lücke E. Holotranscobalamin, a marker of vitamin B-12 status: analytical aspects and clinical utility. The American Journal of Clinical Nutrition. 2011. https://doi.org/10.3945/ajcn.111.013458
- Refsum H, Johnston C, Guttormsen AB, Nexo E. Holotranscobalamin and Total Transcobalamin in Human Plasma: Determination, Determinants, and Reference Values in Healthy Adults. Clinical Chemistry. 2006. https://doi.org/10.1373/clinchem.2005.054619
- Brady J, Wilson L, McGregor L, Valente E, Orning L. Active B12: A Rapid, Automated Assay for Holotranscobalamin on the Abbott AxSYM Analyzer. Clinical Chemistry. 2008. https://doi.org/10.1373/clinchem.2007.096784
- Risch M, Meier DW, Sakem B, et al. Vitamin B12 and folate levels in healthy Swiss senior citizens: a prospective study evaluating reference intervals and decision limits. BMC Geriatrics. 2015. https://doi.org/10.1186/s12877-015-0060-x
- Valente E, Scott JM, Ueland PM, et al. Diagnostic Accuracy of Holotranscobalamin, Methylmalonic Acid, Serum Cobalamin, and Other Indicators of Tissue Vitamin B12 Status in the Elderly. Clinical Chemistry. 2011. https://doi.org/10.1373/clinchem.2010.158154
- Schroder TH, Tan A, Mattman A, et al. Reference intervals for serum total vitamin B12 and holotranscobalamin concentrations and their change points with methylmalonic acid concentration to assess vitamin B12 status during early and mid-pregnancy. Clinical Chemistry and Laboratory Medicine. 2019. https://doi.org/10.1515/cclm-2018-1337
- Morkbak AL, Hvas AM, Milman N, Nexo E. Holotranscobalamin remains unchanged during pregnancy. Longitudinal changes of cobalamins and their binding proteins during pregnancy and postpartum. Haematologica. 2007. https://doi.org/10.3324/haematol.11636
- Knudsen CS, Parkner T, Hoffmann-Lücke E, Abildgaard A, Greibe E. Reference intervals and stability of haptocorrin and holotranscobalamin in Danish children and elderly. Clinica Chimica Acta. 2023. https://doi.org/10.1016/j.cca.2023.117394
- Strand MF, Hager HB, Fredriksen PM, Lindberg M. Reference intervals for holotranscobalamin and total vitamin B12 in 6-12-year-old children in Norway: the Health Oriented Pedagogical Project (HOPP). Scandinavian Journal of Clinical and Laboratory Investigation. 2025. https://doi.org/10.1080/00365513.2025.2475454
- Clarke R, Birks J, Nexo E, et al. Low vitamin B-12 status and risk of cognitive decline in older adults. The American Journal of Clinical Nutrition. 2007. https://doi.org/10.1093/ajcn/86.5.1384
- Hooshmand B, Solomon A, Kåreholt I, et al. Homocysteine and holotranscobalamin and the risk of Alzheimer disease: a longitudinal study. Neurology. 2010. https://doi.org/10.1212/WNL.0b013e3181f88162
- Clarke R, Bennett D, Parish S, et al. Effects of homocysteine lowering with B vitamins on cognitive aging: meta-analysis of 11 trials with cognitive data on 22,000 individuals. The American Journal of Clinical Nutrition. 2014. https://doi.org/10.3945/ajcn.113.076349
- Heil SG, de Jonge R, de Rotte MC, et al. Screening for metabolic vitamin B12 deficiency by holotranscobalamin in patients suspected of vitamin B12 deficiency: a multicentre study. Annals of Clinical Biochemistry. 2012. https://doi.org/10.1258/acb.2011.011039
- Jarquin Campos A, Risch L, Nydegger U, et al. Diagnostic Accuracy of Holotranscobalamin, Vitamin B12, Methylmalonic Acid, and Homocysteine in Detecting B12 Deficiency in a Large, Mixed Patient Population. Disease Markers. 2020. https://doi.org/10.1155/2020/7468506
- Allen LH, Miller JW, de Groot L, et al. Biomarkers of Nutrition for Development (BOND): Vitamin B-12 Review. The Journal of Nutrition. 2018. https://doi.org/10.1093/jn/nxy201
- Brokner M, Hager HB, Lindberg M. Biological variation of holotranscobalamin and cobalamin in healthy individuals. Scandinavian Journal of Clinical and Laboratory Investigation. 2017. https://doi.org/10.1080/00365513.2017.1335881