Holotranscobalamin is the bioactive form of vitamin B12 in circulation, i.e., it is the only form readily taken up and used by cells. HoloTC is recognized as a more sensitive marker of B12 status than serum B12 and may be better than methylmalonic acid or homocysteine. Although holoTC can help identify early B12 deficiency, it is not a “standalone” marker and should be assessed along with serum B12, MMA, and homocysteine. Low holoTC is associated with B12 insufficiency, neuropathy, macrocytosis, neural tube defects, gastrointestinal surgery, and vegetarianism. Elevated holoTC could be a sign of leakage from liver cells.
Standard Range: 25.10 – 165.00 pmol/L
The ODX Range: 54.00 – 165.00 pmol/L
Low active B12 may be associated with B12 insufficiency, B12 deficiency, peripheral neuropathy (Verma 2022), macrocytosis, chemotherapy, inborn errors of metabolism (Hannibal 2016), gastrectomy (Lee 2009), vegetarianism, white brain matter damage (Smith 2018), and neural tube defects (Allen 2018, Ray 2007).
High active B12 levels may be associated with anorexia nervosa and leakage from damaged liver cells (Tam 2022).
Approximately 20-25% of circulating B12 (cobalamin) is bound to transcobalamin in a complex called holotranscobalamin (holoTC). The holoTC is biologically active and readily taken up by the cell. It is considered the most direct measurement of vitamin B12 status (Nexo 2011). Although expected values for holoTC in apparently healthy individuals may range from 35-171 pmol/L (Devalia 2014), a level below 70 pmol/L should be investigated further to rule out B12 insufficiency (Harrington 2017).
HoloTC is one of the four recommended biomarkers used to diagnose B12 deficiency. This 4cB12 index approach includes holoTC, methylmalonic acid (MMA), serum B12, and homocysteine (Jarquin 2020). HoloTC correlates well with intracellular erythrocyte B12 and is a better marker of B12 than homocysteine or MMA in the elderly. A holoTC below 50 pmol/L in the general population should trigger an assessment of methylmalonic acid. A holoTC below 35 pmol/L with an MMA below 271 nmol/L suggests B12 insufficiency; holoTC of 36-50 pmol/L and an MMA below 271 nmol/L suggests an unlikelihood of B12 deficiency, and a holoTC below 50 pmol/L with an MMA above 271 nmol/L likely identifies B12 deficiency. Homocysteine is not as specific for B12 metabolism as MMA is and may or may not be used in a comprehensive evaluation if MMA is available (Hannibal 2016).
The mean corpuscular volume (MCV) may or may not be elevated with B12 deficiency. In a study of 217 subjects having B12 status assessed, MCV was only elevated above the conventional range in 11.4% of those with serum B12 below 200 pg/mL (147.56 pmol/L) and 11.7% in those with a low holoTC below 34 pmol/L (Bondu 2020).
A decrease in holoTC may identify B12 insufficiency despite a “normal” total serum B12. A prospective study of 692 elders found that low holoTC and elevated homocysteine and (MMA) were significantly associated with cognitive decline. Researchers note that the rate of cognitive decline decreased by 30% with a doubling of holoTC and increased by 50% with a doubling of homocysteine or MMA (Smith 2018).
One study of a variety of individuals observed the lowest median holoTC level of 34 pmol/L in vegetarians, a group that also had elevated homocysteine and methylmalonic acid. Median holoTC in healthy controls was 54 pmol/L, while subjects with renal failure had a median level of 70 pmol/L. Requirements for circulating holoTC are higher in renal failure to maintain adequate intracellular B12. Serum holoTC may not be a useful indicator of B12 status in renal dysfunction (Herrmann 2003).
A holoTC below 55.3 pmol/L can increase the risk of neural tube defects (NTD) by at least 3-fold (Allen 2018). HoloTC was significantly lower in pregnancies that resulted in NTDs than in controls in a population-based case-control study of 511 women. Those suffering NTD complications had a mean holoTC of 67.8 pmol/L versus 81.2 pmol/L in controls. Interestingly, serum folate was very similar, at the low end of the conventional range, and below optimal for both groups: 13.3 nmol/L in NTD and 13.9 nmol/L in controls (Ray 2007).
Allen, Lindsay H et al. “Biomarkers of Nutrition for Development (BOND): Vitamin B-12 Review.” The Journal of nutrition vol. 148,suppl_4 (2018): 1995S-2027S. doi:10.1093/jn/nxy201
Bondu, Joseph Dian et al. “Assessing Diagnostic Accuracy of Serum Holotranscobalamin (Active-B12) in Comparison with Other Markers of Vitamin B12 Deficiency.” Indian journal of clinical biochemistry : IJCB vol. 35,3 (2020): 367-372. doi:10.1007/s12291-019-00835-y
Devalia, Vinod et al. “Guidelines for the diagnosis and treatment of cobalamin and folate disorders.” British journal of haematology vol. 166,4 (2014): 496-513. doi:10.1111/bjh.12959
Hannibal, Luciana et al. “Biomarkers and Algorithms for the Diagnosis of Vitamin B12 Deficiency.” Frontiers in molecular biosciences vol. 3 27. 27 Jun. 2016, doi:10.3389/fmolb.2016.00027
Jarquin Campos, Araceli et al. “Diagnostic Accuracy of Holotranscobalamin, Vitamin B12, Methylmalonic Acid, and Homocysteine in Detecting B12 Deficiency in a Large, Mixed Patient Population.” Disease markers vol. 2020 7468506. 7 Feb. 2020, doi:10.1155/2020/7468506
Lee, Young Kyung et al. “Holotranscobalamin as an indicator of vitamin B12 deficiency in gastrectomized patients.” Annals of clinical and laboratory science vol. 39,4 (2009): 361-6.
Nexo, Ebba, and Elke Hoffmann-Lücke. “Holotranscobalamin, a marker of vitamin B-12 status: analytical aspects and clinical utility.” The American journal of clinical nutrition vol. 94,1 (2011): 359S-365S. doi:10.3945/ajcn.111.013458
Ray, Joel G et al. “Vitamin B12 and the risk of neural tube defects in a folic-acid-fortified population.” Epidemiology (Cambridge, Mass.) vol. 18,3 (2007): 362-6. doi:10.1097/01.ede.0000257063.77411.e9
Smith, A David et al. “Vitamin B12.” Advances in food and nutrition research vol. 83 (2018): 215-279. doi:10.1016/bs.afnr.2017.11.005
Tam, Friederike I., et al. "Liver and vitamin B12 parameters in patients with anorexia nervosa before and after short-term weight restoration." Psychiatry Research (2022): 114673.
Verma, Abhishek, et al. "Comparison of Serum Holotranscobalamin with Serum Vitamin B12 in Population Prone to Megaloblastic Anemia and their Correlation with Nerve Conduction Study." Indian Journal of Clinical Biochemistry (2022): 1-9.