Comparison between Serum Holotranscobalamin and Total Vitamin B12 as Indicators of Vitamin B12 Status

This study aims to assess the usefulness of serum holotranscobalamin (holoTC), the fraction of vitamin B12 that is available for tissue uptake, compared with total vitamin B12 in patients investigated for vitamin B12 disorders.

Serum samples were randomly selected from 76 patients (48 females, 28 males; age range 12-69 years) referred to the Clinical Biochemistry Laboratory, Royal Hospital for the assessment of vitamin B12 status. For each patient, serum total vitamin B12 level was determined by chemiluminescent microparticle immunoassay on Architect 2000 analyzer and holoTC (active vitamin B12) level was determined by microparticle enzyme immunoassay on Axsym analyzer (both from Abbott, USA).

Comparison of the data was conducted to reflect the mean, standard deviation (SD) and correlation coefficient between the two groups. The mean (SD) for serum holoTC and total vitamin B12 were 46.5(32.2) pmol/L and 316.3(165.6) pmol/L respectively. There was a significant correlation between holoTC and total vitamin B12 (r= 0.765, P< 0.001) and the regression equation was expressed as; y = a + bx (i.e: holoTC = 1.5 + 0.14 total vitamin B12). Also, the results were assessed for any misclassification when comparing holoTC and the total vitamin B12 in terms of whether each or both values agree or disagree for classifying the patients as having normal or abnormal (low or high) results, based on the cut-off thresholds of the kit's quoted reference range for holo TC of 9 - 123 pmol/L and for total vitamin B12 of 140-600 pmol/L. Accordingly, in 69 (90.8%) samples, there was a parallel agreement/ classification of results, both being normal or abnormal. In 61 (80.4%) patients, both results were normal, whereas in 4 (5.2%) patients, both results were high, and in 4 (5.2%) patients both results were low. However, in 7 (9.8%) cases, there was disagreement/ misclassification of results; 6 (7.8%) patients, holoTC was normal while total vitamin B12 was low, and in 1 (1.4%) patient, holoTC was normal while total vitamin B12 was high.

It can be recommended that holoTC and total vitamin B12, alone and in combination, have almost equal diagnostic efficiency in screening/diagnosing vitamin B12 deficiency for the majority of patients. In very few patients, holoTC appeared to be a better reflector of vitamin B12 status. Further comparison studies based on a gold standard method for classifying vitamin B12 status are worth considering.