Determination of serum holotranscobalamin concentrations with the AxSYM active B(12) assay: cut-off point evaluation in the clinical laboratory

Preliminary evaluation of the diagnostic performance of Roche Elecsys® active vitamin B12 versus total vitamin B12 for vitamin B12 deficiency screening

INTRODUCTION

The prevalence of vitamin B12 deficiency is high in at-risk populations with sometimes irreversible consequences. Beside total B12 (TVB12), active B12 (AVB12) is a promising first-line marker. Only Abbott AVB12 assays were largely evaluated and generally demonstrated benefit in clinical practice. More recently developed Roche AVB12 still requires some investigations.

OBJECTIVES

Our study aimed to evaluate the Roche Elecsys® AVB12 immunoassay performance versus Roche Elecsys® TVB12 competition assay.

DESIGN

and Methods: We included 175 patients at Rouen University Hospital who had a TVB12 value <300 pmol/L. We evaluated performance of AVB12 by comparing the results with TVB12 and MMA values in case of disagreement.

RESULTS

Positive correlation was found between the AVB12 and TVB12. We found a disagreement between TVB12 and AVB12 in 18.8% of cases. Among 33 cases of disagreement, 76% had normal AVB12 but low TVB12, whereas 24% had low AVB12 and normal TVB12. Thirty-one MMA determinations were performed: 71% showed agreement between MMA and AVB12, versus 29% between MMA and TVB12. TVB12 reported a sensitivity (Se) at 66.7%, specificity (Sp) at 20%, positive predictive value (PPV) at 16.7% and negative predictive value (NPV) at 71.4% for the prediction of MMA elevation. We determined an optimized cut-off value of 45.5 pmol/L for AVB12, which reported a Se 66.7%, Sp 60%, PPV 30.7%, and NPV 88.9%.

CONCLUSIONS

Our results provide preliminary evidence that Roche AVB12 may offer better discrimination than Roche TVB12 in the diagnosis of vitamin B12 deficiency. Further more detailed evaluation is warranted.

Perspective: Practical Approach to Preventing Subclinical B12 Deficiency in Elderly Population

Vitamin B12 (also known as cobalamin) is an essential water-soluble vitamin that plays a pivotal role for several physiologic functions during one's lifespan. Only certain microorganisms are able to synthetize B12, thus humans obtain cobalamin exclusively from their diet, specifically from animal-derived foods. Specific sub-group populations are at risk of vitamin B12 subclinical deficiency due to different factors including poor intake of animal source foods and age-dependent decrease in the capacity of intestinal B12 uptake. Consumption of animal products produces some negative health issues and negatively impacts sustainability while a plant-based diet increases the risk of B12 deficiency. Taking a cue from the aforementioned considerations, this narrative review aims to summarize facts about B12 deficiency and the burden of inadequate dietary intake in elderly population, as well as to discuss sustainable approaches to vitamin B12 deficiency in aging population.

Diet and Lifestyle Role in Homocysteine Metabolism in Turner Syndrome

OBJECTIVE

Patients with Turner syndrome (TS) have an unfavorable cardiometabolic profile. Hyperhomocysteinemia is a potential cardiovascular risk factor influenced by genetic and environmental factors, therapies, unbalanced diets and other lifestyle factors. We retrospectively studied the relationship between total plasma homocysteine (Hcy), serum vitamin B12 (B12) and folate concentration in TS patients, taking into account the genetic profile, diet, smoking habits, hormonal therapies and dietary supplements of the subjects.

PATIENTS AND METHODS

We evaluated 50 TS patients (31.5 ± 12.5 years). Medication, including vitamin supplementation, was obtained. Eating habits, cigarette smoking, alcohol and coffee consumption were investigated using phone interviews. Levels of Hcy metabolism parameters were classified by using the relevant cutoff value for an adult population and compared with a reference sample drawn from the general population.

RESULTS

Inadequate Hcy and B12 levels were noted, despite vitamin supplementation. Holotranscobalamin (HoloTC) was above the relevant cutoff in the population, and supplemented subjects showed mean levels lower than nonsupplemented subjects (p = 0.005). Dietary supplementation (p = 0.038), lifestyle (coffee consumption, p = 0.01) and hormonal replacement therapy (p = 0.02) are important factors for Hcy metabolism. No genetic influence on Hcy levels was noted. Multivariable regression analysis identified vitamin supplementation (p = 0.045) as the only independent predictor of increased Hcy levels.

CONCLUSION

Cardiovascular risk in TS can be reduced using educational approaches to a healthy lifestyle with dietary guidelines. Besides this, we also recommend measuring HoloTC for the prompt detection of B12 deficiency and to consider hormone replacement therapy in the biochemical assessment of homocysteine in TS.

Homocysteine metabolism in children and adolescents with epidermolysis bullosa

Background

Epidermolysis bullosa (EB) belongs to a family of rare heterogeneous, genetic disorders characterized by blistering of the skin and mucous membranes in response to minor mechanical trauma. The involvement of the oral mucosa and oesophagus stenosis is suggested to be responsible for severe nutritional deficiencies, but few studies have till now considered this aspect. This observational study aimed to evaluate homocysteine status in children and adolescents with EB by assessing total plasma homocysteine (tHcy) and metabolically related vitamins (B₆, B₁₂, folate) concentrations.

Methods

Twenty EB patients (12 M; age range 0.5−19 years) were evaluated for: plasma tHcy, serum B₁₂ and holotranscobalamin (HoloTC, the active fraction of B₁₂), serum and erythrocyte folate (s-F and Ery-F, respectively), plasma B₆ and serum high sensitive C-reactive-protein (hsCRP) levels. Clinical severity was also evaluated through the Birmingham Epidermolysis Bullosa Severity (BEBS) score. A sex and age well-matched population was also enrolled.

Results

EB patients showed tHcy levels higher (p = 0.04) and B₆ levels lower (p = 0.03) than controls. B₁₂, HoloTC, s-F and ery-F concentrations did not differ between patients and controls. Multiple linear regression analysis showed that tHcy levels were independent of the metabolically related vitamins levels. In addition, serum hsCRP levels were higher in EB patients than in controls (p = 0.003) and correlated negatively with B₆ concentrations (r = -0.6; p = 0.009). BEBS score correlated negatively with HoloTC (p = 0.022) and B₆ (p = 0.005) levels and positively with age (p = 0.031) and hsCRP levels (p < 0.001).

Conclusions

The assessment of tHcy and metabolically related vitamin levels describes an important aspect of EB patients’ nutritional status which can result essential for their long term care. Monitoring B₆ levels in EB patients could be particularly important in order to prevent several complications associated with B₆ deficiency and to avoid a B₆ excess which sustains an inflammatory condition.

Holotranscobalamin (HoloTC, Active-B12) and Herbert's model for the development of vitamin B12 deficiency: a review and alternative hypothesis

The concentration of total vitamin B12 in serum is not a sufficiently sensitive or specific indicator for the reliable diagnosis of vitamin B12 deficiency. Victor Herbert proposed a model for the staged development of vitamin B12 deficiency, in which holotranscobalamin (HoloTC) is the first indicator of deficiency. Based on this model, a commercial immunoassay has been controversially promoted as a replacement for the total vitamin B12 test. HoloTC is cobalamin (vitamin B12) attached to the transport protein transcobalamin, in the serum, for delivery to cells for metabolism. Although there have been many published reports supporting the claims for HoloTC, the results of some studies were inconsistent with the claim of HoloTC as the most sensitive marker of vitamin B12 deficiency. This review examines the evidence for and against the use of HoloTC, and concludes that the HoloTC immunoassay cannot be used to measure vitamin B12 status any more reliably than total vitamin B12, or to predict the onset of a metabolic deficiency, because it is based on an erroneous hypothesis and a flawed model for the staged development of vitamin B12 deficiency. The author proposes an alternative model for the development of vitamin B12 deficiency.

Vitamin B12 and folate levels in healthy Swiss senior citizens: a prospective study evaluating reference intervals and decision limits

BACKGROUND

The vitamin B12 and folate status in nonanaemic healthy older persons needs attention the more so as decrease in levels may be anticipated from reduced haematinic provision and/or impaired intestinal uptake.

METHODS

A total of 1143 subjectively healthy Swiss midlands participants (637 females and 506 males), ≥60 years of age were included in this study. Levels of vitamin B12, holotranscobalamin (holoTC), methylmalonic acid (MMA), homocysteine (Hcy), serum folate, red blood cell (RBC) folate were measured. Further, Fedosov's wellness score was determined. Associations of age, gender, and cystatin C/creatinine-based estimated kidney function, with the investigated parameters were assessed. Reference intervals were calculated. Further, ROC analysis was done to assess accuracy of the individual parameters in recognizing a deficient vitamin B12 status. Finally, decision limits for sensitive, specific and optimal recognition of vitamin B12 status with individual parameters were derived.

RESULTS

Three age groups: 60-69, 70-79 and ≥ 80 had median B12 (pmol/L) levels of 237, 228 and 231 respectively (p = 0.22), holoTC (pmol/L) of 52, 546 and 52 (p = 0.60) but Hcy (μmol/L) 12, 15 and 16 (p < 0.001), MMA (nmol/L) 207, 221 and 244 (p < 0.001). Hcy and MMA (both p < 0.001), but not holoTC (p = 0.12) and vitamin B12 (p = 0.44) were found to be affected by kidney function. In a linear regression model Fedosov's wellness score was independently associated with kidney function (p < 0.001) but not with age. Total serum folate and red blood cell (RBC) folate drift apart with increasing age: whereas the former decreases (p = 0.01) RBC folate remains in the same bandwidth across all age groups (p = 0.12) A common reference interval combining age and gender strata can be obtained for vitamin B12 and holoTC, whereas a more differentiated approach seems warranted for serum folate and RBC folate.

CONCLUSION

Whereas the vitamin B12 and holoTC levels remain steady after 60 years of age, we observed a significant increment in MMA levels accompanied by increments in Hcy; this is better explained by age-related reduced kidney function than by vitamin B12 insufficiency. Total serum folate levels but not RBC folate levels decreased with progressing age.

Oxidative status in different settings and with different methodological approaches compared by Receiver Operating Characteristic curve analysis

OBJECTIVES

To test the performance of different analytical approaches in highlighting the occurrence of deregulated redox status in various physio-pathological situations.

DESIGN AND METHODS

35 light and 61 heavy smokers, 19 chronic renal failure, 59 kidney transplanted patients, and 87 healthy controls were retrospectively considered for the study. Serum oxidative stress and antioxidant status, assessed by spectrophotometric Reactive Oxygen Metabolites (d-ROMs) and Total Antioxidant Capacity (TAC) tests, respectively, were compared with plasma free (F-MDA) and total (T-MDA) malondialdehyde, both quantified by isotope-dilution-gas chromatography-mass spectrometry (ID-GC-MS). Sensitivity, specificity and cut-off points of T-MDA, F-MDA, d-ROMs and TAC were evaluated by both Receiver Operating Characteristic (ROC) analyses and area under the ROC curve (AUC).

RESULTS

Only T-MDA assay showed a clear absence of oxidative stress in controls and significant increase in all patients (AUC 1.00, sensitivity and specificity 100%). Accuracy was good for d-ROMs (AUC 0.87, sensitivity 72.8%, specificity 100%) and F-MDA (AUC 0.82, sensitivity 74.7%, specificity 83.9%), but not high enough for TAC to show in patients impaired antioxidant defense (AUC 0.66, sensitivity 52.0%, specificity 92.9%).

CONCLUSIONS

This study reveals T-MDA as the best marker to detect oxidative stress, shows the ability of d-ROMs to identify modified oxidative status particularly in the presence of high damages, and evidences the poor TAC performance. d-ROMs and TAC assays could be useful for routine purposes; however, for an accurate clinical data evaluation, their comparison versus a "gold standard method" is required.

Improvement of oxidative and metabolic parameters by cellfood administration in patients affected by neurodegenerative diseases on chelation treatment

Objective. This prospective pilot study aimed at evaluating the effects of therapy with antioxidant compounds (Cellfood, and other antioxidants) on patients affected by neurodegenerative diseases (ND), who displayed toxic metal burden and were subjected to chelation treatment with the chelating agent calcium disodium ethylenediaminetetraacetic acid (CaNa₂EDTA or EDTA). Methods. Two groups of subjects were studied: (a) 39 patients affected by ND and (b) 11 subjects unaffected by ND (controls). The following blood parameters were analyzed before and after three months' treatment with chelation + Cellfood or chelation + other antioxidants: oxidative status (reactive oxygen species, ROS; total antioxidant capacity, TAC; oxidized LDL, oxLDL; glutathione), homocysteine, vitamin B12, and folate. Results. After 3-months' chelation + Cellfood administration oxLDL decreased, ROS levels were significantly lower, and TAC and glutathione levels were significantly higher than after chelation + other antioxidants treatment, both in ND patients and in controls. Moreover, homocysteine metabolism had also improved in both groups. Conclusions. Chelation + Cellfood treatment was more efficient than chelation + other antioxidants improving oxidative status and homocysteine metabolism significantly in ND patients and controls. Although limited to a small number of cases, this study showed how helpful antioxidant treatment with Cellfood was in improving the subjects' metabolic conditions.

Holotranscobalamin is not influenced by decreased renal function in elderly men: the MrOS Sweden study

BACKGROUND

Subclinical cobalamin deficiency is common in the elderly, but the sensitivity and specificity of serum total cobalamin for this diagnosis is poor. Serum holotranscobalamin (holoTC), a measure of biologically available cobalamin, is considered a better marker for early cobalamin depletion than total cobalamin. However, in elderly populations, health-related reference intervals for holoTC and correlations to renal function are not entirely clear.

METHODS

HoloTC was determined with an automated microparticle enzyme immunoassay (AxSYM®) in 790 elderly non-vitamin-supplemented Swedish men, median age 75.3 years. Renal function was assessed with creatinine, cystatin C and estimated glomerular filtration rate (eGFR calculated from creatinine).

RESULTS

Median holoTC was 51.8 pmol/L, the health-related reference interval 19.6-132.3 pmol/L. There was no significant difference in mean holoTC in probands with normal compared to high creatinine (P = 0.80) and cystatin C (P = 0.82). No significant differences between the quartiles of creatinine or cystatin C in mean of log holoTC were seen. HoloTC correlated strongly with total cobalamin (r = 0.69, P < 0.001), weaker with eGFRcreatinine (r = -0.09, P < 0.05) and creatinine (r = 0.09, P < 0.05), the latter correlation was only seen in subjects with creatinine <100 µmol/L. HoloTC correlated negatively with plasma total homocysteine (r = -0.24, P < 0.001), but not with cystatin C and age.

CONCLUSIONS

Serum holoTC in healthy elderly men shows the same distribution as earlier described for a younger reference population. In this group of elderly subjects, holoTC did not correlate to reduced renal function. Thus, holoTC appears to be a promising tool for evaluating cobalamin status also in elderly populations.

An encapsulated juice powder concentrate improves markers of pulmonary function and cardiovascular risk factors in heavy smokers

OBJECTIVE

Cigarette smoking is associated with reduced pulmonary function and increased risk factors for cardiovascular disease. This randomized placebo-controlled double-blind study evaluated the effects of two different combinations of mixed fruit and vegetable juice powder concentrate (Juice Plus+, NSA, Collierville, TN) on heavy smokers.

METHODS

At baseline (T 0) and after 3 months' supplementation (T 1), pulmonary function parameters and cardiovascular risk factors-that is, plasma total homocysteine (tHcy) with related B vitamins and cysteine (tCys) concentrations-were assessed in 75 apparently healthy smokers (aged 49.2 ± 10.6 years, >20 cigarettes/d, duration ≥10 years) randomized into 3 groups: placebo (P), fruit/vegetable (FV) and fruit/vegetable/berry (FVB).

RESULTS

T 0: most smokers showed abnormalities in tHcy and tCys concentrations. T 1: respiratory function was unchanged in P and slightly, but not significantly, improved in FV, whereas FVB showed a significant improvement in forced expiratory flow at 25% (FEF25; p < 0.0001 vs P and FV) and significant improvement in CO diffusion lung/alveolar volume (DLCO/VA). FV and FVB (50%) showed significant reduction in tHcy and tCys compared to T 0 ( p < 0.0001) and P ( p < 0.0001).

CONCLUSIONS

At T 1, both supplemented groups, but to a greater extent the FVB group, showed improvements in some pulmonary parameters, cardiovascular risk factors, and folate status. The beneficial effects of Juice Plus+ supplementation could potentially help smokers, even if smoking cessation is advisable.

Holotranscobalamin is a useful marker of vitamin B12 deficiency in alcoholics

Background. Measurement of serum cobalamin (Cbl) levels is the standard investigation for assessing vitamin B12 deficiency. Falsely increased values of Cbl can be caused by alcoholic liver disease. Measurement of total vitamin B12 serum levels might be misleading in alcoholics, because a tissue metabolic deficiency is possible even with normal serum Cbl levels. Holotranscobalamin (HoloTC), the Cbl metabolically active fraction, is considered as a better index of vitamin B12 deficiency. Methods. For assessing vitamin B12 status, we evaluated 22 adult alcoholic male patients by measuring in parallel serum Cbl, serum folate and red blood cell folate levels, HoloTC levels by the AxSYM assay. Results. HoloTC values were reduced in 3 alcoholics with borderline-low Cbl values. Significant positive correlations were found between serum Cbl and HoloTC levels, serum Cbl and gamma-glutamyl transpeptidase (GGT). Conclusion. HoloTC measurement is a useful option for assessing vitamin B12 status in alcoholics, particularly in the subjects with borderline Cbl values and may be considered an early marker of vitamin B12 deficiency.

Screening for metabolic vitamin B12 deficiency by holotranscobalamin in patients suspected of vitamin B12 deficiency: a multicentre study

BACKGROUND

Vitamin B(12) deficiency occurs frequently, especially among the elderly. However, screening for vitamin B(12) deficiency is hampered by poor sensitivity of the existing total vitamin B(12) assay. Methylmalonic acid (MMA) is considered as the most representative indicator of metabolic vitamin B(12) deficiency and is used as such in this study. The aim of this study was to validate the clinical usefulness of holotranscobalamin (holoTC) as an initial screening assay for metabolic vitamin B(12) deficiency in a mixed patient population.

METHODS

Three hundred and sixty blood samples were collected by five Dutch hospitals. Vitamin B(12) and holoTC in serum were measured (AxSYM; Abbott). MMA in serum was measured by tandem mass spectrometry (LC-MS/MS).

RESULTS

Receiver operating curve (ROC) analysis demonstrated a greater area under the curve (AUC) for holoTC than for vitamin B(12) in detecting vitamin B(12) deficiency characterized by three predefined cut-off levels of MMA. A cut-off value of 32 pmol/L of holoTC resulted in the highest sensitivity (83%) with acceptable specificity (60%) in detecting MMA concentrations above 0.45 μmol/L. The combination of vitamin B(12) and holoTC did not improve diagnostic accuracy at this cut-off level.

CONCLUSIONS

HoloTC has a better diagnostic accuracy than vitamin B(12) and can replace the existing vitamin B(12) assay as a primary screening test in patients suspected of vitamin B(12) deficiency. Critical evaluation of cut-off values of holoTC indicated that a cut-off value of 32 pmol/L can be considered in screening for metabolic vitamin B(12) deficiency (defined by MMA > 0.45μmol/L) in a mixed patient population.

The relationship of leukocyte anisocytosis to holotranscobalamin, a marker of cobalamin deficiency

INTRODUCTION

After measurement of the mean volumes of leukocyte subpopulations as well as the distribution widths (DW) of these volumes has become available, we investigated whether such morphometric leukocyte parameters are associated with a commonly used marker of cobalamin deficiency, i.e., holotranscobalamin (HoloTC). Further, we determined reference intervals for these parameters in an elderly population.

METHODS

Consecutive subjectively healthy and volunteering individuals ≥60 years were included. Using the UniCel DxH 800 Coulter Cellular Analysis System MoMV, mean neutrophil volume (NeMV), mean lymphocyte volume (LyMV), monocyte anisocytosis (MoV-DW), neutrophil anisocytosis (NeV-DW), and lymphocyte anisocytosis (LyV-DW) were assessed together with other parameters including HoloTC.

RESULTS

A total of 150 individuals were included in the study. Reference intervals were not dependent on age and gender. MoV-DW (P = 0.002) and NeV-DW (P = 0.02) were significantly lower, and LyMV was significantly higher (P = 0.04) in participants with a HoloTC concentration <28 pm. In contrast, MCV, MoMV, NeMV, and LyV-DW were not associated with HoloTC concentrations. The area under the curve (AUC) in the receiver operating characteristic analysis for detecting a HoloTC <28 pm was 0.81 [95% confidence interval (CI) (0.73, 0.87)] for MoV-DW and 0.73 (0.66, 0.80) for NeV-DW.

CONCLUSION

In this collective of subjectively healthy elderly individuals, monocyte anisocytosis, neutrophil anisocytosis and mean lymphocyte volume were associated with decreased HoloTC.

Maternal one-carbon metabolism, MTHFR and TCN2 genotypes and neural tube defects in India

BACKGROUND

Neural tube defects (NTDs) are among the most common severe congenital malformations, representing a long-term public health burden in India. A deranged one-carbon metabolism and genes regulating this metabolism have been linked to NTDs. Vitamin B(12) deficiency is reported to be more prevalent than folate deficiency in the Indian population. We investigated the role of maternal nutritional and genetic markers related to one-carbon metabolism in the etiology of NTDs.

METHODS

We conducted a multicenter case-control study to compare plasma folate, vitamin B(12) , homocysteine and holo-transcobalamin levels, and polymorphisms in methylenetetrahydrofolate reductase (MTHFR, 677C>T, 1298A>C, 1781G>A and 236+724A>G) and transcobalamin (TCN2, 776C>G) genes, in 318 women with NTD-affected offspring (cases) and 702 women with apparently healthy offspring (controls). The samples were collected at diagnosis in cases and at delivery in controls.

RESULTS

We observed a significant association of high maternal plasma homocysteine concentrations with NTDs in the offspring (p = 0.026). There was no association of maternal folate or B(12) levels with NTDs (p > 0.05) but low maternal holo-transcobalamin predicted strong risk of NTDs in the offspring (p = 0.003). The commonly associated maternal polymorphism 677C>T in the MTHFR gene did not predict risk of NTDs in the offspring (p > 0.05) and 1298A>C and 1781G>A polymorphisms in MTHFR were protective (p = 0.024 and 0.0004 respectively). Maternal 776C>G polymorphism in TCN2 was strongly predictive of NTD in the offspring (p = 0.006).

CONCLUSION

Our study has demonstrated a possible role for maternal B(12) deficiency in the etiology of NTDs in India over and above the well-established role of folate deficiency.

Assessment of the vitamin B12 status of pregnant women in Nigeria using plasma holotranscobalamin

Maternal vitamin B12 deficiency during pregnancy is an independent risk factor for neural tube defects and other neurological problems in infants. We determined the vitamin B12 status of 143 pregnant women in Nigeria representing all trimesters who presented to an antenatal clinic in Jos, Nigeria, using holotranscobalamin II levels (holoTCII), which is a measure of the vitamin B12 that is available for uptake into tissues. The holoTCII concentration ranged from 13 to 128 pmol/L. Using a cutoff of 40 pmol/L, 36% of the women were classified as vitamin B12-deficient. HoloTCII concentrations correlated negatively with plasma homocysteine levels (r = −0.24, P = 0.003) and positively with red blood cell folate concentrations (r = 0.28, P < 0.001). These data underscore the importance of supplementing pregnant women in Nigeria with vitamin B12 in order to ensure adequate vitamin B12 status and decrease the risk for neural tube defects.

Holotranscobalamin, a marker of vitamin B-12 status: analytical aspects and clinical utility

Approximately one-quarter of circulating cobalamin (vitamin B-12) binds to transcobalamin (holoTC) and is thereby available for the cells of the body. For this reason, holoTC is also referred to as active vitamin B-12. HoloTC was suggested as an optimal marker of early vitamin B-12 deficiency >20 y ago. This suggestion led to the development of suitable assays for measurement of the compound and clinical studies that aimed to show the benefit of measurement of holoTC rather than of vitamin B-12. Today holoTC can be analyzed by 3 methods: direct measurement of the complex between transcobalamin and vitamin B-12, measurement of vitamin B-12 attached to transcobalamin, or measurement of the amount of transcobalamin saturated with vitamin B-12. These 3 methods give similar results, but direct measurement of holoTC complex is preferable in the clinical setting from a practical point of view. HoloTC measurement has proven useful for the identification of the few patients who suffer from transcobalamin deficiency. In addition, holoTC is part of the CobaSorb test and therefore useful for assessment of vitamin B-12 absorption. Clinical studies that compare the ability of holoTC and vitamin B-12 to identify individuals with vitamin B-12 deficiency (elevated concentration of methylmalonic acid) suggest that holoTC performs better than total vitamin B-12. To date, holoTC has not been used for population-based assessments of vitamin B-12 status, but we suggest that holoTC is a better marker than total vitamin B-12 for such studies.