Unreliability of current assays to detect cobalamin deficiency: “nothing gold can stay”

High frequency of vitamin B12 deficiency in a Brazilian population

OBJECTIVE

There are few studies regarding vitamin B12 deficiency in developing countries. In Brazil, a late diagnosis of vitamin B12 deficiency progressing to severe neurological damage is common. Thus, the aim of the present study was to verify the frequency of vitamin B12 deficiency in two Brazilian populations (elderly and adult participants) and to compare different methods of vitamin B12 deficiency detection.

DESIGN

Five hundred participants were recruited from health centres from south-east Brazil and were separated into two groups: 60 years old or more and 30-59 years old. Vitamin B12 and folate concentrations were measured using electrochemiluminescence immunoassay (ECI) and RIA. Methylmalonic acid (MMA) was measured by LC coupled to tandem MS. Full blood counts were acquired using standard methods.

RESULTS

All participants had normal blood count results and mean cell volume less than 99 fl; none of them presented folate deficiency according to the results, which were all greater than 3 ng/ml. Cobalamin levels less than 200 pmol/l were identified by one of the two or by both methods in 7.2 % of the participants aged 60 years or more and 6.4 % of the participants aged 30-59 years. MMA levels were higher in older subjects (P = 0.007) compared with younger subjects. A greater correlation of MMA v. RIA was observed than of MMA v. ECI (P = 0.0017 v. P = 0.014). MMA quantification estimated that cobalamin deficiency was present in more than 11 % of the subjects for both studied groups.

CONCLUSIONS

The study shows that vitamin B12 deficiency is frequent in Brazilian adults and suggests that RIA is more sensitive than ECl for measuring cobalamin levels.

Indicators for assessing folate and vitamin B12 status and for monitoring the efficacy of intervention strategies

Deficiencies of folate or of vitamin B12 are widespread and constitute a major global burden of morbidity affecting all age groups. Detecting or confirming the presence of folate or vitamin B12 deficiency and distinguishing one from the other depends, ultimately, on laboratory testing. Tests to determine the presence of folate or vitamin B12 deficiency are used singly or in combination to establish the nutritional status and prevalence of deficiencies of the vitamins in various populations. The efficacy of interventions through the use of fortification or supplements is monitored using the same laboratory tests. Tests currently in use have limitations that can be either technical or have a biological basis. Consequently, each single test cannot attain perfect sensitivity, specificity, or predictive value. Laboratory indicators of vitamin B12 or folate status involve measurement of either the total or a physiologically relevant fraction of the vitamin in a compartment such as the blood. Thus, assays to measure vitamin B12 or folate in plasma or serum as well as folate in red blood cells are in widespread use, and more recently, methods to measure vitamin B12 associated with the plasma binding protein transcobalamin (holotranscobalamin) have been developed. Alternatively, levels of surrogate biochemical markers that reflect the metabolic function of the vitamin can be used. Surrogates most commonly used are plasma homocysteine, for detection of either vitamin B12 or folate deficiency and methylmalonic acid for detection of vitamin B12 deficiency. The general methods as well as their uses, indications, and limitations are presented.

Folic acid and cognition in older persons

Folic acid supplementation has drawn much attention in recent years for the prevention of Alzheimer's disease and cognitive decline. In this review, the authors describe how current evidence does not support the use of folic acid supplements to protect against cognitive decline. Although a few studies suggest that folic acid supplementation may provide neuroprotection among persons who are folate deficient, there is also data to indicate that supplementation in persons without folate deficiency may pose a risk to neurological function. Vitamin B12 deficiency is common in old age and may not be easy to recognise. Folic acid supplementation may mask the anaemia associated with vitamin B12 deficiency and, therefore, may delay treatment while allowing progression of neurological symptoms. Whether or not folic acid supplementation exacerbates neurological symptoms of vitamin B12 deficiency is not clear. Further studies are needed to determine the possible risks and benefits of folic acid supplementation in older persons.

Measurement of Total Vitamin B12 and Holotranscobalamin, Singly and in Combination, in Screening for Metabolic Vitamin B12 Deficiency

Background: The standard screening test for vitamin B12 deficiency, measurement of total plasma vitamin B12, has limitations of sensitivity and specificity. Plasma vitamin B12 bound to transcobalamin (holoTC) is the fraction of total vitamin B12 available for tissue uptake and therefore has been proposed as a potentially useful alternative indicator of vitamin B12 status.

Methods: We compared the diagnostic accuracy of total vitamin B12, holoTC, and a combination of both measures to screen for metabolic vitamin B12 deficiency in an elderly cohort (age ≥60 years). Plasma methylmalonic acid and homocysteine were used as indicators of vitamin B12 deficiency.

Results: Low total vitamin B12 (<148 pmol/L) and low holoTC (<35 pmol/L) were observed in 6.5% and 8.0%, and increased methylmalonic acid (>350 nmol/L) and homocysteine (>13 μmol/L) were observed in 12.1% and 17.0% of the study participants. In multiple regression models, holoTC explained 5%–6% more of the observed variance in methylmalonic acid and homocysteine than did total vitamin B12 (P ≤0.004). ROC curve analysis indicated that total vitamin B12 and holoTC were essentially equivalent in their ability to discriminate persons with and without vitamin B12 deficiency. Individuals with low concentrations of both total vitamin B12 and holoTC had significantly higher concentrations of methylmalonic acid and homocysteine than did individuals with total vitamin B12 and/or holoTC within the reference intervals (P <0.001).

Conclusions: HoloTC and total vitamin B12 have equal diagnostic accuracy in screening for metabolic vitamin B12 deficiency. Measurement of both holoTC and total vitamin B12 provides a better screen for vitamin B12 deficiency than either assay alone.