Controversy in the Measurement of Vitamin B12 Levels
How do you know you have a nutrient deficiency? In the past, people had clear clinical signs of deficiency, or suffered with non-specific symptoms for years with no one knowing what the problem was. Nowadays, various biochemical tests can identify if nutrient levels are low. For each micronutrient, there appears to be an array of different testing methods that can be employed. The choice of test can depend on whether current or long-term assessment of nutrient status is needed, convenience and cost. Choice sparks controversy.
For vitamin B12, the following tests are routinely used to assess status: serum vitamin B12, holotranscobalamin, homocysteine and methylmalonic acid. Each test measures something different, and each has its own advantages and disadvantages (see Herrmann and Obeid, and Oberley and Yang for recent updates).
Serum vitamin B12 is a direct measure of the amount of the vitamin in the blood. This test is often used as the first stage of testing, such that very low levels (<148 pmol/L) are normally considered to be indicative of deficiency, and sub-clinical deficiency (148–221 pmol/L) requires further testing, although there is considerable variation in the levels used (Allen, CDC). The serum vitamin B12 test reflects recent intakes, and is also influenced by a host of other factors including certain illnesses, use of oral contraceptives, pregnancy and low folate status. In addition, some people can be deficient despite having “normal” serum levels, which could be related to where the vitamin is stored or genetic factors. Unless very low, serum vitamin B12 is not normally used to diagnose deficiency.
Holotranscobalamin II testing has been more recently developed and is considered by some to be a more useful measure of vitamin B12 status. This blood binding protein transports vitamin B12 and is considered “the metabolically active fraction of B12” according to Sobczyńska-Malefora and co-workers. It is less likely to be influenced by pregnancy or renal function, which are disadvantages of the other tests, and may have a better predictive value. The method is still undergoing testing.
Methylmalonic acid is considered to be the current “gold standard” measure of vitamin B12 deficiency. This metabolite increases in concentration when vitamin B12 is deficient. The enzyme methylmalonyl-CoA mutase requires vitamin B12 to convert methylmalonic acid to succinyl-CoA, important in cellular energy production; when vitamin B12 levels are low, the enzyme has a reduced activity and methylmalonic acid builds up. The test is a measure of functional deficiency. In patients with renal failure or poorer renal function, the test may generate false positives. Part of the increase in methylmalonic acid levels in the elderly is attributed to decreasing renal function. An analysis of the cost-effectiveness of vitamin B12 testing found the test to be considerably more costly than serum vitamin B12 (Health Quality Ontario). An advantage is that methylmalonic acid can also be measured in urine.
Homocysteine is considered to be a non-specific marker of vitamin B12 deficiency. This means that although homocysteine is elevated when vitamin B12 is deficient, homocysteine is also elevated for other reasons, which include folate deficiency and a genetic tendency for elevated levels.
From the discussions held during the session held on “B12 Methodologies and Clinical Diagnosis of Status” during the 9th International Conference on Homocysteine and One Carbon Metabolism, it is clear that there is no consensus yet on the best testing strategy. The controversy appears to come from which test is the best one to use, and what levels signify deficiency. Use of the serum vitamin B12 test alone is likely to miss some people who are deficient, and also over-diagnose others. Holotranscobalamin tests may be superior to serum B12, however the test has to be fully tested before it is used routinely, and it should also be comparable in ease of measurement and cost to the vitamin B12 test, or at least more specific and accurate to justify extra effort and cost. Methylmalonic acid testing is the gold standard, however clinicians need to be aware of patient characteristics, particularly the increase in levels that occurs with renal function. The higher cost may be prohibitive for screening, although justified if clinical symptoms indicate that testing is needed. Homocysteine cannot stand on its own as a measure of vitamin B12 status but must be accompanied by another measure.
There is still work ongoing to determine the best strategy to use, as found by Bailey and colleagues in a recent publication that modeled the association between serum vitamin B12 and methylmalonic acid. Ultimately, no single test can diagnose vitamin B12 deficiency at the moment. Medical professionals need to consider the overall health, diet, and results of other laboratory and clinical testing to determine the cause of deficiency and the best way to treat it.
Allen LH. How common is vitamin B-12 deficiency? Am J Clin Nutr. 2009 Feb;89(2):693S-6S. doi: 10.3945/ajcn.2008.26947A. Epub 2008 Dec 30. http://www.ncbi.nlm.nih.gov/pubmed/19116323
Bailey RL, Durazo-Arvizu RA, Carmel R, Green R, Pfeiffer CM, Sempos CT, Carriquiry A, Yetley EA. Modeling a methylmalonic acid-derived change point for serum vitamin B-12 for adults in NHANES. Am J Clin Nutr. 2013 Aug;98(2):460-7. doi: 10.3945/ajcn.113.061234. Epub 2013 Jun 26. http://www.ncbi.nlm.nih.gov/pubmed/23803883
Herrmann W, Obeid R. Utility and limitations of biochemical markers of vitamin B12 deficiency. Eur J Clin Invest. 2013 Mar;43(3):231-7. doi: 10.1111/eci.12034. Epub 2013 Jan 18.
Oberley MJ, Yang DT. Laboratory testing for cobalamin deficiency in megaloblastic anemia. Am J Hematol. 2013 Jun;88(6):522-6. doi: 10.1002/ajh.23421. Epub 2013 Mar 15. http://www.ncbi.nlm.nih.gov/pubmed/23423840
Sobczyńska-Malefora A, Gorska R, Pelisser M, Ruwona P, Witchlow B, Harrington DJ. An audit of holotranscobalamin ("Active" B12) and methylmalonic acid assays for the assessment of vitamin B12 status: Application in a mixed patient population. Clin Biochem. 2013 Aug 18. pii: S0009-9120(13)00385-8. doi: 10.1016/j.clinbiochem.2013.08.006. [Epub ahead of print] http://www.ncbi.nlm.nih.gov/pubmed/23965230