Limitations of Current Lab Tests for B12 Deficiency

Doctors working with B12 deficiency have noted the following limitations of current lab tests for B12 deficiency:

∞ ∞ ∞

According to Ralph Green, M.D., Ph.D.: “The startling and disturbing findings in this study are that all assays [tests for B12 deficiency] showed considerable variability before any treatment was initiated and that the results of these assays taken singly or in combination often did not reliably predict or preclude a response to specific treatment with vitamin B12. Taken at face value and given the general reliance placed on the clinical reliability of these tests for identification of cobalamin deficiency, these findings are extremely troubling … At this stage it would be prudent to conclude that the currently available assays for identifying or excluding cobalamin deficiency, though potentially useful, should be used with full awareness of their possible limitations, at least until unresolved issues have been settled.” (Green, R. Unreliability of current assays to detect cobalamin deficiency: “nothing gold can stay.” Blood, 2005;105: 910-911 – as reported in Could It Be B12?, 2nd Edition, pp. 216 and 217)

∞ ∞ ∞

“One problem is that there is limited information on patients with clinical findings of B12 deficiency but normal serum B12 and metabolite levels. Lawrence Solomon, M.D., of Yale University Health Services, evaluated thirty-seven patients who responded to vitamin B12 therapy and found that pretreatment values of serum B12 and Hcy [homocysteine] were normal in about 50 percent of the subjects and MMA [methylmalonic acid] values were normal in 25 percent.” (Solomon, L. R. Cobalamin-responsive disorders in the ambulatory care setting: unreliability of cobalamin, methylmalonic acid, and homocysteine testing. Blood 2005; 105:978-985 – as reported in Could It Be B12?, 2nd Edition, p. 217)

∞ ∞ ∞

“Over-diagnosis of B12 deficiency is essentially innocuous; but as this report [that of Ralph Green, M.D. quoted above] notes, ‘missed diagnosis is quite clearly, a matter of greater gravity, particularly since the risk of formidable devastation from neurologic damage that results from uncorrected cobalamin deficiency is preventable.'” (Could It Be B12?, 2nd Edition, p. 216)

∞ ∞ ∞

According to Dr. I. Chanarin, M.D., FRCPath: “In the absence of a single perfect diagnostic test, the importance of clinical criteria to classify patients as deficient or non deficient in Cobalamin cannot be understated.” 

∞ ∞ ∞


The essence of this important message is that if you have signs and symptoms of a B12 deficiency, do not ignore them, even if all the lab tests are “normal.” A therapeutic trial of B12 with monitoring of the response could prevent irreversible physical and mental problems. If you are interested in possible specific limitations of current testing, read on.


Serum blood test for B12 deficiency

Limitations may include, but not be limited to, the following:

1. The lower limit of the “normal range” is now considered too low. It was initially set at 200 pg/ml because that was the limit that had to be maintained to prevent abnormalities in the blood (pernicious anemia).

The many roles that B12 plays, in addition to preventing pernicious anemia, were not appreciated at that time. It is now recognized  (and even noted on some lab reports) that the range between 200 pg/ml and 450 pg/ml is considered a “grey area” where a certain percentage of people will experience symptoms caused by B12 deficiency. Deficiencies in the cerebrospinal fluid (CSF) begin to appear at serum B12 levels below 550 pg/ml. (VanTiggelen, C. J. M. et al. Assessment of vitamin-B12 status in CSF. American Journal of Psychiatry 141, 1:136-7, 1984 – as reported in Could It Be B12?, 2nd Edition, p. 218)

In fact, the lower limit of “normal” in Japan and some European countries is 500 – 550 pg/ml, based on the level that needs to be maintained to help prevent cognitive manifestations such as dementia and memory loss (Goodman, Mark, et al. Are U.S. lower normal B-12 limits too low? Journal of the American Geriatrics Society, Vol. 44, No. 10, October 1996, pp. 1274-75).

According to the authors of Could It Be B12?, “For brain and nervous system health and prevention of disease in older adults, serum B12 should be maintained near or above 1,000 pg/ml.” (Could It Be B12?, 2nd Edition, p. 11)


2. Vitamin B12 is present in blood bound to two transport proteins, transcobalamine II (TC) and haptocorrin (HC). Only Vitamin B12 bound to transcobalamine II (holoTC) is “active” and biologically functional. “Only around 20% of total serum B12 is in the active form our bodies use.” (Could It Be B12?, 2nd Edition, p. 13)

The serum B12 test does not reflect the percentage of the total B12 level that is actually usable by the body.

One study showed that “Patients with primarily neurological problems had significantly higher inactive B12 analogue levels … than did patients with primarily blood problems.” (Carmel R, Karnaze DS, Weiner JM. Neurologic abnormalities in cobalamin deficiency are associated with higher cobalamin ‘analogue’ values than are hematologic abnormalities. J Lab Clin Med. 1988 Jan;111(1):57-62.)


3. Serum B12 levels can be measured in the lab using R-protein or human intrinsic factor. According to Carmel et al. in the study cited above, “33 of the 76 patients with neurological symptoms had a normal serum B12 when measured with R-protein. But when measured with intrinsic factor, many of these patients had much lower serum B12 levels.”

So a “normal” level might reflect the way the serum B12 test was performed by the lab rather than the actual active B12 status of the individual being tested. 


4. “Patients with underlying liver disease, alcoholism, myeloproliferative disorders, lymphoma, or intestinal bacterial overgrowth often have falsely elevated serum B12 levels.” (from Could It Be B12?, 2nd Edition, p. 216) A normal, or even abnormally high, serum B12 value may reflect these underlying problems rather than adequate B12 levels. 




Blood or Urine Methylmalonic Acid (MMA)

Holotranscobalamin (HoloTC) Test

High levels of homocysteine could reflect inadequate B12, B6, or folate.

The MMA test is often considered the Gold Standard of B12 testing since high levels suggest that active B12 is lacking. Normal MMA levels suggest that adequate B12 is active and functioning in the general tissues of the body. However, according to Peter Hinde, the Blood Brain Barrier makes it more difficult for B12 to get into the nervous tissue. A test of the cerebrospinal fluid would be necessary to determine adequate levels of B12 in the nervous tissue. In light of the invasive nature of a spinal tap to withdraw cerebrospinal fluid for analysis, it makes sense to follow the recommendation of Pacholok and Stuart (Could It Be B12?) to give any individual with neurological symptoms, no matter what the test results, a therapeutic trial of B12.

The Holotranscobalamin (HoloTC) Test detects the amount of active B12. It has been termed investigational until recently, but is now available through Specialty Laboratories in Santa Monica, California. “A group of researchers concluded that the HoloTC and the serum B12 test had equal diagnostic accuracy in screening for metabolic B12 deficiency. They found that both tests used in combination provided a better screen than either assay alone.” (Miller, J. W., et al. Measurement of total vitamin B12 and holotranscobalamin, singly and in combination, in screening for metabolic vitamin B12 deficiency. Clinical Chemistry (2006) 52:2;278-285 – as reported in Could It Be B12, 2nd Edition, p. 13).