|MP 2.04.39||Serum Holotranscobalamin as a Marker of Vitamin B12 (Cobalamin) Status|
|Original Policy Date
|Last Review Status/Date
Reviewed with literature search/8:2012
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Holotranscobalamin (holo-TC) is a transcobalamin-vitamin B12 complex which has been investigated as a diagnostic test for vitamin B12 deficiency in symptomatic and at-risk populations, as well as a assay for monitoring response to therapy.
Vitamin B12 (cobalamin) is an essential vitamin that is required for DNA synthesis affecting red blood cell formation and methionine synthesis affecting neurologic functioning. Cobalamin deficiency can result from nutritional deficiencies or malabsorption. Dietary insufficiency is most common among vegetarians and elderly people. Malabsorption of vitamin B12 may be associated with autoantibodies, as in pernicious anemia, or can occur after gastrectomy, or in other gastrointestinal tract conditions, such as celiac disease, Whipple’s disease, and Zollinger-Ellison syndrome. Clinical signs and symptoms of cobalamin deficiency include megaloblastic anemia, paresthesias and neuropathy, and psychiatric symptoms, such as irritability, dementia, depression, or psychosis. While the hematologic abnormalities promptly disappear after treatment, neurologic disorders may become permanent if treatment is delayed.
The diagnosis of cobalamin deficiency has traditionally been based on low levels of total serum cobalamin, typically less than 200 pg/mL, in conjunction with clinical evidence of disease. However, this laboratory test has been found to be poorly sensitive and specific. Therefore, attention has turned to measuring metabolites of cobalamin as a surrogate marker. For example, in humans only 2 enzymatic reactions are known to be dependent on cobalamin: the conversion of methylmalonic acid (MMA) to succinyl-CoA, and the conversion of homocysteine and folate to methionine. Therefore, in the setting of cobalamin deficiency, serum levels of MMA and homocysteine are elevated and have been investigated as surrogate markers.
There also is interest in the direct measurement of the subset of biologically-active cobalamin. Cobalamin in serum is bound to 2 proteins, transcobalamin and haptocorrin. Transcobalamin-cobalamin complex (called holotranscobalamin, or holo-TC) functions to transport cobalamin from its site of absorption in the ileum to specific receptors throughout the body. Less than 25% of the total serum cobalamin exists as holo-TC, but this is considered the clinically relevant biologically active form. Serum levels of holo-TC can be measured using a radioimmunoassay or enzyme immunoassay.
In January 2004, the device HoloTC RIA (Axis-Shield plc, Dundee, UK) is an example of a radioimmunoassay for holo-TC that was cleared for marketing by the U.S. Food and Drug Administration (FDA) through the 510(k) process. The FDA determined that this device was substantially equivalent to existing devices for use in: “quantitative measurement of the fraction of cobalamin (vitamin B12) bound to the carrier protein transcobalamin in the human serum or plasma. Measurements obtained by this device are used in the diagnosis and treatment of vitamin B12 deficiency.”
In November 2006, the device Axis-Shield HoloTC Assay (Axis-Shield, Dundee, UK), an enzyme immunoassay for holo-TC, was cleared for marketing by the FDA through the 510(k) process. The FDA determined that this device was substantially equivalent to existing devices for use in: “quantitative determination of holotranscobalamin…in human serum and plasma on the AxSym® System. HoloTC is used as an aid in the diagnosis and treatment of vitamin B12 deficiency.”
Measurement of holotranscobalamin is considered investigational in the diagnosis and management of Vitamin B12 deficiency.
In July 2005, the following CPT category III code describing holo-TC became available:
0103T: Holotranscobalamin, quantitative
BlueCard/National Account Issues
State or federal mandates (e.g., FEP) may dictate that all devices approved by the U.S. Food and Drug Administration (FDA) may not be considered investigational and thus these devices may be assessed only on the basis of their medical necessity.
This policy was created in 2005 and updated regularly with searches of the MEDLINE database, most recently for the period of March 2009 through June 2010. There were no clinical trials identified that directly evaluated the utility of testing cobalamin status with serum holotranscobalamin. There were also no trials that evaluated the benefit of treatment in individuals with subclinical cobalamin deficiency. The diagnostic performance and operating characteristics continue to be an area of active research. One systematic review and several randomized controlled trials (RCTs) have been identified addressing this area.
Review articles highlight the analytical aspects and clinical utility of the use of holo-TC. (1,2)
Validation of the clinical use of any diagnostic test focuses on 3 main principles: 1) the technical feasibility of the test; 2) the diagnostic performance of the test, such as sensitivity, specificity, and positive and negative predictive value in different populations of patients and compared to the gold standard; and 3) the clinical utility of the test, i.e., how the results of the diagnostic test will be used to improve the management of the patient.
The serum measurements of holo-TC involve the use of standard laboratory immunoassay techniques. In the first step, holo-TC in the serum sample is separated by magnetic microspheres coated with monoclonal antibodies to human transcobalamin. The cobalamin bound to the holo-TC is then released and measured by a competitive binding radioimmunoassay or fluorescence, depending on the device used.
The diagnostic performance must be compared to the established gold standard for measuring cobalamin deficiency. This is particularly problematic, since there is currently no established gold standard. As noted in the Description section, serum levels of total cobalamin are considered poorly sensitive and specific, and holo-TC measurements are not independent of total cobalamin measures, leading to a potential bias in the estimate of the test’s diagnostic power. There have been several reports proposing serum measures of methylmalonic acid (MMA) and homocysteine as an alternative gold standard (3-5); their specificity has been questioned. (6,7)
According to the U.S. Food and Drug Administration (FDA) decision summary, the cut-off values for holo-TC were based on a normal population instead of a population of those with known cobalamin deficiency. For example, the low value of holo-TC, 37 pmol/L, was based on a study of 303 normal Finnish individuals. This study has also been published by Loikas and colleagues in the peer-reviewed literature. (8) Participants included 226 normal elderly subjects and 80 normal, non-elderly adult subjects. Patients were excluded from the trial if they had hyperhomocysteinemia, evidence of a possible cobalamin deficiency. In addition, patients in the lowest one third of holo-TC results underwent additional testing with methylmalonic acid (MMA); those with elevated MMA levels were also excluded. In the normal reference population, the holo-TC range was 25–254 pmol/L with a 95% central reference interval of 37–171 pmol/L. Therefore, the cut-off value for a low result was established at 37 pmol/L. This cut-off value was then applied to the results of 107 patients with presumed cobalamin deficiency, as evidenced by different combinations of an increased plasma homocysteine or MMA level, or a low total serum cobalamin level, defining patients with potential, possible, or probable cobalamin deficiency. A total of 48% of those with presumed deficiency had a holo-TC below 37 pmol/L. The frequencies of low holo-TC levels increased with increasing pretest probability of cobalamin deficiency. For example, among the 16 patients thought to have the highest pretest probability of cobalamin deficiency, based on elevated levels of homocysteine and MMA, 100% had low levels of holo-TC. Therefore, this study used levels of homocysteine and MMA as the gold standard. Based on this standard, the sensitivity of the test was only 48% among those with cobalamin deficiency rated as either potential, possible, or probable. The authors conclude that further studies are needed to confirm the clinical utility and specificity of holo-TC in diagnosis of subclinical cobalamin deficiency. Also, these values for a homogeneous population of Finnish subjects with a diet high in fish might not be able to be extrapolated to the heterogeneous American population and diet. Furthermore, these cut-off points require confirmation in a larger population of patients whose cobalamin status is unknown.
In April 2009, Hoey and colleagues published a systematic review of the response of various biomarkers to treatment with vitamin B12. (9) Only one RCT utilizing holo-TC was identified for the review, therefore the authors concluded that data were insufficient to draw conclusions about the effectiveness of serum holo-TC as a biomarker for vitamin B12 status. The included RCT follows:
In a double-blind trial to determine the effects of B12 supplementation of cognitive functioning in older adults, Eussen and colleagues measured holo-TC at baseline, 12, and 24 weeks in 195 subjects randomized to three groups: cobalamin, cobalamin plus folate supplementation, or placebo. The primary outcome measure was cognitive improvement. (10) The results did not support a significant difference in cognitive functioning. The authors noted a significant time-treatment interaction after 12 weeks in both treatment arms of holo-TC for all biomarkers measured (vitamin B12, MMA, holo-TC, homocysteine and red blood cell folate [p <0.0002]). Specifically for holo-TC, in the vitamin B12 group, mean levels increased from 58 ± 21 to 183 ± 124 (p <0.05 for difference from baseline). In the folate and vitamin B12 supplementation group, holo-TC increased from 68 ± 33 to 222 ± 133 (p <0.05 for difference from baseline). Comparatively, the placebo group’s levels did not significantly change, from 70 ± 39 to 65 ± 43 (p <0.05 for difference from treatment groups). Further changes did not occur between 12 and 24 weeks of supplementation.
Eussen and colleagues published a smaller trial in 2008. (10) Once again, patients were randomly assigned to cobalamin, cobalamin plus folate, or placebo supplementation in subjects with known mild cobalamin deficiency. Along with serum cobalamin and MMA levels, holo-TC was utilized to assess deficiency status and did rise in response to therapy. Other recent studies have utilized holo-TC as one of a number of measures of cobalamin status. (11-15) However, these studies do not attempt to assess the independent predictive capacity of the test and therefore do not add to the evidence base for this policy.
Valente and colleagues reported on the diagnostic accuracy of holotranscobalamin, MMA, serum cobalamin, and other indicators of tissue vitamin B12 status in an elderly population. (16) Elderly subjects (n =700), age range 63-97 years, were recruited from an ongoing observational cohort study to collect data on 2,000 individuals older than 60 years with mild to moderate cognitive impairment. A separate reference population of 120 healthy volunteers, age 18-62 years, was used to determine a reference interval for the red cell cobalamin assay. The cut-offs for deficiency were defined as 20 pmol/L for holo-TC, 123 pmol/L for serum cobalamin, and less than 33 pmol/L for red cell cobalamin. The red cell lower limit of 33 pmol/L packed red cells was used to dichotomize the concentrations into deficient and nondeficient vitamin B12 status for the construction of receiver operating characteristic (ROC) plots. The areas under the curve (AUC) showed that serum holo-TC was the best predictor with AUC 0.90 (95% confidence interval [CI]: 0.86 -0.93), and this was significantly better (p <0.0002) than the next best predictors serum cobalamin 0.80 (95% CI: 0.75 -0.85), and MMA 0.78 (95% CI 0.72-0.83). For these 3 analytes, the authors constructed a 3-zone partition of positive and negative zones and a deliberate indeterminate zone between. The boundaries were values of each test that resulted in a posttest probability of deficiency of 60% and a posttest probability of no deficiency of 98%. The proportion of indeterminate observations for holo-TC, cobalamin, and MMA was 14%, 45%, and 50%, respectively.
Advocates of holo-TC testing suggest that this laboratory test can identify early subclinical stages of cobalamin deficiency or other conditions, permitting prompt initiation of treatment, specifically supplementary cobalamin dietary supplementation. Further, this reasoning suggests that early diagnosis will lead to an improvement in health outcome in patients. This hypothesis was not directly tested in any of the identified published literature. In the absence of a gold standard, the clinical significance of subclinical cobalamin deficiency must be further studied by understanding the natural history of this condition. Does subclinical deficiency inevitably progress to clinical deficiency? Does cobalamin supplementation normalize the values? How variable are cobalamin levels within patients? These clinical issues have not been well addressed in the literature. Finally, for all patients at risk, i.e., vegetarians, the elderly, and post-gastrectomy patients, the recommended treatment of subclinical disease is further dietary supplementation of cobalamin. This recommendation is appropriate, regardless of the level of measured cobalamin.
Heil and colleagues aimed to validate the clinical usefulness of holo-TC as an initial screening assay for metabolic vitamin B12 deficiency in a mixed patient population. (17) Three hundred and sixty blood samples were collected by 5 Dutch hospitals, and vitamin B12 and holo-TC in serum were measured. MMA in serum was measured by tandem mass spectrometry. Receiver-operating-curve analysis demonstrated a greater area under the curve for holo-TC than for vitamin B12 in detecting vitamin B12 deficiency characterized by 3 predefined cut-off levels of MMA. A cut-off value of 32 pmol/L of holo-TC resulted in the highest sensitivity (83%) with acceptable specificity (60%) in detecting MMA concentrations above 0.45 μmol/L. The combination of vitamin B12 and holo-TC did not improve diagnostic accuracy at this cut-off level. The authors concluded that holo-TC has a better diagnostic accuracy than vitamin B12 and can replace the existing vitamin B12 assay as a primary screening test in patients suspected of vitamin B12 deficiency.
There are inadequate data to establish holotranscobalamin testing as an alternative to either total serum cobalamin, or levels of MMA or homocysteine in the diagnosis of vitamin B12 deficiency. While technically feasible, and likely to have diagnostic performance that approaches that of currently utilized tests, no evidence of clinical utility has been demonstrated, neither as a screening tool in the general or at-risk population, nor as a diagnostic tool in symptomatic individuals. Evidence of the clinical utility of the test is currently lacking, and therefore the test remains investigational.
Technology Assessment, Guideline and Position Statements
Many societies have recommended vitamin B12 supplementation for specific clinical conditions or evaluation for vitamin B12 deficiency in the workup for clinical indication without specifying a methodology. An exception is in a practice parameter for peripheral neuropathy by the American Academy of Neurology (AAN), who has specified a methodology (evidence level C): “serum B12 level with metabolites (methylmalonic acid with or without homocysteine)” in the evaluation for vitamin B12 deficiency. (18)
Medicare National Coverage
No national coverage determination
|ICD-9 Diagnosis||Investigational for all codes|
|ICD-10-CM (effective 10/1/13)||Investigational for all codes|
|ICD-10-PCS (effective 10/1/13)||Not applicable. No ICD procedure codes for laboratory tests.|
Holo-TC, Vitamin B12 Deficiency
Vitamin B12 Deficiency, Transcobalamin
|04/1/05||Add policy to Medicine section, Pathology/ Laboratory subsection||New policy|
|04/25/06||Replace policy||Policy updated with literature search; no change in policy statement|
|03/13/08||Replace policy||Policy updated with literature search, no change to policy statement. No references added.|
|08/12/10||Replace policy||Policy updated with literature search, no change in policy statement. References 4-5 and 7-15 added.|
|8/11/11||Replace policy||Policy updated with literature search, no change to policy statement. References 1-2 added and references reordered|
|08/09/12||Replace policy||Policy updated with literature search, no change to policy statement. Reference 17 added.|