Soluble transcobalamin receptor, sCD320, is present in human serum and relates to serum cobalamin - establishment and validation of an ELISA.

BACKGROUND: The cell-surface receptor, TCblR/CD320, binds and internalizes transcobalamin-bound cobalamin. So far, no assay has been developed in order to quantify the soluble form of the receptor. METHODS: This study developed an ELISA employing a monoclonal antibody that recognized the extracellular domain of CD320 as capture antibody and a biotinylated polyclonal antibody for detection. Serum samples were analyzed from blood donors (n==153) and samples from patients referred for analysis of serum cobalamin [n=757, serum cobalamin 75-2000 pg/mL (55-1476 pmol/L)]. RESULTS: The ELISA specifically measured soluble CD320 with a quantification limit of 1.6 arbitrary units and a total variation of 4%-8%. Size exclusion chromatography was used which showed that the immunoreactive protein observed in serum eluted in the same position as soluble CD320 in the calibrator. Male donors (n=76) showed significantly higher median concentration [18 arbitrary units (arb.u.)] than female donors (n=77, median 16 arb.u.), but the absolute difference was marginal and suggested that a common reference interval of 12-97 arb.u. can be used. A positive correlation was observed between levels of soluble CD320 and serum cobalamin with median levels increasing from 17 arb.u. [n=203, cobalamin 75-270 pg/mL (55-200 pmol/L)] to 23 arb.u. [n=350, cobalamin 814-2000 pg/mL (601-1476 pmol/L)], p<0.0001. CONCLUSIONS: An ELISA was established for measurement of soluble CD320 and it was shown that the protein is present in human serum. The serum concentrations of soluble CD320 correlated positively with serum cobalamin levels.

Elevated plasma vitamin B12 levels as a marker for cancer: a population-based cohort study.

BACKGROUND: A substantial proportion of patients referred for plasma vitamin B12 (cobalamin [Cbl]) measurement present with high Cbl levels, which have been reported in patients with different cancer types. However, the cancer risk among patients with newly diagnosed high Cbl levels has not been adequately examined. METHODS: We conducted this cohort study using population-based Danish medical registries. Patients referred for Cbl measurement with levels greater than the lower reference limit (≥200 pmol/L) were identified from the population of Northern Denmark during the period of 1998 to 2009 using a database of laboratory test results covering the entire population. Data on cancer incidence (follow-up 1998-2010), Cbl treatment, and prior diagnoses were obtained from medical registries. Patients receiving Cbl treatment were excluded. Cancer risks were calculated as standardized incidence ratios (SIRs) with 95% confidence intervals (CIs), stratified by plasma Cbl levels. All statistical tests were two-sided. RESULTS: We identified 333 667 persons without prevalent cancer and not receiving Cbl treatment. Six percent had Cbl levels greater than the upper reference limit (≥601 pmol/L). Cancer risk increased with higher Cbl levels and was highest during the first year of follow-up (Cbl 601-800 pmol/L: SIR = 3.44, 95% CI = 3.14 to 3.76; Cbl >800 pmol/L: SIR = 6.27, 95% CI = 5.70 to 6.88; both P < .001). The risks were particularly elevated for hematological and smoking- and alcohol-related cancers for persons with high Cbl levels. CONCLUSIONS: High Cbl levels were associated with the risk of subsequently diagnosed cancer, mostly within the first year of follow-up. This may have clinical implications for the interpretation of high Cbl levels.

Positive predictive value of the diagnosis coding for vitamin B12 deficiency anemia in the Danish National Patient Register.

PURPOSE: The aim of this validation study was to assess the positive predictive value (PPV) of the International Classification of Diseases, 10th revision (ICD-10) diagnosis of vitamin B12 (cobalamin [Cbl]) deficiency anemia in the Danish National Patient Register (DNPR). PATIENTS AND METHODS: We identified all patients above 18 years of age recorded in the DNPR with a diagnosis of Cbl deficiency anemia (ICD-10 code: D51) admitted to two Danish university hospitals and one Danish regional hospital from 2000 through 2009. We assessed the PPV using biochemical parameters on Cbl deficiency with or without anemia as reference standards. These data were obtained from the Laboratory Information Systems Database. Data on prescriptions with Cbl supplementation drugs were obtained from the Aarhus University Prescription Database. RESULTS: We identified 1089 patients coded with a Cbl deficiency anemia diagnosis in the DNPR. The PPV was 31.5% (95% confidence interval [CI]: 28.8%-34.3%) and 36.8% (95% CI: 34.0%-39.7%) depending on definitions of Cbl deficiency with anemia. When using Cbl deficiency without anemia as a reference standard, the PPV was 51.3% (95% CI: 48.4%-54.3%). The PPV for Cbl supplemented patients was 22.2% (95% CI: 18.0%-26.9%) and for non-Cbl supplemented patients 63.9% (95% CI: 60.5%-67.3%). CONCLUSION: The PPVs of the ICD-10 diagnosis coding for Cbl deficiency anemia were generally low in the DNPR. Therefore, this register should be used with caution to study patients with Cbl deficiency anemia.