Effects of Immune Complexes on Holotranscobalamine Assay of Vitamin B-12 Deficiency in Myeloproliferative Disorders

Abstract

In myeloproliferative disorders (MPDs), vitamin B-12 levels are measured falsely elevated with conventional methods due to increased carrier protein synthesis. HoloTranscobalamine (HoloTC) assay is a first-choice method for detecting true vitamin B-12 deficiency in MPDs. Our aim was to determine effects of immune complexes on HoloTC assay. This is a cross-section study. Vitamin B-12 levels in 61 patients with myeloproliferative disorders were measured by both electrochemical immunoassay and HoloTC assay. The HoIoTC cutoff was greater than 35 pmol/L. HoloTC assay for each sample were repeated after polyethylene glycol (PEG) treatment to exclude IgG, IgA and IgM type immune complexes. Also, methylmalonic acid, folate, homocystein, liver, and kidney function tests were obtained. Methylmalonic acid test showed that 42 patients (68.9%) had vitamin B-12 deficiency. Vitamin B-12 levels by HoIoTC assay decreased by 19.2 +/- 11.28% in essential thrombocytosis, 40.0 +/- 9.39% in chronic myeloid leukemia, 30.9 +/- 14.62% in myelofibrosis and 21.2 +/- 11.55% in polycythemia vera patients after PEG treatment. There was significant difference between the averages of groups (p< 0.01). Methylmalonic Acid Test was used as the B-12 status variable. The comparison of ROC curves of HoIoTC before and after PEG showed no statistically significance between area under curves. The optimum cut-off points for both HoloTC before and after PEG were 40.6 pmol/L and 32.1 pmol/L, respectively. Immune complexes may have some effect on HoIoTC assay which has been recently reported to have a superior diagnostic accuracy for vitamin Biz deficiency in patients with MPDs. Although exclusion of immune complexes did not improve its diagnostic performances, effects of exclusion were significantly different between subgroups of MPDs.