Macro vitamin B12: an underestimated threat

Background: The correct identification of the macro-B12 interference (macroforms) is paramount to avoid potential erroneous clinical decisions. Our objectives were to determine whether immunoassays are affected by the presence of macro-B12 and to validate a polyethylene glycol (PEG) precipitation procedure to detect it. Methods: Sixty-two serum samples obtained from healthy volunteers were analyzed to determine recovery and reference intervals (RIs) following PEG precipitation. Thereafter, 50 serum samples with very high levels of B12 (>1476 pmol/L) were randomly selected to search for macro-B12 interferences. Serum samples obtained from healthy volunteers and related PEG aliquots were analyzed on a Cobas (R) immunoassay. Patients' samples were analyzed on both Cobas (R) and Architect (R) immunoassays. Finally, samples suspected to contain macro-B12 were analyzed by size-exclusion chromatography (SEC) to confirm the presence of macro-B12. Results: Recovery and post-PEG RIs determined on a Cobas 8000 (R) in healthy volunteers ranged from 68.3% to 108.4% and from 122.1 to 514.4 pmol/L, respectively. Fifteen samples (30%) were found to show macro-B12 while using the recovery criteria, and nine samples (18%) while using the post-PEG RI. The other immunoassay ran on the Architect i2000 (R) was also affected by the presence of macro-B12. Size-exclusion chromatography studies confirmed the presence of macro-B12 (immunoglobulin-B12 complexes). Conclusions: The prevalence of macro-B12 in elevated B12 samples is high. We suggest to systematically screen for the presence of macro-B12 with PEG precipitation procedure in samples with elevated B12 levels to avoid potential misdiagnosis or harmful clinical consequences.