Screening for metabolic vitamin B12 deficiency by holotranscobalamin in patients suspected of vitamin B12 deficiency: a multicentre study

Background: Vitamin B-12 deficiency occurs frequently, especially among the elderly. However, screening for vitamin B-12 deficiency is hampered by poor sensitivity of the existing total vitamin B-12 assay. Methylmalonic acid (MMA) is considered as the most representative indicator of metabolic vitamin B-12 deficiency and is used as such in this study. The aim of this study was to validate the clinical usefulness of holotranscobalamin (holoTC) as an initial screening assay for metabolic vitamin B-12 deficiency in a mixed patient population. Methods: Three hundred and sixty blood samples were collected by five Dutch hospitals. Vitamin B-12 and holoTC in serum were measured (AxSYM; Abbott). MMA in serum was measured by tandem mass spectrometry (LC-MS/MS). Results: Receiver operating curve (ROC) analysis demonstrated a greater area under the curve (AUC) for holoTC than for vitamin B-12 in detecting vitamin B-12 deficiency characterized by three predefined cut-off levels of MMA. A cut-off value of 32 pmol/L of holoTC resulted in the highest sensitivity (83%) with acceptable specificity (60%) in detecting MMA concentrations above 0.45 mu mol/L. The combination of vitamin B-12 and holoTC did not improve diagnostic accuracy at this cut-off level. Conclusions: HoloTC has a better diagnostic accuracy than vitamin B-12 and can replace the existing vitamin B-12 assay as a primary screening test in patients suspected of vitamin B-12 deficiency. Critical evaluation of cut-off values of holoTC indicated that a cut-off value of 32 pmol/L can be considered in screening for metabolic vitamin B-12 deficiency (defined by MMA > 0.45 mu mol/L) in a mixed patient population.