Holotranscobalamin, a New Early Marker Improves the Diagnostic Utility of the Macrocytic Anemia Panel Assays Performed on the Abbott AxSYM® Analyser.
Authors: Bamonti Fabrizia , Fracchiolla Nicola 
Published: October 2019

A diagnosis of macrocytic anemia, a common blood disorder presenting in many forms, requires additional tests to determine the underlying cause. In particular, the prevalence of a sub-clinical functional Vitamin B12 (B12) deficiency in the general population is higher than expected. Treatment of B12 deficiency is important to prevent neurological and/or hematological complications but requires a reliable diagnosis. Total serum cobalamin concentrations may not reliably indicate B12 status. Several recent studies have reported that holotranscobalamin (HoloTC), the bio-available cobalamin fraction, would be an early indicator of B12 status with improved accuracy compared to total serum B12. We evaluated the diagnostic value of measurements of a macrocytic anemia panel (total B12, HoloTC, Folate and Homocysteine) in order to characterize populations at risk of metabolic cobalamin deficiency. Our observational study included 66 myelofibrosis patients (MF), 56 neurological patients (26 probable Alzheimer’s disease, AD; 17 Mild Cognitive Impairment, MCI; 13 Vascular Disease,VaD), 20 vegans (V) and 16 apparently healthy volunteers (C) matched for age and sex. Serum concentrations of B12, Folate, and HoloTC were determined by microparticle enzyme immunoassay (MEIA); plasma Hcy levels were measured by fluorescence polarization immunoassay (FPIA) all on the Abbott AxSYM analyser. Hyperhomocysteinemia (>12 micromol/L) was present in 33.3% of MF (median 10.4, range 3.9–30.9); 38.5% of AD (median 11.4, range 7.1–46.5); 41.2% of MCI (median 12.0, range 8.3–16.4); 69.2% of VaD (median 15.0, range 6.6–23.6); 76.5% of V (median 14.2, range 5.9–31.5) and 33.3% of C (median 10.0, range 5.7–15.8). Low levels of B12 (< 180 pmol/L) were found in 10.6% of MF (median 471, range 137–959); 11.5% of AD (median 275, range 78–695); 11.8% of MCI (median 259, range 127–648); 15.4% of VaD (median 283, range 114–591); 10.6% of V (median 231, range 172–883) and 11.1 % of C (median 237, range 177–795). Low levels of HoloTC (< 35 pmol/L) were found in 43.9% of MF (median 46, range 14.9–123); 46.1% of AD (median 43, range 6.0–185); 52.9% of MCI (median 37, range 12–114); 23.1% of VaD (median 56, range 31–105); 82.3% of V (median 15.3, range 4.1–197) and 33.3 % of C (median 45, range 29–75). Vegans showed HoloTC levels significantly lower than all the other groups (p<0.02, Mann Whitney U test). Folate levels were considered not to add information in this context. Our preliminary findings clearly showed that HoloTC together with Hcy can help detect subjects likely to develop neurological and/or hematologic complications, and thereby likely to benefit from early tailored pharmacological treatment. In conclusion, HoloTC determination may be included as a complementary or alternative diagnostic strategy.

Anemia Panel Results In At-Risk Groups Group Hcy>12μmol/L(%) B12<180pmol/L (%) HoloTC<35pmol/L (%) MF 33.3 10.6 43.9 AD 38.5 11.5 46.1 MCI 41.2 11.8 52.9 VaD 69.2 15.4 23.1 V 76.5 10.6 82.3 C 33.3 11.1 33.3