Determination of 'total' and 'true' cobalamin with the SimulTRAC Assay

The determination of 'total' and 'true' cobalamin (the SimulTRAC Assay, Becton Dickinson) has been compared with other laboratory tests and with clinical symptoms typical of cobalamin deficiency. Patient specimens (n = 5709) were analysed for total cobalamin which was measured by its binding to a crude hog intrinsic factor preparation in the radioisotope assay. Specimens with borderline values (150-225 pmol/l) were reanalysed for true cobalamin in order to find out whether there were patients with cobalamin deficiency and a low true cobalamin content in serum. Of 803 patient sera with total cobalamin values between 150-225 pmol/l two patients had low true cobalamin values (less than 110 pmol/l) and typical laboratory and clinical signs of cobalamin deficiency. On the other hand three patients in this range had laboratory and clinical signs of cobalamin deficiency and significantly higher values with true than with total cobalamin. Measurement of true cobalamin is therefore not an unequivocal way to detect patient sera from patients with a cobalamin responsive anaemia. Analysis of true cobalamin does not seem to increase the sensitivity and specificity for cobalamin deficiency.