RESUMO
This study aims to assess the usefulness of serum holotranscobalamin [holo TC], the fraction of vitamin B12 that is available for tissue uptake, compared with total vitamin B12 in patients investigated for vitamin B12 disorders. Serum samples were randomly selected from 76 patients [48females, 28 males; age range 12-69 years] referred to the Clinical Biochemistry Laboratory, Royal Hospital for the assessment of vitamin B12 status. For each patient, serum total vitamin B12 was determined by chemiluminescent microparticle immunoassay on Architect 2000 analyzer and holo TC[active vitamin B12] level was determined by microparticle enzyme immunoassay on Axsym analyzer [both from Abbott, USA]. Comparison of the data was conducted to reflect the mean, standard deviation [SD] and correlation coefficient between the two groups. The mean [SD] for serum holo TC and total vitamin B12 were 46.5[32.2] pmol/L and 316.3[165.6] pmol/L respectively. There was a significant correlation between holo TC and total vitamin B12 [r=0.765, P<0.001] and the regression equation was expressed as; y= a+bx [i.e: holot TC = 1.5+0.14 total vitamin B12]. Also, the results were assessed for any misclassification when comparing holo TC and the total vitamin B12 in terms of whether each or both values agree or disagree for classifying the patients as having normal or abnormal [low or high] results, based on the cut-off thresholds of the kit's quoted reference range for holo TC of 9-123 pmol/L and for total vitamin B12 of 140-600 pmol/L. Accordingly, in69 [90.8%] samples, there was a parallel agreement/ classification of results, both being normal or abnormal. In 61 [80.4%] patients, both results were normal, whereas in 4 [5.2%] patients, both results were high, and in 4 [5.2%] patients both results were low. However, in 7[9.8%] cases, there was disagreement/ misclassification of results; 6 [7.8%] patients, holo TC was normal while total vitamin B12 was low, and in 1 [1.4%] patient, holo TC was normal while total vitamin B12 was low, and in 1 [1.4%] patient, holo TC was normal while total vitamin B 12 was high. It can be recommended that holo TC and total vitamin B 12, alone and in combination, have almost equal diagnostic efficiency reflector of vitamin B 12 status. Further comparison studies based on a gold standard method for classifying vitamin B 12 status are worth considering