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See TDL Laboratory Guide 2013 Pages 19,23,90
Conventional tests for Vitamin B12 measure total serum Vitamin B12, not metabolically active B12. It is better understood now that total B12 levels are not as clearly correlated with clinical symptoms as they might be. Clinically significant vitamin B12 deficiency can occur, and B12 status misclassified, even with Total Vitamin B12 levels apparently within normal range. Measuring Active B12 is diagnostically more accurate for detecting B12 deficiency that requires therapy.
It is fair to say that Active B12 and Total B12 do show good agreement at the extremes (ie “very deficient” or “not at all deficient”) but there is a large grey zone of indeterminate range between normal and abnormal which is likely to be misclassified if total serum B12 alone is relied upon. It is therefore expected that by testing with Active B12, findings will be less in number, but more clinically relevant.
All Vitamin B12s will be replaced by Active B12 whether requested as a single test, or requested with red cell or serum folate. The sample type and turnaround time remain unchanged.
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For further information see: www.active-b12.com