The limitations of serum B12 testing have been reported for some time.

When testing for deficiency, Serum B12 does not give an accurate picture unless there is an overt deficiency and even then not consistently.  This is because the majority (approx.70%) of B12 in the blood is attached to haptocorrin, which is unable to enter cells, and is termed inactive B12 (Lloyd-Write et al 2003).

Dr David Spenser a neurologist in the stroke prevention and atherosclerosis research center in Ontario Canada, was interviewed recently on Radio National Health Report.  His findings suggest that if serum B12 levels are below 400pmol/L both methylmalonic acid and homocysteine begin to rise, causing a decrease in metabolic function of B12 for suppressing these two factors.

The normal range of Serum B12 is 160-600 pm/L, which is clearly too large a range and is likely to be missing many functional B12 deficiencies, especially in the elderly where B12 digestion and absorption is often compromised.  He concluded that any serum B12 result <400pmol/L is ambiguous and therefore requires clarification with the more reliable Active B12 assay (Transcobalamin II).

This is at odds with current recommendations of Australian pathology companies.