Vitamin B12 deficiency is a major public health issue and is widespread. Among the population groups at risk are the elderly, pregnant women and their offspring, vegetarians, and patients with intestinal diseases. The classic haematological signs and symptoms may be absent or non-specific. Neurological symptoms may be the only manifestation of deficiency and can be irreversible if treatment is delayed. Early and accurate detection of Vitamin B12 deficiency is therefore important. The current widely used test, total serum B12, is now recognised as being a late marker, with sub-optimal sensitivity and specificity. This diagnostic challenge has led to the search for earlier and more accurate biochemical markers for Vitamin B12 deficiency. Holotranscobalamin, or Active-B12, has been shown to be an early marker of Vitamin B12 deficiency and may fulfill this role.