I believe it is important to note that suboptimal levels of B12 may be present long before symptoms serious enough to seek healthcare. Tests for B12 deficiency are also likely to miss B12 deficiency until it progress to stage 3 or 4.

• Stage 1 – “declining blood levels of the vitamin” – signs and symptoms often go unnoticed or neglected and may only be detectable by measuring holoTC (reduced in deficiency) in serum, plasma, or urine.
• Stage 2 – “low cellular concentrations of the vitamin” – signs and symptoms often go unnoticed or neglected and may only be detectable by measuring holoTC (reduced in deficiency) in serum, plasma, or urine.
• Stage 3 – “increased blood level of homocysteine and a decreased rate of DNA synthesis” – may produce stronger signs and symptoms and may be detectable by homocysteine and blood methylmalonic acid (both elevated in deficiency).
• Stage 4 – “pernicious anemia” – where signs and symptoms commonly associated with B12 deficiency are often seen. This stage is where markers such as hemoglobin and red blood cells are likely to present as anemia and serum B12 levels are likely to be reduced to below “normal” levels.

Sources:

• RHR: What Causes Neuropathy—and How To Treat It (2014)
• B12 deficiency: a silent epidemic with serious consequences (2012)
• Vegetarianism and Nutrient Deficiencies (2009)