I found this 2003 article which makes a couple of interesting points:
1. The use of low serum B12 (< 200 pg/mL) as a sole means of diagnosis may miss up to half of the patients who have actual tissue B12 deficiency (they were looking at elderly patients)
2. If B12 is between 100 - 400 pg/mL, then MMA and homocysteine should be checked and a B12 deficiency is indicated if either of these are high (this logic is in the flowchart graphic, Figure 3)
They back these up with statistics from studies.
Article is here:
Title: Vitamin B12 Deficiency
Authors: ROBERT C. OH, CPT, MC, USA, U.S., Army Health Clinic, Darmstadt, Germany
DAVID L. BROWN, MAJ, MC, USA, Madigan Army Medical Center, Fort Lewis, Washington
Publication: Am Fam Physician. 2003 Mar 1;67(5):979-986.