What vitamin B12 level is considered as vitamin B12 deficiency?

Serum vitamin B12 levels below approximately 170-250 pg/mL indicate deficiency in adults. Vitamin B12 deficiency is most often caused by malabsorption from food, pernicious anemia (an autoimmune disease that affects gastric mucosa), postsurgical malabsorption, and dietary deficiency. However, for many deficient individuals the cause is unknown.