Laboratory diagnosis of vitamin B12 and folate deficiency: a guide for the primary care physician.

At one time, the diagnosis of a deficiency of vitamin B12 or folate was considered to be relatively straightforward. As knowledge has accumulated, the limitations of such tests as serum vitamin level measurements and the Schilling test have become apparent. With the development of newer tests, atypical and subclinical deficiency states have been recognized. In this review, available tests used in the diagnosis of vitamin B12 and folate deficiency are discussed, and a rational approach to the diagnosis of these deficiency states is presented.