The frequency of "routine" low serum cobalamin levels in populations over age 50 years is high, being about 5% in one recent large midwestern study.1 Because a low serum cobalamin level in someone over age 50 years raises the diagnostic probability of pernicious anemia (PA), ie, the vitamin B12 deficiency disease due to an inadequate or absent gastric secretion of intrinsic factor (IF) of "unknown" cause, a classic Schilling test is often done in such cases to determine if the diagnosis is in fact early PA in the premegaloblastic anemia stage.2
See also p 1712.
Since the Schilling test results are usually normal, the conclusion is drawn that the low serum cobalamin level was an artifact (an error). In fact, often it is the normal Schilling test result that is misleading, and the low cobalamin level was in fact a signal that PA was developing, but in
Victor Herbert. Don't Ignore Low Serum Cobalamin (Vitamin B12) Levels. Arch Intern Med. 1988;148(8):1705–1707. doi:10.1001/archinte.1988.00380080009003