a highly specific assay for plasma vitamin b 12 levels in humans is disclosed . the assay employs intrinsic factor preparations obtained from mouse or rat stomachs , which intrinsic factors are free of r - protein and thus react specifically with vitamin b 12 in a competitive binding assay . detection of vitamin b 12 levels is utilized in the diagnosis of several clinical disorders , such as pernicious anemia , a disease state which results in reduced circulating levels of vitamin b 12 in the blood .