Use of radioisotope techniques in the clinical evaluation of patients with megaloblastic anemia

Because virtually all cases of vitamin B 12 deficiency seen in this country are due to malabsorption, the availability of radioactive vitamin B 12 for direct measurement of absorption of this essential nutrient has proved to be of great clinical value. These tests are useful not only in demonstrating vitamin B 12 malabsorption but also often in defining the pathophysiological mechanism responsible for this abnormality. The urinary excretion test of Schilling remains the most useful test for vitamin B 12 absorption. Minor precautions and modifications in technique make the test results more reliable and easier to interpret. The 8-hr plasma test for vitamin B 12 absorption can no longer be considered acceptable. Some patients with vitamin B 12 malabsorption have results in the normal range when studied by this method. Serum vitamin B 12 assays utilizing radioactive vitamin B 12 and the isotope dilution principle are now widely used and are useful screening tests. Low normal or borderline results observed in patients with clinical evidence suggestive of vitamin B 12 deficiency should be interpreted with caution or confirmed by radioactive vitamin B 12 absorption studies. Radioactive vitamin B 12 can also be used for rapid, reliable assay of gastric intrinsic factor, antibody to intrinsic factor and unsaturated vitamin B 12 serum binding capacity. Methods using radioactive folate compounds for similar in vivo and in vitro studies are not yet applicable for routine use in nuclear medicine laboratories.