Studies on a radioassay for intrinsic factor antibody: comparison of methods and false positive results due to elevated serum B12 levels.

Availability of a kit method (Corning Immo Phase IFAB) for intrinsic factor antibody (IFAB) has made it possible for a routine radioimmunoassay (RIA) laboratory to test for the antibody, thereby providing another aid in diagnosing pernicious anemia. Comparison of data from a charcoal method with the kit method was favorable, each method detecting 35 (74%) positives and 12 (26%) negatives of 47 pernicious anemia patients. Compared with a charcoal method study the kit method had fewer false positives due to elevated serum B12. False positive results occurred for only 24 hours after a 1-mg injection of B12, and results remained negative the following seven days. The authors' studies supported the manufacturer's statement that results are unreliable when the serum B12 level exceeds 3,500 pg/ml. Clinical experience with the Corning Immo Phase IFAB test and false positive results is summarized.