Specific, sensitive, and quantitative enzyme-linked immunosorbent assay for human immunoglobulin G antibodies to anthrax toxin protective antigen

The bioterrorism-associated human anthrax epidemic in the fall of 2001 highlighted the need for a sensitive, reproducible, and specific laboratory test for the confirmatory diagnosis of human anthrax. The Centers for Disease Control and Prevention developed, optimized, and rapidly qualified an enzyme-linked immunosorbent assay (ELISA) for immunoglobulin G (IgG) antibodies to Bacillus anthracis protective antigen (PA) in human serum. The qualified ELISA had a minimum detection limit of 0.06 µg/mL, a reliable lower limit of detection of 0.09 µg/mL, and a lower limit of quantification in undiluted serum specimens of 3.0 µg/mL anti-PA IgG. The diagnostic sensitivity of the assay was 97.8%, and the diagnostic specificity was 97.6%. A competitive inhibition anti-PA IgG ELISA was also developed to enhance diagnostic specificity to 100%. The anti-PA ELISAs proved valuable for the confirmation of cases of cutaneous and inhalational anthrax and evaluation of patients in whom the diagnosis of anthrax was being considered. aturally occurring anthrax is a zoonotic disease of herbivores, with low-level sporadic infection of humans. Since 1950, human anthrax in the United States was confined to those occupationally at risk, with only 235 confirmed cases, mostly cutaneous, reported from 1955 to 2002 (1–3). The occurrence of human anthrax in the country and the public perception of the disease changed dramatically in the fall of 2001, with the first successful bioterrorist anthrax attack on the U.S. civilian population. This event necessitated the simultaneous development and application of qualified laboratory assays— including serologic assays—to evaluate patients suspected of having anthrax. The major obstacle to serologic analysis of human anthrax