Abstract
A competitive ELISA for the measurement of antithrombin III antigen was developed using a pure preparation of antithrombin. This two step immunoassay requires only one antibody binding step and avoids the binding of anti-antithrombin antibody to the solid phase. Plasma samples measured by this ELISA correlated well with measurements taken with immune electrophoresis measurements (r = 0.933); the ELISA was also shown to be both sensitive and precise.