A Solid‐Phase Radioassay for the Quantitative Determination of Antiplasmin Activity in Human Plasma

Summary. The caseinolytic activity of one CTA (Committee on Thrombolytic Agents) unit of human plasmin is inhibited by a series of plasma dilutions containing antiplasmin. The neutralization of the standard plasmin by increasing amounts of antiplasmin shows a steeper linear decrease of plasmin activity between 1.0 and 0.5 CTA units and a much smaller further inactivation below 0.5 CTA units. It is thought that the standard plasmin is partially damaged at the antiplasmin combining site during the purification procedure and might be responsible for the differences in plasmin-antiplasmin neutralization in the standard curve. Using the steeper slope of the plasmin neutralization curve, an average of 8.6±1.0 CTA units plasmin neutralizing activity per ml human plasma was found in 36 healthy donors. The difficulty of obtaining ‘native’ standard plasmin with full antiplasmin combining capacity represents the main problem of a reproducible reliable antiplasmin assay.