Problems associated with the radioimmunoassay of serum trypsin

A commercial trypsin radioimmunoassay (RIA) kit was used for its ability to measure trypsin bound to the serum protease inhibitors, alpha 2 macroglobulin (alpha 2 M) and alpha 1 anti-trypsin (alpha 1 AT). Only 20% of trypsin bound to alpha 2 M and 70% bound to alpha 1 AT was detected by the assay system. Recovery of trypsin added to human serum varied from 0 to 20%. Standard curves prepared from purified human cationic trypsin did not exhibit parallelism with the kit standard curves. Inclusion of horse serum in the standard solutions improved the parallelism observed. Immunoreactive trypsin (IRT) levels obtained for serum samples were found to vary considerably depending on the standard curve used to calculate the assay results. Lower IRT levels were observed when trypsin standards prepared in the absence of horse serum were used as reference.