Factor IX antigen by a rapid staphylococcal protein A-membrane binding radioimmunoassay: results in haemophilia B patients and carriers and in fetal samples.

Staphylococcal protein A-membranes have been used with isolated, radiolabelled factor IX and specified rabbit antisera for modification of a radioimmunoassay. The current method is a rapid 4 h procedure and dilution curves of plasma parallel those of isolated, unlabelled protein. Non-specific binding is 5%; the assay readily detects concentrations as low as 0.6 u/dl. Carrier detection of haemophilia B was improved and/or confirmed by the demonstration of factor IX antigen in excess of clotting activity in nine of 15 women tested from pedigrees in which the affected members had excess circulating antigen. Of 15 new haemophilia B pedigrees examined, 13 had antigen levels which were in two-fold or greater excess over their clotting activities; all but three were considerably below normal, however. To diagnose haemophilia B in newborns at risk, levels in three cord blood samples were tested; two were positive and the third was normal. Six fetal blood samples were assayed and contained from 4 to 20 u/dl factor IX antigen; levels correlated with fetal age.