A radioimmunoassay for tetanus antibodies using protein A - containing Staphylococcus aureus

To measure tetanus antibodies a trace amount of 125I-labeled tetanus toxin is mixed with appropriate dilutions of human serum or blood. The labeled antigen-antibody complexes are adsorbed to heat-killed staphylococci (Cowan I) via their surface protein A. The radioactivity of the washed solid phase is a function of the initial antibody concentration. The test allows the measurement of 6 X 10(-0) U of tetanus antitoxin in a volume of 0.03 ml. In order to avoid possible interferences, serum has to be diluted 20-fold before use. Taking that into account, the real border limit of sensitivity is 4 X 10(-3) U/ml serum. Antibodies may be measured in serum, in plasma, and even in heparinized blood. As to its sensitivity, the test compares well with the toxin neutralization procedure. It is superior to the previous radioimmunologic, enzymoimmunologic, and hemagglutination techniques with respect to sensitivity and reproducibility. It reflects the values obtained in the toxin neutralization test better than the other in vitro procedures, as shown by parallel assays of 17 sera.