Two-site enzyme immunoassay for pregnancy associated plasma protein A (PAPP-A)

Pregnancy associated plasma protein A (PAPP-A) is one of the newly isolated placental proteins whose concentration in the maternal circulation increases during pregnancy. The limited sensitivity of the immunoelectrophoretic techniques first developed to measure PAPP-A meant that assays could only be done in the second half of pregnancy but now radioimmunoassays have been set up in order to determine the concentration of this protein in early pregnancy as well. The two-site enzymometric immunoassay described here is characterised by a sensitivity and a precision which are comparable to the results obtained by radioimmunoassay. Neither the structurally similar alpha 2-macroglobulin, nor other pregnancy associated proteins interfered in the assay. The tracer, monospecific rabbit anti-PAPP-A coupled covalently to horseradish peroxidase, is stable for months when stored adequately.