A sensitive enzyme immunoassay for pregnancy-associated plasma protein A (PAPP-A): a possible first trimester method of screening for Down syndrome and other trisomies.

Pregnancy-associated plasma protein A (PAPP-A) is a large glycoprotein produced mainly by the trophoblast during pregnancy and released into the maternal circulation. Its biological function is unknown. In the second trimester i.e. when Down syndrome (DS) screening is routinely performed, the level of maternal serum PAPP-A was found to be within the normal range in pregnancies affected by fetal trisomy 21. However, PAPP-A was shown to be a potent marker for DS before 14 weeks of gestation. Only radioimmunoassays (RIAs) based on labelled antigen competition reached the required sensitivity for early pregnancy PAPP-A determinations; but they have a very short shelf life due to inherent tracer half-life and, in the case of PAPP-A, instability of the labelled antigen after three weeks. We describe a convenient and novel enzyme immunoassay (ELISA) with high sensitivity and a long shelf life.