The development and validation of a radioimmunoassay for human pregnancy-associated plasma protein A (PAPP-A)

A specific and sensitive radioimmunoassay for human pregnancy-associated plasma protein A (PAPP-A) is described. This assay utilized tracer repurified by affinity chromatography on heparin-Sepharose. The most sensitive assay developed had a detection limit of 5 microliters PAPP-A in serum, and this protein could not be detected in sera from non-pregnant subjects. During pregnancy, mean circulating levels rose from 0.6 mg/l at 6 weeks gestation to 110 mg/l at term, and PAPP-A was detected in 44 of 101 umbilical blood samples. The use of the anticoagulants EDTA and acid citrate dextrose resulted in significantly different results in the plasma compared to the corresponding serum sample. PAPP-A was also measured in amniotic fluid.