Improved immunoassay for the determination of surfactant protein A (SP-A) in human amniotic fluid.

A simple, improved immunoassay for the determination of human surfactant protein A (SP-A) in human amniotic fluid was developed. The immunoreaction with a monoclonal antibody PC6-immobilized plastic bead, peroxidase-labeled monoclonal antibody PE10 and amniotic fluid sample diluting in the buffer containing 0.6% sodium dodecyl sulfate/2% Triton X-100, was carried out simultaneously at 45 degrees C for 30 min in test tubes. After washing the bead with 2% skimmilk in phosphate buffered saline containing 1% Triton X-100, the peroxidase reaction was developed by adding the substrate reagent and the absorbance was measured. The amniotic fluid obtained at full term was used as a standard instead of purified SP-A, because of the stability of the antigenicity. This immunoassay method was used to measure SP-A in 69 samples of amniotic fluid from 22 to 41 weeks of gestation. The result indicated that the SP-A values obtained by the present immunoassay can be used for predicting the fetal lung maturity. This simplified monoclonal immunoassay completed the measurement within 1 hr, and so it could be used routinely in clinical laboratory.