A double antibody solid phase radioimmunoassay for placental alkaline phosphatase.

A solid-phase "direct" radioimmunoassay for determination of placental alkaline phosphatase in serum was developed. The assay is highly sensitive, with a minimum detectable dose of 9 ng protein per ml. The specificity for the placental type of the enzyme, in comparison to other serum alkaline phosphatases, was found to be convincing. The choice of genetic types of either placental alkaline phosphatase or antiserm was found not to influence the radioimmunoassay. With the described technique the serum concentration of this enzyme during normal pregnancy was measured. A 25-fold increase from low levels during the first trimester up to 252 +/- 70 ng/ml (mean +/- S.D.) in gestation week 40 was observed.