ELISA test for the detection of an immunological blocking factor in human pregnancy serum

We have previously shown that progesterone-treated lymphocytes of healthy pregnant women can produce a 34 kDa protein, progesterone-induced blocking factor (PIBF) capable of blocking lymphocyte function in vitro. Lymphocytes of women with idiopathic threatened pre-term delivery failed to produce this factor. On the assumption that the PIBF appears in the serum of healthy pregnant women but not in that of women at risk for immunologically based abortion of pre-term delivery, an ELISA has been designed for detecting the blocking factor in serum samples. Sera obtained at the time of delivery, as well as those from women with pre-term deliveries or miscarriages, contained significantly less PIBF than sera of 209 healthy pregnant women. Data obtained by testing sera of women at 16 weeks gestation whose pregnancies ended in spontaneous abortion showed that the predictive value of the test depended on the time interval between blood sampling and the onset of abortion. In 11 of 13 women with uterine contractions the outcome of pregnancy was predictable by normal or lower than normal levels of PIBF in the sera taken at the time the symptoms were presented. The data suggest that determination of PIBF in pregnancy serum might contribute to the diagnosis of immunologically bases pre-term disruption of pregnancy.