[Reliability of the detection of fetal fibronectin using monoclonal antibody FDC-6]

Detection of amniotic fluid in the vagina is commonly used to diagnosed rupture of amniotic membranes. In 1991 an immunoassay was introduced which is based on the detection of a fetal fibronectin isoform using the monoclonal antibody FDC-6. Fetal fibronectin is said to be absent from normal adult tissues and plasma, but present in fetal tissues. The low concentration of fetal fibronectin in maternal plasma is said not to interfere with the test results. Theoretically, demonstration of significant amounts of fetal fibronectin in vaginal secretions may be considered an unequivocal sign of leakage of amniotic fluid or, at least, of pre-rupture stretching of the membranes. We evaluated results of the membrane immunoassay and enzyme immunoassay, which are both based on the monoclonal antibody FDC-6. Fetal fibronectin has been determined in a clinically unambiguous context in amniotic fluid as well as in vaginal swabs and blood samples. In amniotic fluid, all membrane immunoassays performed were positive. The concentration of fetal fibronectin during the third trimester of pregnancy was lower than in the second trimester. Only 9/15 cases were positive for fetal fibronectin if vaginal swabs were taken 12 hours after unequivocal rupture of membranes and no amniotic fluid was visible in the vagina. Vaginal swabs of women with regular contractions showing cervical dilation of > 2 cm were positive for fetal fibronectin in all cases (20/20) whereas no fetal fibronectin could be determined in vaginal swabs obtained from clinically inapparent pregnant women. In all blood samples of pregnant women fetal fibronectin was detected and, surprisingly, also in blood samples of non-pregnant women, as well as in vaginal swabs of non-pregnant women with vaginal bleeding. In cases of unequivocal rupture or intactness of membranes, the results of both assays correspond well with the clinical findings. 12 hours after rupture of membranes the test may become negative if no amniotic fluid is visible in the vagina. As all blood samples had been positive for fetal fibronectin we conclude that vaginal bleeding will probably interfere with the test. The most surprising result is the apparent presence of fetal fibronectin in the blood samples of non-pregnant women. This observation can only be explained by a cross-reaction of the FDC-6 antibody with hepatic fibronectin.