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Fetal Fibronectin

Fetal fibronectin (fFN) is a large glycoprotein located in the extracellular matrix of the choriodecidual junction between the maternal decidua and the fetal membranes. It is an adhesive glycoprotein that crosslinks collagen and binds cells together. Some research has suggested that it plays a role in implantation. fFN can be detected in cervicovaginal secretions during the first 22 weeks of pregnancy until the fetal membranes completely fuse to the maternal decidua. fFN levels reach their peak of approximately 4000 ng/mL between 10 and 12 weeks gestation, fall to below 50 ng/mL by 22 weeks, and remain at undetectable levels until 36 to 37 weeks. By 37 weeks gestation, fFN becomes more heavily glycosylated, loses its adhesive properties and is again detectable in cervicovaginal secretions. Mechanical stress caused by uterine contractions and local inflammation lead to separation of the choriodecidiual interface and promote the release of fetal fibronectin into the vagina. Between 24 and 37 weeks gestation, fFN should normally not be present in the cervix and vagina. Detection of fFN in cervicovaginal secretions at a concentration of >50 ng/mL indicates the patient is at high risk for preterm labor and subsequent early birth.

The fFN test is most useful in differentiating “true” preterm labor from false labor in symptomatic women presenting with contractions between 24 to 34.9 weeks gestational age. Patients who present with contractions but insignificant cervical dilation or effacement can often have their therapy modified based on the presence or absence of fetal fibronectin. Without fFN testing, 20% of these women deliver preterm if sent home. For predicting delivery within 7 days in symptomatic women at 24 to 34.9 weeks gestation, fFN’s sensitivity is 57-93% and specificity is 73-92%. The positive predictive value (PPV) is only 13%, but the negative predictive value is 97-99.6%. FFN works well for predicting when a symptomatic woman will not deliver, but not for predicting when she will deliver.

FDA has approved fFN for the diagnosis of impending premature delivery in symptomatic women who are at 24.0 to 34.9 weeks’ gestation. For asymptomatic women who are at 22 to 39 weeks’ gestation, fFN is FDA approved for predicting the risk of preterm delivery.

The American College of Obstetrics and Gynecology has recommended the use of this test if the following criteria are met (Int’l J Obstet Gynecol 1997; 59:164):

  • Amniotic membranes are intact
  • Cervical dilatation <2 cm and effacement <80%
  • Sampling is performed no earlier than 24 weeks, 0 days and no latter than 34 weeks, 6 days of gestation (using a swab of the vaginal vault).

Amniotic fluid is rich in fFN, therefore, the test cannot be used in women with ruptured membranes. Specimens should not be collected from patients with known placental abruption, placenta previa, ruptured membranes, vaginal bleeding, or intercourse in the last 24 hours because these factors can cause false positive results. Multiple pregnancies cause elevated FFN concentrations.

False Negative Results

False Positive Results

Lubricants

Blood

Soaps

Amniotic fluid

Disinfectants

Recent intercourse

Cerclage

Digital exam within 24 hours

Fetal fibronectin is sampled from cervical and vaginal secretions. Serum samples are not used due to the presence of fFN produced by hepatocytes. FFN should be collected at the beginning of the speculum examination before any lubricants are used and before the digital examination. Lubricating ointments and Betadine should be avoided because they can cause false negative results. A special Dacron polyester swab and transport tube must be used. Any cervical vaginal mucus can be used; however, the best sampling site is the cervical os. During a sterile speculum examination, the swab is gently rolled over the exocervix and dipped into the posterior fornices, but not placed into the cervical canal. When fully saturated, the swab contains approximately 150 uL of secretions.

Reference value is negative. The optimal cutoff for FFN is 50 ng/mL to determine preterm delivery. A negative test indicates that the FFN level is <50 ng/mL.

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