Foetal Fibronectin Test

Foetal fibronectin is a glycoprotein that sits between the foetal membranes and the decidua. Its presence in cervicovaginal secretions is associated with the separation of membranes, and it is used to assess the risk of spontaneous preterm birth in women with symptoms.

A negative test has a high negative predictive value for delivery within about one to two weeks, while a positive test has low specificity. The test is generally used to reduce unnecessary admissions in women with symptoms, not to diagnose preterm labour.

Usage. It is used in women with signs of possible preterm labour between 22 and 34 weeks of gestation, with intact membranes and a cervix that is not dilated beyond the threshold.

Indications. Symptoms suggestive of preterm labour between 22 and 34 weeks, with intact membranes and a cervix that is not dilated beyond the local threshold, commonly 3 cm.

Preparation and technique. A swab is taken from the posterior fornix of the vagina, after speculum examination and before digital examination. The swab is placed in the transport medium, and the immunoassay is performed by the laboratory.

Contraindications and precautions. Vaginal bleeding, ruptured membranes, cervical dilatation beyond the threshold, recent sexual intercourse within the preceding 24 hours, and recent digital examination or vaginal ultrasound, all of which can give false positives.

Interpretation.

• A negative result shows foetal fibronectin under the cut off, with a high negative predictive value for delivery within about one to two weeks.
• A positive result shows foetal fibronectin over the cut off, with a low positive predictive value.
• Predictive values depend on the prevalence of preterm birth in the population studied.

Causes of a false positive result. Vaginal bleeding, recent intercourse, digital examination, semen, vaginal ultrasound, and infection.

Causes of a false negative result. Testing at the wrong time or after prolonged rupture, and sampling technique.

Critical values and findings needing immediate action. A positive result in a woman with active labour requires immediate assessment and management of preterm labour. A negative result does not exclude preterm labour and does not justify discharge without clinical assessment.

Limitations and artefacts. The test has a low specificity in the general population and in women with symptoms. Bleeding, intercourse and examination shortly before the test produce false positives.

Follow up tests. Transvaginal cervical length measurement, which is more accurate in many settings, and serial clinical assessment of cervical change and contractions.

Special groups. Multiple pregnancy changes the baseline risk of preterm birth, and the interpretation of a result is adjusted. Women with previous preterm birth have a higher prior risk. The test is not used beyond 34 weeks of gestation.